Acta Crystallographica Section B via MedWorm.com

Crystal growth and twinned crystal structure of Sr2CaWO6

Acta Crystallographica Section B — Wed, 17 Mar 2010 16:21:37 +0100

Single crystals of Sr2CaWO6 have been prepared by sintering at high temperature. Powder samples were compressed into rods and heated up to 1953 K. This seems a promising new route for further studies of the structure and physical properties of double perovskites. The structural model of Sr2CaWO6 includes a quantitative description of the twinning shown by the diffraction pattern that should be present in almost any single-crystal specimen for this type of compound. (Source: Acta Crystallographica Section B)

l-2-Aminobutyric acid: two fully ordered polymorphs with Z′ = 4

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

The crystal structure of l-2-aminobutyric acid, an l-alanine analogue with an ethyl rather than a methyl side chain, has proved elusive owing to problems growing diffraction quality crystals. Good diffraction data have now been obtained for two polymorphs, in space groups P21 and I2, revealing surprisingly complex, yet fully ordered crystalline arrangements with Z′ = 4. The closely related structures are divided into hydrophilic and hydrophobic layers, the latter being the thinnest ever found for an amino acid (other than α-glycine). The hydrophobic layers furthermore contain conspicuous pseudo-centers-of-symmetry, leading to overall centrosymmetric intensity statistics. Uniquely, the four molecules in the asymmetric unit can be divided into two pairs that each forms an independent hydr...

Universal prediction of intramolecular hydrogen bonds in organic crystals

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

A complete exploration of intramolecular hydrogen bonds (IHBs) has been undertaken using a combination of statistical analyses of the Cambridge Structural Database and computation of ab initio interaction energies for prototypical hydrogen-bonded fragments. Notable correlations have been revealed between computed energies, hydrogen-bond geometries, donor and acceptor chemistry, and frequencies of occurrence. Significantly, we find that 95% of all observed IHBs correspond to the five-, six- or seven-membered rings. Our method to predict a propensity for hydrogen-bond occurrence in a crystal has been adapted for such IHBs, applying topological and chemical descriptors derived from our findings. In contrast to intermolecular hydrogen bonding, it is found that IHBs can be predicted across the ...

Predicted crystal structures of tetramethylsilane and tetramethylgermane and an experimental low-temperature structure of tetramethylsilane

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

No crystal structure at ambient pressure is known for tetramethylsilane, Si(CH3)4, which is used as a standard in NMR spectroscopy. Possible crystal structures were predicted by global lattice-energy minimizations using force-field methods. The lowest-energy structure corresponds to the high-pressure room-temperature phase (Pa\overline{3}, Z = 8). Low-temperature crystallization at 100 K resulted in a single crystal, and its crystal structure has been determined. The structure corresponds to the predicted structure with the second lowest energy rank. In X-ray powder analyses this is the only observed phase between 80 and 159 K. For tetramethylgermane, Ge(CH_3)_4, no experimental crystal structure is known. Global lattice-energy minimizations resulted in 47 possible crystal structures w...

More examples of the 15-crown-5...H2O—M—OH2...15-crown-5 motif, M = Al3+, Cr3+ and Pd2+

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

Five structures of co-crystals grown from aqueous solutions equimolar in 15-crown-5 (or 15C5) and [M(H2O)6](NO3)n, M = Al3+, Cr3+ and Pd2+, are reported. The hydrogen-bonding patterns in all are similar: metal complexes including the fragment trans-H2O—M—OH2 alternate with 15C5 molecules, to which they are hydrogen bonded, to form stacks. A literature survey shows that this hydrogen-bonding pattern is very common. In each of the two polymorphs of the compound [Al(H2O)6](NO3)3·15C5·4H2O there are two independent cations; one forms hydrogen bonds directly to the 15C5 molecules adjacent in the stack, while the other cation is hydrogen-bonded to two water molecules that act as spacers in the stack. These stacks are then crosslinked by hydrogen bonds formed by the three nitrate counterion...

Change in electronic structure in a six-coordinate copper(II) complex accompanied by an anion order/disorder transition

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

A variable-temperature crystallographic study of [Cu(LOH)2][ClO4]2·2(CH3)2CO [LOH = 2,6-bis(hydroxyiminomethyl)pyridine] between 30 and 300 K is presented. The complex exhibits an unusual electronic structure at room temperature with a {d_{z^2}}1 ground state, corresponding to an axially compressed ligand coordination geometry about the copper ion. This reflects a suppression of the pseudo-Jahn–Teller distortion that is normally shown by copper(II) compounds with this ligand geometry [Halcrow et al. (2004). New J. Chem. 28, 228–233]. On cooling the compound undergoes an abrupt structural change at 157 ± 3 K, that does not involve a change in the space group (P\bar 1), but causes significant changes to c and the unit-cell angles. This reflects a conformational rearrangement of...

1,1′-Fc(4-C6H4CO2Et)2 and its unusual salt derivative with Z′ = 5, catena-[Na+]2[1,1′-Fc(4-C6H4CO2−)2]·0.6H2O [1,1′-Fc = (η5-(C5H4)2Fe]

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

The neutral diethyl 4,4′-(ferrocene-1,1′-diyl)dibenzoate, Fe[η5-(C5H4)(4-C6H4CO2Et)]2 (I), yields (II) (following base hydrolysis) as the unusual complex salt poly[disodium bis[diethyl 4,4′-(ferrocene-1,1′-diyl)dibenzoate] 0.6-hydrate] or [Na+]2[Fe{η5-(C5H4)-4-C6H4CO_2^-}2]·0.6H2O with Z′ = 5. Compound (I) crystallizes in the triclinic system, space group P\bar 1, with two molecules having similar geometry in the asymmetric unit (Z′ = 2). The salt complex (II) crystallizes in the orthorhombic system, space group Pbca, with the asymmetric unit comprising poly[decasodium pentakis[diethyl 4,4′-(ferrocene-1,1′-diyl)dibenzoate] trihydrate] or [Na+]10[Fe{η5-(C5H4)-4-C6H4CO_2^-}2]5·3H2O. The five independent 1,1′-Fc[(4-C6H4CO2)−]2 dianions stack in an offset ladder (stepp...

Inorganic structures in space group P31m; coordinate analysis and systematic prediction of new ferroelectrics

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

The 62 entries listed in ICSD release 2009/1 under polar space group P31m correspond to 31 families of inorganic crystal structures, some with only one member. Coordinate analysis reveals, over a wide confidence range, 11 of these families as ferroelectric candidates. One includes the well known improper ferroelectric GASH (guanidinium aluminum sulfate hexahydrate), [(C(NH2)3)Al(SO4)2(H2O)6], another the previously predicted ferroelectric CsNO3 phase II. Those remaining include K3Nb3B2O12, the minerals schairerite, galeite and lizardite 1T, LaNi5D6 and γ-CaNi5D6.1, Ca(OCl)2Ca(OH)2, [N(CH3)4]2Mo3S13, Li17Ag3Sn6 and Cs3As5O9. Candidate selection is based upon detecting an approach by the reported atomic arrangement to the symmetry of a corresponding nonpolar supergroup. A further 13 familie...

Modulation functions of incommensurately modulated Cr2P2O7 studied by the maximum entropy method (MEM)

Acta Crystallographica Section B — Tue, 16 Mar 2010 00:00:00 +0100

The maximum entropy method (MEM) has been used to determine electron density in superspace of incommensurately modulated chromium pyrophosphate from X-ray diffraction data measured by Palatinus et al. [(2006), Acta Cryst. B62, 556–566]. Chromium pyrophosphate, Cr2P2O7, contains ordered regions (83% of the volume) and regions with disorder. Analysis of the MEM density has allowed the determination of the displacive modulation functions within ordered regions. The disordered regions can be described as the alternate occupation of two conformations of the pyrophosphate group and two positions of the chromium atom, with occupational probabilities that depend continuously on the phase of modulation t. A structure model based on the interpretation of the MEM density provides a fit to the diffr...

Superspace description of the homologous series Ga2O3(ZnO)m

Acta Crystallographica Section B — Sat, 27 Feb 2010 16:01:31 +0100

A unified description for the structures of the homologous series Ga2O3(ZnO)m, gallium zinc oxide, is presented using the superspace formalism. The structures were treated as a compositely modulated structure consisting of two subsystems. One is constructed with metal ions and the other with O ions. The ideal model is given, in which the displacive modulations of ions are well described by the zigzag function with large amplitudes. Alternative settings are also proposed which are analogous to the so-called modular structures. The validity of the model has been confirmed by refinements for phases with m = 6 and m = 9 in the homologous series. A few complex phenomena in real structures are taken into account by modifying the ideal model. (Source: Acta Crystallographica Section B)

New RE microporous heteropolyhedral silicates containing 41516182 tetrahedral sheets

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

Four heteropolyhedral microporous silicates, A3RESi6O15·2.25H2O, crystallizing in the Cmm2 space group and based on 41516182 tetrahedral sheets [A3 = Na2.74K0.26, RE = Ce, abbreviated as TR05; TR06: A3 = Na2.72K0.28, RE = La; TR07: A3 = Na3, RE = La; TR08: A3 = Na2.74(H3O)0.26, RE = La0.68Eu0.32] have been hydrothermally synthesized in Teflon-lined autoclaves at 503 K and structurally characterized using X-ray diffraction single-crystal data. Except for TR05, diffraction data have been collected on {001} twins by merohedry. The four structures are isotypic and based on strongly corrugated 41516182 silicate sheets interconnected along [010] by seven-coordinated RE polyhedra to form a microporous heteropolyhedral framework. The framework is crossed by three systems of ellipsoidal channels...

Two new silicate structures based on a rhodesite-type heteropolyhedral microporous framework

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

Two new members of the mero-plesiotype rhodesite series [Sr2Na2(Si8O19)·4H2O, abbreviated as TR09; SrNa4(Si8O19)·4H2O, TR10] have been hydrothermally synthesized in Teflon-lined autoclaves at 503 K and structurally characterized using X-ray diffraction single-crystal data. The crystal structures were solved by direct methods and refined to R = 0.021 [TR09; 3317 reflections with Io > 2σ(Io)] and R = 0.033 [TR10; 5007 reflections with Io > 2σ(Io)]. Both structures are based on a rhodesite-type microporous heteropolyhedral framework, where two types of channels are within the double silicate layer that alternates with an `octahedral' O sheet. The large Sr2+ cation constrains to the roughly ellipsoidal shape of the channels. The H2O molecules are located both in the O sheets and in the c...

Proton-transfer dynamics in the (HCO3−)2 dimer of KHCO3 from Car–Parrinello and path-integrals molecular dynamics calculations

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

The proton motion in the (HCO_3^-)2 dimer of KHCO3 at 298 K has been studied with Car–Parrinello molecular dynamics (CPMD) and path-integrals molecular dynamics (PIMD) simulations. According to earlier neutron diffraction studies at 298 K hydrogen is disordered and occupies two positions with an occupancy ratio of 0.804/0.196. A simulation with only one unit cell is not sufficient to reproduce the disorder of the protons found in the experiments. The CPMD results with four cells, 0.783/0.217, are in close agreement with experiment. The motion of the two protons along the O...O bridge is highly correlated inside one dimer, but strongly uncoupled between different dimers. The present results support a mechanism for the disorder which involves proton transfer from donor to acceptor and ...

Re-investigation of the structure and crystal chemistry of the Bi2O3–W2O6 `type (Ib)' solid solution using single-crystal neutron and synchrotron X-ray diffraction

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

Single crystals of composition Bi35.66W4.34O66.51 (or Bi8.2WO15.3, bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi2O3–WO3 system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I41. Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-i...

Mullite-type Ga4B2O9: structure and order–disorder phenomenon

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

Ga4B2O9, an aluminium-free mullite-type compound, was prepared by a boric-acid flux method and its structure was determined using powder X-ray diffraction techniques, in combination with transmission electron microscopy, solid-state 11B MAS-NMR and IR spectroscopies. GaO6 octahedra share edges in a trans-manner forming one-dimensional chains along the b direction, and the chains are further cross-linked by GaO5, BO3 and BO4 groups into a three-dimensional mullite-type structure. The disorder of the inter-chain groups results in a small unit cell for Ga4B2O9 compared with that for Al4B2O9, an ordered compound with a superstructure. By deconstructing the structure of Ga4B2O9, we were able to identify the fundamental building units and their linking rules which can be used to reconstruct the ...

Precursor effects of the orthorhombic to monoclinic phase transition in benzocaine form (II) revealed by X-ray diffuse scattering

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

We described the development of a Monte Carlo computer model for the room-temperature form (II) polymorph of benzocaine that incorporates, on a local scale, structural features derived from the low-temperature form (III) polymorph. The introduction of this extra information convincingly reproduces those observed diffraction features that an earlier harmonic model was unable to achieve. In both form (I) and form (II) the hydrogen-bonded chains of molecules that extend along the respective a axes tend to slide backward and forward along their lengths. While in form (I) the motion is well modelled by a harmonic potential in form (II) there is a degree of anharmonicity that means that some intermolecular contact vectors, which are identical in the average structure, are distributed bimodally w...

Maximum entropy method and charge flipping, a powerful combination to visualize the true nature of structural disorder from in situ X-ray powder diffraction data

Acta Crystallographica Section B — Fri, 26 Feb 2010 00:00:00 +0100

In a systematic approach, the ability of the Maximum Entropy Method (MEM) to reconstruct the most probable electron density of highly disordered crystal structures from X-ray powder diffraction data was evaluated. As a case study, the ambient temperature crystal structures of disordered α-Rb2[C2O4] and α-Rb2[CO3] and ordered δ-K2[C2O4] were investigated in detail with the aim of revealing the `true' nature of the apparent disorder. Different combinations of F (based on phased structure factors) and G constraints (based on structure-factor amplitudes) from different sources were applied in MEM calculations. In particular, a new combination of the MEM with the recently developed charge-flipping algorithm with histogram matching for powder diffraction data (pCF) was successfully introduced...

Polysomatic apatites

Acta Crystallographica Section B — Fri, 22 Jan 2010 16:10:41 +0100

This article examines the state-of-the-art in polysomatic apatite synthesis and crystallochemical design. It also presents X-ray and neutron powder diffraction investigations for several polysome chemical series and examines the prevalence of stacking disorder by electron microscopy. These insights into the structure-building principles of apatite polysomes will guide their development as functional materials. (Source: Acta Crystallographica Section B)

Notes for authors 2010

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

(Source: Acta Crystallographica Section B)

A list of organic kryptoracemates

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformatio...

Structures of dipeptides: the head-to-tail story

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

The hydrogen-bonding patterns in crystal structures of unprotected, zwitterionic dipeptides are dominated by head-to-tail chains involving the N-terminal amino groups and the C-terminal carboxylate groups. Patterns that include two concomitant chains, thus generating a hydrogen-bonded layer, are of special interest. A comprehensive survey shows that dipeptide structures can conveniently be divided into only four distinct patterns, differing by definition in the symmetry of the head-to-tail chains and amide hydrogen-bonding type, but also in other properties such as peptide conformation and the propensity to include solvent water or various organic guest molecules. Upon crystallization, the choice of pattern for a specific dipeptide is not random, but follows from the amino acid sequence. (...

Ab initio structure determination of phase II of racemic ibuprofen by X-ray powder diffraction

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

Annealing of the quenched ibuprofen at 258 K yielded a new crystalline form, called phase II. Powder X-ray diffraction patterns of this phase II were recorded with a laboratory diffractometer equipped with an INEL G3000 goniometer and a curved position-sensitive detector CPS120. The starting structural model was found by a Monte-Carlo simulated annealing method. The final structure was obtained through Rietveld refinements with rigid-body constraints for the phenyl group and soft restraints on the other interatomic bond lengths and bond angles. The cell volume is 5% larger than that of the conventional phase I at 258 K. It is also shown that the orientation of the propanoic acid group is drastically changed with respect to phase I, leading to strong modifications of the orientation of ...

Concomitant polymorphic behavior of di-μ-thiocyanato-κ2N:S;κ2S:N-bis[bis(tri-p-fluorophenylphosphine-κP)silver(I)]

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

The structures of two polymorphs, both monoclinic P21/n [polymorph (I)] and P21/c [polymorph (II)], of di-μ-thiocyanato-κ2N:S;κ2S:N-bis[bis(tri-p-fluorophenylphosphine-κP)silver(I)] complexes have been determined at 100 K. In both polymorphs the complex has a dinuclear structure where the silver(I) coordinates to two phosphine ligands and two bridging thiocyanate anions to form complexes with distorted tetrahedral geometry. Polymorph (I) has just one half of the [Ag2(SCN)2{P(4-FC6H4)3}4] molecule at (0, ½, 0) from the origin in the asymmetric unit. Polymorph (II) has one and a half molecules of [Ag2(SCN)2{P(4-FC6H4)3}4] in the asymmetric unit; the half molecule is situated at (0, 1, ½), while the full molecule is located at (1/3, ½, 1/3) from the origin. The Ag—P bond distances ...

Revision of the structure of Cs2CuSi5O12 leucite as orthorhombic Pbca

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

The crystal structure of a hydrothermally synthesized leucite analogue Cs2CuSi5O12 has been determined and refined using the Rietveld method from high-resolution synchrotron X-ray and neutron powder diffraction data. This structure is based on the topology and cation-ordering scheme of the Pbca leucite structure of Cs2CdSi5O12, and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs2CuSi5O12 is topologically identical to other known leucite structures and is different from that originally proposed by Heinrich & Baerlocher [(1991), Acta Cryst. C47, 237–241] in the tetragonal space group P4_12_12. The crystal structure of a dry-synthesized leucite analogue Cs2CuSi5O12 has also been refined; this has the Ia\bar 3d cubic pollucite struc...

Orientational disorder and phase transitions in crystals of dioxofluoromolybdate, (NH4)2MoO2F4

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

Dioxotetrafluoromolybdate, (NH4)2MoO2F4, was synthesized in a single-crystal form and its structures [(I) at 297 K and (II) at 223 K] were determined by X-ray diffraction. Two independent states of a cis-MoO2F4 octahedron are characteristic of static and dynamic disorder in structure (I). The dynamically disordered Mo atom is displaced from the symmetry axis producing four possible orientations of an anion that allow O and F atoms to be identified in separate orientations owing to the inherent differences between the Mo—O and Mo—F bonding. After the phase transition at lower temperature, (I) transforms into the statically disordered structure (II) with three possible orientations of the cis-MoO2F4 octahedron. In this case, it also seemed possible to distinguish between O and F atom...

Structures of incommensurate and commensurate composite crystals RbxMnO2 (x = 1.3711, 1.3636)

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

Rb1.3711MnO2 (Rb11Mn8O16) has been synthesized via the azide/nitrate route from a stoichiometric mixture of the precursors Mn2O3, RbNO3 and RbN3. The structure of this extremely air- and moisture-sensitive compound can best be described in terms of an incommensurate composite structure, built up by a honeycomb-like framework of Rb ions, as one subsystem and by a second subsystem of chains, consisting of edge-sharing MnO4/2 tetrahedra. These two composite substructures interpenetrate in such a way that the manganate chain polyanions centre the channels of the Rb-honeycomb framework. Crystals transform by an aging process into Rb1.3636MnO2 (Rb15Mn11O22), which has a similar structure but a different commensurate modulation. Two reasons can be established for the origin of the modulations: th...

A complicated quasicrystal approximant ∊16 predicted by the strong-reflections approach

Acta Crystallographica Section B — Fri, 22 Jan 2010 00:00:00 +0100

The structure of a complicated quasicrystal approximant ∊16 was predicted from a known and related quasicrystal approximant ∊6 by the strong-reflections approach. Electron-diffraction studies show that in reciprocal space, the positions of the strongest reflections and their intensity distributions are similar for both approximants. By applying the strong-reflections approach, the structure factors of ∊16 were deduced from those of the known ∊6 structure. Owing to the different space groups of the two structures, a shift of the phase origin had to be applied in order to obtain the phases of ∊16. An electron-density map of ∊16 was calculated by inverse Fourier transformation of the structure factors of the 256 strongest reflections. Similar to that of ∊6, the predicted structu...

Structure solution of the new titanate Li4Ti8Ni3O21 using precession electron diffraction

Acta Crystallographica Section B — Sat, 12 Dec 2009 15:34:59 +0100

A sample having stoichiometry Li[Ti1.5Ni0.5]O4 has been synthesized to obtain a spinel structure. The resulting crystalline powder revealed a multiphase nature with spinel as the minor phase. The main phase is a new trigonal phase having a = 5.05910 (1), c = 32.5371 (1) Å. The structure has been solved by direct methods working on a three-dimensional set of intensities obtained from a precession electron-diffraction experiment, and refined on synchrotron powder diffraction data in the space group P\bar 3c1. The model consists of hexagonal layers of edge-sharing octahedra occupied either by the heavy cations Ti and Ni, or preferentially by Li. On the basis of cation-site occupancies the stoichiometry becomes Li4Ti8Ni3O21, which is compatible with the microanalysis results. (Source: A...

Octahedral tilting in cation-ordered Jahn–Teller distorted perovskites – a group-theoretical analysis

Acta Crystallographica Section B — Sat, 12 Dec 2009 00:00:00 +0100

Computer-based group-theoretical methods are used to enumerate structures arising in A2BB′X6 perovskites, with either rock-salt or checkerboard ordering of the B and B′ cations, under the additional assumption that one of these two cations is Jahn–Teller active and thereby induces a distortion of the BX6 (or B′X6) octahedron. The requirement to match the pattern of Jahn–Teller distortions to the cation ordering implies that the corresponding irreducible representations should be associated with the same point in the Brillouin zone. Effects of BX6 (and B′X6) octahedral tilting are included in the usual way. Finally, an analysis is presented of more complex models of ordering and distortion as might lead to the doubling of the long axis of the common Pnma perovskite, observed in ...

Zur Kristallchemie von Graphen und Graphit

Acta Crystallographica Section B — Sat, 12 Dec 2009 00:00:00 +0100

The crystal structure of single-layer graphene in comparison to graphite is discussed with regard to its crystallographic and chemical properties. In both of these polymorphs of carbon, the atomic volume of carbon, reduced to the closest packing of atoms, is practically the same and considerably smaller than in diamond. This indicates pentavalent carbon in graphene as well as in graphite. The observed elastic corrugations of the graphene layers which probably cause their amazing rigidity seem to be due to numerous weak chemical bonds within the layers. (Source: Acta Crystallographica Section B)

Growth and single-crystal refinement of phase-III potassium nitrate, KNO3

Acta Crystallographica Section B — Thu, 19 Nov 2009 15:58:27 +0100

Oriented single crystals of the high-temperature phase of KNO3 (phase III), a ferroelectric compound that may also occur as an atmospheric aerosol particle, were grown at room temperature and pressure by atomizing a solution of KNO3 in water and allowing droplets to dry on a glass substrate. The crystals are up to 1 mm across and are stable unless mechanically disturbed. There is no evidence of the spontaneous transformation of phase III to the room-temperature stable phase (phase II), even after several months. Single-crystal structure determinations of phase III were obtained at 295 and 123 K. The unit cell regained its room-temperature dimensions after warming from 123 K. The phase-III KNO3 structure can be viewed as the stacking parallel to the c axis of alternating K atoms and p...

Clara Brink Shoemaker (1921–2009)

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

(Source: Acta Crystallographica Section B)

[Ru(py)4Cl(NO)](PF6)2·0.5H2O: a model system for structural determination and ab initio calculations of photo-induced linkage NO isomers. Erratum

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

In the paper by Cormary et al. [Acta Cryst. (2009), B65, 612–623] two authors were inadvertently omitted from the author list and one name was given incorrectly. (Source: Acta Crystallographica Section B)

Towards a more reliable symmetry determination from powder diffraction: a redetermination of the low-temperature structure of 4-methylpyridine-N-oxide

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

The low-temperature structure of 4-methylpyridine-N-oxide was previously determined in symmetry P41 [Damay et al. (2006), Acta Cryst. B62, 627–633]. Using a recently published symmetry-determination method it was found that the true symmetry of the structure is P41212. The structure was refined in the new space group using X-ray and neutron data. The previously published structure is close to the newly refined structure, but the new structure is in agreement with the results of rotational tunneling spectroscopy, and, in contrast to the structure in symmetry P41, does not require a twofold degeneracy of the tunneling bands. (Source: Acta Crystallographica Section B)

Space groups P1 and Cc: how are they doing?

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

A survey of recent entries in the Cambridge Structural Database suggests that the percentage of structures described in space groups of inappropriately low symmetry has decreased from about 10% in the early 2000s to less than 5% today for space group Cc, but that for space group P1 it remains close to 8%. (Source: Acta Crystallographica Section B)

How to easily replace the independent atom model – the example of bergenin, a potential anti-HIV agent of traditional Asian medicine

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

Bergenin, which has been isolated from a variety of tropical plants, has several pharmacological applications in traditional Asian medicine. Its electron-density distribution was obtained from a room-temperature low-resolution X-ray data set measured with point detection making use of multipole populations from the invariom library. Two refinement models were considered. In a first step, positional parameters and ADPs were refined with fixed library multipoles (model E1). This model was suitable to be input into a second refinement of multipoles (model E2), which converged smoothly although based on Cu Kα room-temperature data. Quantitative results of a topological analysis of the electron density from both models were compared with Hartree–Fock and density-functional calculations. Wi...

Comparison of the effects of pressure on three layered hydrates: a partially successful attempt to predict a high-pressure phase transition

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

We report the effect of pressure on the crystal structures of betaine monohydrate (BTM), l-cysteic acid monohydrate (CAM) and S-4-sulfo-l-phenylalanine monohydrate (SPM). All three structures are composed of layers of zwitterionic molecules separated by layers of water molecules. In BTM the water molecules make donor interactions with the same layer of betaine molecules, and the structure remains in a compressed form of its ambient-pressure phase up to 7.8 GPa. CAM contains bi-layers of l-cysteic acid molecules separated by water molecules which form donor interactions to the bi-layers above and below. This phase is stable up to 6.8 GPa. SPM also contains layers of zwitterionic molecules with the waters acting as hydrogen-bond donors to the layers above and below. SPM undergoes a singl...

Molecular and crystalline structures of three (S)-4-alkoxycarbonyl-2-azetidinones containing long alkyl side chains from synchrotron X-ray powder diffraction data

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

The (S)-4-alkoxo-2-azetidinecarboxylic acids are optically active β-lactam derivatives of aspartic acid, which are used as precursors of carbapenem-type antibiotics and poly-β-aspartates. The crystal structures of three (S)-4-alkoxo-2-azetidinecarboxylic acids with alkyl chains with 10, 12 and 16 C atoms were solved using parallel tempering and refined against the X-ray powder diffraction data using the Rietveld method. The azetidinone rings in the three compounds display a pattern of asymmetrical bond distances and an almost planar conformation; these characteristics are compared with periodic solid-state, gas-phase density-functional theory (DFT) calculations and MOGUL average bond distances and angles from the CSD. The compounds pack along [001] as corrugated sheets separated by appro...

Experimental and theoretical charge-density study of a tetranuclear cobalt carbonyl complex

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

Details of the complex bonding environment present in the molecular centre of an alkyne-bridged dicobalt complex have been examined using a combination of experimental and theoretical charge-density modelling for two compounds which share a central Co2C2 tetrahedral moiety as their common motif. Topological analysis of the experimental electron density illustrates the problem of separating the Co—C bond-critical points (b.c.p.s) from the intervening ring-critical point (r.c.p.), due largely to the flat nature of the electron density in the CoC2 triangles. Such a separation of critical points is immediately obtained from a topological analysis of the theoretical electron density as well as from the multipole-projected theoretical density; however, the addition of random noise to the theor...

Cluster analyses of metal-organic fragments using the dSNAP software

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

The dSNAP computer program has been used to classify searches of the Cambridge Structural Database for two ligands: —O—CH2—CH2—O— and N(CH2CH2O—)3 commonly found in metal-organic systems. The clustering method used is based on total geometries (i.e. all the lengths and angles involving all the atoms in the search fragment, whether bonded or not) and proved capable of distinguishing in a wholly automatic, objective way between different types of metal complex purely on the basis of the geometry of the ligand and the relative positions of the O atoms to the metals. (Source: Acta Crystallographica Section B)

Isomorphism in monomeric 1:3 complexes of silver(I) salts with tri-p-tolylphosphine

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

Reaction of silver(I) salts with three equivalents of tri-p-tolylphosphine in CH3CN resulted in a series of isomorphous complexes [AgX{P(4-MeC6H4)3}3] (X = Br, SCN, ClO4). These complexes all crystallize in the orthorhombic space group Pna21. The complexes with X = Br, SCN are distorted tetrahedral around the silver(I) atom, whereas the ClO4− complex is distorted trigonal planar around the silver. The new complexes are compared with each other using r.m.s. overlay calculations as well as half-normal probability plot analysis and with the previously reported isomorphous chloride, bromide as well as the non-isomorphous iodide complexes. (Source: Acta Crystallographica Section B)

(111)p microtwinning in SrRuO3 thin films on (001)p LaAlO3

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

SrRuO3 (SRO) thin films grown on (001)p (p = pseudocubic) oriented LaAlO3 (LAO) by pulsed laser deposition have been characterized using transmission electron microscopy. Observations along the 〈100〉p directions suggests that although the SRO layer maintains a pseudocube-to-pseudocube orientation relationship with the underlying LAO substrate, it has a ferroelastic domain structure associated with a transformation on cooling to room temperature to an orthorhombic Pbnm phase (a−a−c+ Glazer tilt system). In addition, extra diffraction spots located at ±1/6(ooo)p and ±1/3(ooo)p (where `o' indicates an index with an odd number) positions were obtained in 〈110〉p zone-axis diffraction patterns. These were attributed to the existence of high-density twins on {111}p pseudocubic plane...

Magnetic behaviour of synthetic Co2SiO4

Acta Crystallographica Section B — Mon, 16 Nov 2009 00:00:00 +0100

Synthetic Co2SiO4 crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50 K. We have investigated the temperature variation of the Co2SiO4 magnetic structure by means of non-polarized and polarized neutron diffraction for single crystals. Measurements with non-polarized neutrons were made at 2.5 K (below TN), whereas polarized neutron diffraction experiments were carried out at 70 and 150 K (above TN) in an external magnetic field of 7 T parallel to the b axis. Additional accurate non-polarized powder diffraction studies were performed in a broad temperature range from 5 to 500 K with small temperature increments. Detailed symmetry analysis of the Co2SiO4 magnetic structure sh...

On the valences of bonds in the oxycomplexes of Sn2+

Acta Crystallographica Section B — Tue, 27 Oct 2009 16:02:22 +0100

The differences between Wang and Liebau's [Wang & Liebau (2007). Acta Cryst. B63, 216–228] stoichiometric valence (atomic valence) and structural valence (bond-valence sum) observed in Sn2+ and other lone-pair cation oxycomplexes arises from their use of the Brese & O'Keeffe bond-valence parameters which are based on the assumption that the bond-valence parameter b = 0.37 Å applies to all bond types. According to the theory of the bond-valence model, the bond-valence sum is necessarily equal to the ionic charge, implying that in the Wang and Liebau model the ionic charges are equal to the structural valence. If charges are chosen equal to the stoichiometric valence, the bond-valence parameters for Sn2+—O bonds are R0 = 1.859 Å, b = 0.55 Å. While both models are theoretically v...

Weak intra- and intermolecular interactions in a binaphthol imine: an experimental charge-density study on (±)-8′-benzhydrylideneamino-1,1′-binaphthyl-2-ol

Acta Crystallographica Section B — Tue, 27 Oct 2009 00:00:00 +0100

The charge density in (±)-8′-benzhydrylideneamino-1,1′-binaphthyl-2-ol (1) has been studied experimentally using Mo Kα X-ray diffraction at 100 K, and by theory using density-functional thoery (DFT) calculations at the B3LYP/6-311++G** level. The nature of the weak intramolecular peri-C...N, CH...π, H...H and C(π)...C(π) interactions has been examined by topological analysis using the Quantum Theory of Atoms in Molecules (QTAIM) approach. An analysis of the density ρ(r), the Laplacian of the density ∇2ρ(rb) and other topological properties at the bond-critical points were used to classify these interactions. The study confirms the presence of the intramolecular CH...π interaction in (1), which was previously suspected on geometrical grounds. An analysis of the ellipticity...

Octahedral tilt twinning and compositional modulation in NaLaMgWO6

Acta Crystallographica Section B — Tue, 27 Oct 2009 00:00:00 +0100

A combination of selected-area electron diffraction (SAED), neutron powder diffraction (NPD) and high-resolution transmission electron microscopy (HRTEM) reveals a complex superstructure in the ordered perovskite NaLaMgWO6. Through indexing of SAED patterns the unit-cell dimensions are found to be 46.8 × 7.8 × 7.9 Å, which corresponds to a 12ap × 2ap × 2ap superstructure of the simple Pm\overline 3 m perovskite unit cell. HRTEM images reveal the formation of an unmistakable stripe contrast that repeats with the same periodicity. Doubling of the b and c axes is brought about by a combination of layered ordering of Na and La, rock-salt ordering of Mg and W, and octahedral tilting. The a axis repeat distance results from a one-dimensional twinning of the octahedral tilts in combination...

A conformational polymorphic transition in the high-temperature ∊-form of chlorpropamide on cooling: a new ∊′-form

Acta Crystallographica Section B — Tue, 27 Oct 2009 00:00:00 +0100

Structural changes in the high-temperature ∊-polymorph of chlorpropamide, 4-chloro-N-(propylaminocarbonyl)benzenesulfonamide, C10H13ClN2O3S, on cooling down to 100 K and on reverse heating were followed by single-crystal X-ray diffraction. At temperatures below 200 K the phase transition into a new polymorph (termed the ∊′-form) has been observed for the first time. The polymorphic transition preserves the space group Pna21, is reversible and is accompanied by discontinuous changes in the cell volume and parameters, resulting from changes in molecular conformation. As shown by IR spectroscopy and X-ray powder diffraction, the phase transition in a powder sample is inhomogeneous throughout the bulk, and the two phases co-exist in a wide temperature range. The cell parameters and t...

Structure and conformational analysis of a bidentate pro-ligand, C21H34N2S2, from powder synchrotron diffraction data and solid-state DFTB calculations

Acta Crystallographica Section B — Wed, 16 Sep 2009 23:00:00 +0100

The molecular and crystalline structure of ethyl 1′,2′,3′,4′,4a′,5′,6′,7′-octahydrodispiro[cyclohexane-1,2′-quinazoline-4′,1′′-cyclohexane]-8′-carbodithioate (I) was solved and refined from powder synchrotron X-ray diffraction data. The initial model for the structural solution in direct space using the simulated annealing algorithm implemented in DASH [David et al. (2006). J. Appl. Cryst. 39, 910–915] was obtained performing a conformational study on the fused six-membered rings of the octahydroquinazoline system and the two spiran cyclohexane rings of (I). The best model was chosen using experimental evidence from 1H and 13C NMR [Contreras et al. (2001). J. Heterocycl. Chem. 38, 1223–1225] in combination with semi-empirical AM1 calculations. In the refined s...

[Ru(py)4Cl(NO)](PF6)2·0.5H2O: a model system for structural determination and ab initio calculations of photo-induced linkage NO isomers

Acta Crystallographica Section B — Wed, 16 Sep 2009 23:00:00 +0100

Structure analysis of ground state (GS) and two light-induced (SI and SII) metastable linkage NO isomers of [Ru(py)4Cl(NO)]·(PF6)2·0.5H2O is presented. Illumination of the crystal by a laser with λ = 473 nm at T = 80 K transfers around 92% of the NO ligands from Ru—N—O into the isomeric configuration Ru—O—N (SI). A subsequent irradiation with λ = 980 nm generates about 48% of the side-on configuration \rm Ru \lt ^N_O (SII). Heating to temperatures above 200 K or irradiation with light in the red spectral range transfers both metastable isomers reversibly back to the GS. Photodifference maps clearly show the N—O configurations for both isomers and they could be used to find a proper starting model for subsequent refinements. Both metastable isomers have slightly but si...

Superspace description of the modulated structure of the metal-salt-hybrid Bi7 − δNi2Br5 − 2δ (δ = 1/9)

Acta Crystallographica Section B — Wed, 16 Sep 2009 23:00:00 +0100

The compound Bi7 − δNi2Br5 − 2δ = (Bi3Ni)2[Bi1 − δBr4]Br1 −2δ (δ = 1/9) is a sub-bromide of the intermetallic phase Bi3Ni. Its crystal structure contains metallic rods, ^1_{\infty}[Bi3Ni], which are embedded in a salt-like matrix of bromido-bismuthate(III) and bromide anions. The non-stoichiometry originates from the variation of the number n of trans edge-sharing octahedra in the [BinBr4n + 2](n + 2)− oligomers (3 ≤ n ≤ 5), as well as from vacancies on the sites of the isolated Br atoms. The simplified structure is described in the orthorhombic space group Cmcm with a = 4.0660 (4), b = 23.305 (3), c = 17.130 (2) Å. It shows a statistical distribution of vacancies and orientational disorder of the concatenated octahedra. By choosing the mod...

Modulated structure and phase transitions of Sr10Ga6O19

Acta Crystallographica Section B — Fri, 28 Aug 2009 13:11:36 +0100

The crystal structure of Sr10Ga6O19 was investigated by in situ single-crystal X-ray diffraction in the temperature range 298–673 K. At ambient conditions the compound shows a (3 + 1)-dimensional modulated structure in the superspace group C2/c(0β0)s0 [a = 34.9145 (13), b = 7.9369 (2), c = 15.9150 (7) Å and β = 103.551 (3)°] with a modulation wavevector of q = 0.4288 (2)b*. Whereas the presented structural model uses first-order harmonic modulation functions only, some features of the modulations are discussed utilizing an electron density derived by the maximum entropy method. Furthermore, two phase transitions were identified: between 453 and 503 K the incommensurate superstructure is replaced by a doubling of the a and b lattice constants, and between 503 and 6...

Ternary fluorides BaMF4 (M = Zn, Mg and Mn) at low temperatures

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

Ternary fluorides BaMF4 (M = Zn, Mg, Mn) have been studied in the temperature range from 300 to 10 K using synchrotron and laboratory powder and single-crystal diffraction. The first two compounds are stable down to 10 K, while the third one undergoes a phase transition to an incommensurately modulated structure at approximately 250 K. The modulated phase is stable down to 10 K. The magnetic anomalies at 45 and 27 K observed previously in BaMnF4 are exclusively reflected in the behavior of the γ component of the q vector, which assumes an irrational value of approximately 0.395 Å−1 at the temperature corresponding to the onset of the magnetic ordering and then stays constant down to 10 K. Mn—Mn distances do not indicate any structural response to the magnetic ordering. ...

Eu3Si15 − xAl1 + xOxN23 − x (x ≃ 5/3) as a commensurate composite crystal

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

A new Eu-SiAlON crystal, Eu3Si15 − xAl1 + xOxN23 − x (x ≃ 5/3), was found and the structure was determined by an X-ray diffraction technique using a twinned sample. The structure consists of a host framework, which is constructed by the connection of MX4 tetrahedra (M: Si or Al; X: O or N), and Eu ions as the guest ions. The structure is considered to be a commensurate composite crystal. The basic vectors are a1 = a/3, b and c for the first substructure, and a2 = a/5, b and c for the second substructure. The first substructure consists of part of the host framework and the Eu ions, while the remainder of the host structure is taken as the second substructure. Possible phases belonging to the series are proposed using the composite crystal model in (3 + 1)-dimensional ...

Structure of caesium disulfate at 120 and 273 K

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

The crystal structures of Cs2S2O7 at 120 and 273 K have been determined from X-ray single-crystal data. Caesium disulfate represents a new structure type with a uniquely high number of independent formula units at 120 K: In one part caesium ions form a tube surrounding the disulfate ions, [Cs8(S2O7)6+]n; in the other part a disulfate double-sheet sandwiches a zigzagging caesium ion chain, [Cs2(S2O7)6−]n. Caesium disulfate shows an isostructural order–disorder transition between 230 and 250 K, where two disulfate groups become partially disordered above 250 K. The Cs+-ion arrangement shows a remarkable similarity to the high-pressure RbIV metal structure. (Source: Acta Crystallographica Section B)

The experimental and theoretical QTAIMC study of the atomic and molecular interactions in dinitrogen tetroxide

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

The atomic and molecular interactions in a crystal of dinitrogen tetraoxide, α-N2O4, have been studied in terms of the quantum topological theory of molecular structure using high-resolution, low-temperature X-ray diffraction data. The experimental electron density and electrostatic potential have been reconstructed with the Hansen–Coppens multipole model. In addition, the three-dimensional periodic electron density of crystalline α-N2O4 has been calculated at the B3LYP/cc-pVDZ level of theory with and without the geometry optimization. The application of the quantum theory of atoms in molecules and crystals (QTAIMC) recovered the two types of intermolecular bond paths between O atoms in crystalline α-N2O4, one measuring 3.094, the other 3.116 Å. The three-dimensional distribution ...

Weak interactions in chain polymers [M(μ-X)2L2]∞ (M = Zn, Cd; X = Cl, Br; L = substituted pyridine) – an electron density study

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

The experimental electron-density distributions in crystals of five chain polymers [M(μ-X)2(py)2] (M = Zn, Cd; X = Cl, Br; py = 3,5-substituted pyridine) have been obtained from high-resolution X-ray diffraction data sets (sin θ/λ > 1.1 Å−1) at 100 K. Topological analyses following Bader's `Atoms in Molecules' approach not only confirmed the existence of (3, −1) critical points for the chemically reasonable and presumably strong covalent and coordinative bonds, but also for four different secondary interactions which are expected to play a role in stabilizing the polymeric structures which are unusual for Zn as the metal center. These weaker contacts comprise intra- and inter-strand C—H...X—M hydrogen bonds on the one hand and C—X...X—C interhalogen contacts on the ot...

Modulated crystal structure of incommensurate low tridymite

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

The incommensurately modulated crystal structure of low SiO2 tridymite was refined based on single-crystal X-ray diffraction data in the superspace group Cc(α0γ)0. The data set consists of 885 main reflections, 1751 first-order, 924 second-order and 119 third-order satellite reflections with I > 3σ(I). The modulation is mainly made up from cooperative twistings of the rigid SiO4 tetrahedra. Two orders of displacement waves are used to describe the modulation of the Si atoms and three orders for the O atoms. The maximal amplitudes are ca 0.6 Å. O atoms bridging pairs of tetrahedra in cis and trans configurations show different positional modulation patterns. The anisotropic displacement parameters (ADPs) are also modulated. A correlation between ADP modulations and positional modulati...

Topological properties of hydrogen bonds and covalent bonds from charge densities obtained by the maximum entropy method (MEM)

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

Charge densities have been determined by the Maximum Entropy Method (MEM) from the high-resolution, low-temperature (T ≃ 20 K) X-ray diffraction data of six different crystals of amino acids and peptides. A comparison of dynamic deformation densities of the MEM with static and dynamic deformation densities of multipole models shows that the MEM may lead to a better description of the electron density in hydrogen bonds in cases where the multipole model has been restricted to isotropic displacement parameters and low-order multipoles (lmax = 1) for the H atoms. Topological properties at bond critical points (BCPs) are found to depend systematically on the bond length, but with different functions for covalent C—C, C—N and C—O bonds, and for hydrogen bonds together with covalent C�...

Order–disorder of oxygen anions and vacancies in solid solutions of La2TiO5 and La4Ga2O9

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

Successful Rietveld refinements of seven compounds with the formulae La2Ti(1−x)GaxO(5−x/2), where x = 0.00, 0.20, 0.50, 0.70, 0.90, 0.95 and 1.00, were performed in order to describe the solid solubility between orthorhombic (Pnam) La2TiO5 and monoclinic (P21/c) La4Ga2O9. The relationship between the end-member structures, which are already known, is discussed; the space-group change is a consequence of ordering the oxygen vacancies that become more numerous as Ga is substituted for Ti. The structures of the solid solutions are also described. The lengths of cell edges obey Vegard's rule. (Source: Acta Crystallographica Section B)

Incommensurately modulated lanthanide coinage-metal diarsenides. II. GdCuAs2, GdAu1−δAs2 and TbAu1−δAs2 – new distortion variants of the HfCuSi2 type with irregularly stacked zigzag chains of arsenic atoms

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

GdCuAs2, GdAu1−δAs2 and TbAu1−δAs2 crystallize as incommensurately modulated variants of the HfCuSi2 type. Structure models have been developed in the monoclinic superspace group P121/m1(α0γ)00 (No. 11.1). The components of the modulation wavevectors q = αa* + 0b* + γc* are α = 0.04 (1) and γ = 0.48 (1) for GdCuAs2, α = 0.03 (1) and γ = 0.48 (1) for GdAu1−δAs2 and α = 0.02 (1) and γ = 0.46 (1) for TbAu1−δAs2. The predominant effect of the positional modulation is the distortion of a square net of arsenic atoms, which results in planar zigzag chains. Rod groups and layer groups of the respective structure motifs are identified and discussed. (Source: Acta Crystallographica Section B)

Incommensurately modulated lanthanide coinage-metal diarsenides. I. CeAu1−δAs2 [δ = 0.015 (2)] – a new distortion variant of the HfCuSi2 type with irregularly stacked cis–trans chains of arsenic atoms

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

Rare-earth metal coinage-metal diarsenides LnTAs2 (Ln = Y, La, Ce–Lu; T = Cu, Ag, Au) are known to crystallize in structures closely related to the HfCuSi2 type, which comprises a stacking sequence of puckered TAs layers and planar square As nets, separated by the Ln atoms. CeAu1−δAs2, with δ = 0.015 (2), shows an incommensurate positional modulation of the arsenic atoms in the planar As nets. Based on X-ray diffraction data on a twinned crystal, a structure model in the monoclinic superspace group P121/m1(α0γ)00 (No. 11.1) with basic unit-cell parameters of a = 5.804 (1), b = 5.814 (1), c = 10.179 (1) Å and β = 90.09 (8)° is presented. The components of the modulation wavevector q = αa* + 0b* + γc* are α = 0.08 (1) and γ = 0.39 (1). The structure comprises c...

Modulation of atomic positions in CaCuxMn7−xO12 (x ≤ 0.1)

Acta Crystallographica Section B — Thu, 27 Aug 2009 23:00:00 +0100

The modulation of atomic positions in CaCuxMn7−xO12 (x = 0 and 0.1) was studied using synchrotron radiation powder diffraction below 250 and 220 K, respectively. The copper-rich member CaCuxMn7−xO12 (x = 0.23) does not show any modulation of the atomic positions at temperatures as low as 10 K. Using low-temperature neutron powder diffraction the modulation of the magnetic moments of Mn ions in CaCuxMn7−xO12 (x = 0, 0.1 and 0.23) has been investigated. Long-range modulated magnetic ordering in CaCuxMn7−xO12 (x = 0, 0.1 and 0.23) is observed below 90.4, 89.2 and 78.1 K. (0,0,qp) and (0,0,qm) are the propagation vectors describing the modulations of the atomic positions and the magnetic moments. For CaCuxMn7−xO12 (x = 0 and 0.1) the magnetic modulation and atomic modulation le...

Electron diffraction of ABX3 perovskites with both layered ordering of A cations and tilting of BX6 octahedra

Acta Crystallographica Section B — Tue, 21 Jul 2009 12:04:38 +0100

It is shown that 21 ABX3 perovskites with tilted BX6 octahedra and layered ordering of A cations can be generated on the basis of group–subgroup relations. These structures (with 16 different space groups) are classified into ten diffraction types in terms of the conditions for superstructure reflections caused by the ordering of A cations, tilting of BX6 octahedra and structural absences. SAED (selected-area electron diffraction) allows the distinction of seven of the 21 different perovskites, while additional symmetry analysis by CBED (convergent-beam electron diffraction) is needed for the remaining 14 structures. The space groups of lithium lanthanum titanate pseudomorphs (with discrete chemical compositions) are successfully deduced by electron diffraction experiments. (Source: Acta...

Structures of strontium diformate and strontium fumarate. A synchrotron powder diffraction study

Acta Crystallographica Section B — Wed, 15 Jul 2009 23:00:00 +0100

The crystal structures of strontium diformate in space groups P212121 (α form, 295 K), P41212 (β form, 334 and 540 K) and I41/amd (δ form, 605 K), and strontium fumarate in space groups Fddd (β form, 105 K) and I41/amd (α form, 293 K) have been determined from synchrotron X-ray powder diffraction data. Except for the α-strontium diformate, all the structures are based on a diamond-like Sr-ion arrangement, as in strontium acetylene dicarboxylate. The formate ions are disordered in the δ phase owing to steric hindrance. The fumarate ions are disordered over four (α) or two (β) symmetry-equivalent orientations. α-Strontium fumarate crystallizes with a unique 90° carboxylate dihedral angle, and is stable up to 773 K. (Source: Acta Crystallographica Section B)

Structural elucidation of the Bi2(n + 2)MonO6(n + 1)(n = 3, 4, 5 and 6) family of fluorite superstructures by transmission electron microscopy

Acta Crystallographica Section B — Wed, 15 Jul 2009 23:00:00 +0100

The cationic framework structure of a whole new family of compounds with the general formula Bi2(n + 2)MonO6(n + 1) (n = 3, 4, 5 and 6) has been elucidated by transmission electron microscopy (TEM) methods. High-resolution transmission electron microscopy (HRTEM) has been used to postulate heavy-atom models based on the known structure of the n = 3 phase, Bi10Mo3O24. These models were tested by HRTEM image simulation, electron diffraction and powder X-ray diffraction simulation methods which agreed with the experimental results. The four known phases of this family correspond to n = 3, 4, 5 and 6 members and all show fluorite superstructures. They consist of a common δ-Bi2O3 fluorite-type framework, inside of which are distributed ribbons of {MoO4} tetrahedra which are infinite al...

Trigonal structures of ABe2BO3F2 (A = Rb, Cs, Tl) crystals

Acta Crystallographica Section B — Wed, 15 Jul 2009 23:00:00 +0100

Several interesting fluoroberyllium borates were synthesized hydrothermally and characterized by single-crystal X-ray diffraction. The crystal structures of RbBe2BO3F2 (RBBF; rubidium fluoroberyllium borate) and CsBe2BO3F2 (CBBF; caesium fluoroberyllium borate), previously determined in the space group C2, were reinvestigated for higher symmetry and found to have more suitable solutions in the space group R32. TlBe2BO3F2 (TBBF; thallium fluoroberyllium borate) was synthesized as a novel compound also having this trigonal structure type. Details of the space-group determination and unique structural features are discussed. These crystal structures were compared with that of KBe2BO3F2, revealing interesting structural trends within this family of compounds that are also discussed. A crystall...

Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucites

Acta Crystallographica Section B — Wed, 15 Jul 2009 23:00:00 +0100

The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined stru...

Synthesis, crystal structure and spectroscopic properties of a novel carbacylamidophosphate: N-(3-nitrobenzoyl)-N′,N′′-bis(tert-butyl)phosphoric triamide

Acta Crystallographica Section B — Fri, 10 Jul 2009 23:00:00 +0100

The new compound N-(3-nitrobenzoyl)-N′,N′′-bis(tert-butyl)phosphoric triamide was synthesized by reacting 3-nitrobenzoyl phosphoramidic dichloride and tert-butyl amine, and characterized by multinuclear (1H, 13C and 31P) NMR and FTIR spectroscopy techniques. Structural and conformational properties were analyzed using single-crystal X-ray diffraction, vibrational spectra and theoretical calculations. The crystal structure contains three symmetry-independent disordered molecules, connected via intermolecular N—H...O=P and N—H...O=C hydrogen bonds to form a centrosymmetric hexameric chain extended along the [2,1,\bar 1] direction. The disorder is mainly caused by rotation of the tert-butyl groups around the C—N bonds. (Source: Acta Crystallographica Section B)

Polymorphism in the spin-crossover ferric complexes [(TPA)FeIII(TCC)]PF6

Acta Crystallographica Section B — Fri, 10 Jul 2009 23:00:00 +0100

We have identified two polymorphs of the molecular complex [(TPA)Fe(III)(TCC)]PF6 [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF6 anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented. (Source: Acta Crystallographica Section B)

Inverse bilayer structure of mononuclear CoII and NiII complexes of the type M(H2O)3(SO4)(4-CNpy)2

Acta Crystallographica Section B — Fri, 10 Jul 2009 23:00:00 +0100

Two new metal compounds of the formula [M(H2O)3(SO4)(4-CNpy)2]·H2O [M = Ni (1) and Co (2), 4-CNpy = 4-cyanopyridine] have been prepared and studied by X-ray diffraction. In both of these compounds the 4-CNpy ligands are coordinated via pyridyl-N atoms to the metal ions in a cis fashion. The neutral complexes along with the uncoordinated H2O molecules are glued together preferentially into inverse bilayers by non-covalent interactions, including unique interlayer π–π interactions between antiparallel nitrile groups. Hartree–Fock and density-functional theory (DFT) calculations indicate that the π–π interactions are energetically significant. The unit-cell similarity index (Π) of 0.0046 for the compounds suggests their isostructurality, which is also supported by their X-ray powd...

Atomic displacements at and order of all phase transitions in multiferroic YMnO3 and BaTiO3

Acta Crystallographica Section B — Fri, 10 Jul 2009 23:00:00 +0100

Coordinate analysis of the multiple phase transitions in hexagonal YMnO3 leads to the prediction of a previously unknown aristotype phase, with the resulting phase-transition sequence: P63′cm′(e.g.) ↔ P63cm ↔ P63/mcm ↔ P63/mmc ↔ P6/mmm. Below the Néel temperature TN ≃ 75 K, the structure is antiferromagnetic with the magnetic symmetry not yet determined. Above TN the P63cm phase is ferroelectric with Curie temperature TC ≃ 1105 K. The nonpolar paramagnetic phase stable between TC and ∼ 1360 K transforms to a second nonpolar paramagnetic phase stable to ∼ 1600 K, with unit-cell volume one-third that below 1360 K. The predicted aristotype phase at the highest temperature is nonpolar and paramagnetic, with unit-cell volume reduced by a further factor of 2. C...

The incommensurately modulated crystal structure of β-Pb2BiVO6: interpretation of the phase transition α → β → δ and conduction properties of related materials

Acta Crystallographica Section B — Fri, 10 Jul 2009 23:00:00 +0100

A new polymorph of Pb2BiVO6 was prepared under ambient conditions and its crystal structure was determined by single-crystal X-ray diffraction. The phase transitions α → β and β → δ were identified in the mother phase; the high-temperature form δ-Pb2BiVO6 eventually decomposes at 753 K to a mixture of Pb4BiVO8 and the high-temperature form of PbBiVO5 before showing recombination at 923 K. β-Pb2BiVO6 has an incommensurate monoclinic modulated structure. This crystal structure is twinned and complementary structural investigations of a powder sample of β-Pb2BiVO6 by TEM diffraction studies confirmed the lattice and incommensurate modulation character. Log σ = f(T−1) dependences for Pb2BiVO6-related materials (6% M-for-V-substituted compositions; M = Cr, Mn, P) are reported,...

Molecular Clusters. A Bridge to Solid-State Chemistry. By Thomas P. Fehlner, Jean-François Halet and Jean-Yves Saillard. Pp. xii + 378. Cambridge University Press, 2007. Price £70. ISBN 978-0-521-85236-4

Acta Crystallographica Section B — Fri, 12 Jun 2009 23:00:00 +0100

(Source: Acta Crystallographica Section B)

Structures of the pseudo-trigonal polymorphs of Cu2(OH)3Cl. Corrigendum

Acta Crystallographica Section B — Fri, 12 Jun 2009 23:00:00 +0100

An error in the paper by Malcherek & Schlüter (2009), Acta Cryst. B65, 334–341, is corrected. (Source: Acta Crystallographica Section B)

On the polymorphism of benzocaine; a low-temperature structural phase transition for form (II)

Acta Crystallographica Section B — Fri, 12 Jun 2009 23:00:00 +0100

A low-temperature structural phase transition has been observed for form (II) of benzocaine (BZC). Lowering the temperature doubles the b-axis repeat and changes the space group from P212121 to P1121 with γ now 99.37 °. The structure is twinned, the twin rule corresponding to a 21 screw rotation parallel to a. The phase transition is associated with a sequential displacement parallel to a of zigzag bi-layers of ribbons perpendicular to b*. No similar phase transition was observed for form (I) and this was attributed to the different packing symmetries of the two room-temperature polymorphic forms. (Source: Acta Crystallographica Section B)

Transferability and reproducibility in electron-density studies – bond-topological and atomic properties of tripeptides of the type l-alanyl-X-l-alanine

Acta Crystallographica Section B — Fri, 12 Jun 2009 23:00:00 +0100

In the last decade three different data bank approaches have been developed that are intended to make electron-density examinations of large biologically important molecules possible. They rely on Bader's concept of transferability of submolecular fragments with retention of their electronic properties. Therefore, elaborate studies on the quantification of transferability in experiment and theory are still very important. Tripeptides of the type l-alanyl-X-l-alanine (X being any of the 20 naturally encoded amino acids) serve as a model case between amino acids and proteins. The two experimental electron-density determinations (l-alanyl-l-histidinyl-l-alanine and l-alanyl-l-phenylalanyl-l-alanine, highly resolved synchrotron X-ray diffraction data sets) performed in this study and theoretic...

Analysis of ion-migration paths in inorganic frameworks by means of tilings and Voronoi–Dirichlet partition: a comparison

Acta Crystallographica Section B — Fri, 12 Jun 2009 23:00:00 +0100

Two methods using Voronoi–Dirichlet polyhedra (Voronoi–Dirichlet partition) or tiles (tiling) based on partitioning space are compared to investigate cavities and channels in crystal structures. The tiling method was applied for the first time to study ion conductivity in 105 ternary, lithium–oxygen-containing compounds, LiaXbOz, that were recently recognized as fast-ion conductors with the Voronoi–Dirichlet partition method. The two methods were found to be similar in predicting the occurrence of ionic conductivity, however, their conclusions on the dimensionality of conductivity were different in two cases. It is shown that such a contradiction can indicate a high anisotropy of conductivity. Both advantages and restrictions of the methods are discussed with respect to fast-ion co...

A non-mathematical introduction to the superspace description of modulated structures

Acta Crystallographica Section B — Sun, 24 May 2009 19:06:29 +0100

The X-ray analysis of (6R,7aS)-6-(tert-butyl-dimethylsilanyloxy)-1-hydroxy-2-phenyl-5,6,7,7a-tetrahydropyrrolizin-3-one, C19H27NO3Si, revealed a diffraction pattern which is typical for modulated structures: strong Bragg peaks surrounded by weaker reflections which cannot be indexed with the same three reciprocal lattice vectors that are used to describe the strong peaks. For this class of crystal structures the concept of superspace has been developed which, however, for many crystallographers still constitutes a Gordian Knot. As a possible tool to cut this knot the crystal structure of the above-mentioned tetrahydropyrrolizinone derivative is presented as an illustrative example for handling and describing the modulated structure of a typical pharmaceutical (i.e. molecular) compound. Hav...

Structural Crystallography of Inorganic Oxysalts. By S. V. Krivovichev. Vol. 22, IUCr Monographs on Crystallography. Oxford University Press, 2009. Price (hardback) GBP 65.00. ISBN 978-0-19-921320-7.

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

(Source: Acta Crystallographica Section B)

A solution to the observed Z′ = 2 preference in the crystal structures of hydrophobic amino acids

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

Chiral amino acids without functional groups in their side chains (hydrophobic amino acids) systematically form crystals with two molecules in the asymmetric unit. In contrast, racemates of the same compounds form crystals with Z′ = 1. The present investigation addresses the origin of this important difference between enantiomeric and racemic crystals. Through a series of ab initio calculations on infinite two-dimensional slabs, derived from crystal structures, as well as calculations on full crystal structures it is shown that it is indeed possible to explain the observed behaviour. Additionally, the (not unexpected) observation that amino acids usually form racemates in the solid phase rather than undergoing racemic separation upon crystallization is rationalized on the basis of energy...

Single-crystal diffuse scattering studies on polymorphs of molecular crystals. I. The room-temperature polymorphs of the drug benzocaine

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

The drug benzocaine (ethyl 4-aminobenzoate), commonly used as a local anaesthetic, is a bimorphic solid at room temperature. Form (I) is monoclinic P21/c, while the metastable form (II) is orthorhombic P212121. Three-dimensional diffuse X-ray scattering data have been collected for the two forms on the 11-ID-B beamline at the Advanced Photon Source (APS). Both forms show strong and highly structured diffuse scattering. The data have been interpreted and analysed using Monte Carlo (MC) modelling on the basis that the scattering is purely thermal in origin and indicates the presence of highly correlated molecular motions. In both forms (I) and (II) broad diffuse streaks are observed in the 0kl section which indicate strong longitudinal displacement correlations between molecules in the 〈03...

Racemic calcium tartrate tetrahydrate [form (II)] in rat urinary stones

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

The title compound, [Ca(C4H4O6)]·4H2O, calcium tartrate tetrahydrate, is a new triclinic centrosymmetric form identified in rat kidney calculus. The crystal structure was determined from powder and single-crystal X-ray diffraction. The four water molecules belong to one square face of the Ca-atom coordination (a square antiprism), the four O atoms of the second square face coming from two tartrate anions, building infinite chains alternating Ca atom polyhedra and tartrate anions along a, with the chains cross-linked by a network of hydrogen bonds. (Source: Acta Crystallographica Section B)

SiBr4 – prediction and determination of crystal structures

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

For SiBr4 no crystal structures have been reported yet. In this work the crystal structures of SiBr4 were predicted by global lattice-energy minimizations using force-field methods. Over an energy range of 5 kJ mol−1 above the global minimum ten possible structures were found. Two of these structures were experimentally determined from X-ray synchrotron powder diffraction data: The low-temperature β phase crystallizes in P21/c, the high-temperature α phase in Pa\overline{3}. Temperature-dependant X-ray powder diffraction shows that the phase transition occurs at 168 K. (Source: Acta Crystallographica Section B)

Structures of the pseudo-trigonal polymorphs of Cu2(OH)3Cl

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

The crystal structure of Cu2(OH)3Cl has been determined using two natural samples with almost ideal stoichiometry. While one of the samples exhibits a twinned clinoatacamite structure, the other sample is characterized by the appearance of additional weak diffraction maxima at half integer positions of h and k. Structure refinement was carried out with the space group P\bar 1. The relationship between the triclinic phase, clinoatacamite, paratacamite and the herbertsmithite structure is discussed in terms of symmetry as a function of Cu concentration. (Source: Acta Crystallographica Section B)

Characterization of the pressure-induced second-order phase transition in the mixed-valence vanadate BaV6O11

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

The pressure dependence of the structure of the mixed-valence vanadate BaV6O11 was studied with single-crystal X-ray diffraction in a diamond–anvil cell. The compressibility data could be fitted with a Murnaghan equation of state with the zero-pressure bulk modulus B0 = 161 (7) GPa and the unit-cell volume at ambient pressure = 387.1 (3) Å^3 (B′ = 4.00). A phase transition involving a symmetry reduction from P63/mmc to P63mc can be reliably detected in the high-pressure data. The estimated transition pressure lies in the range 1.18 (Source: Acta Crystallographica Section B)

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

This is the second of two papers, where we discuss the cluster structures of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al63.6Ta36.4 (AT-19), a = 19.1663 (1) Å, V = 7040 Å3, cF(5928 − x)-Al56.6Cu3.9Ta39.5, x = 20 (ACT-45), a = 45.376 (1) Å, V = 93 428 Å3 and cF(23, 256 − x)-Al55.4Cu5.4Ta39.1, x = 122 (ACT-71), a = 71.490 (4) Å, V = 365 372 Å3. The space group is F\bar 43m in all three cases. The structures can be described as packings of clusters such as fullerenes, dodecahedra, pentagonal bifrusta and Friauf polyhedra. A characteristic feature of the two larger structures are nets of hexagonal bipyramidal Ta clusters (h.b.p.). The extremely short distance of 2.536–2.562 Å between their apical Ta at...

Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. I. Structure solution and refinement

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

This is the first of two parts, where we report the structure determination of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al63.6Ta36.4 (AT-19), a = 19.1663 (1) Å, V = 7040 Å3, cF(5928 − x)-Al56.6Cu3.9Ta39.5, x = 20 (ACT-45), a = 45.376 (1) Å, V = 93 428 Å3 and cF(23 256 − x)-Al55.4Cu5.4Ta39.1, x = 122 (ACT-71), a = 71.490 (4) Å, V = 365 372 Å3. The space group is F{\bar 4}3m in all three cases. These cluster-based structures are closely related to the class of Frank–Kasper phases. It is remarkable that all three structures show the same average structure that resembles the cubic Laves phase. (Source: Acta Crystallographica Section B)

Structures of K0.05Na0.95NbO3 (50–300 K) and K0.30Na0.70NbO3 (100–200 K)

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

Rietveld refinement using neutron powder diffraction data is reported for the potential lead-free piezoelectric material KxNa1 − xNbO3 (x = 0.05, x = 0.3) at low temperatures. The structures were determined to be of rhombohedral symmetry, space group R3c, with the tilt system a−a−a− for both compositions. It was found that some of the structural parameters differ significantly in the two structures, and particularly the NbO6 octahedral strains as a function of temperature. The 300 K profile for K0.05Na0.95NbO3 shows the coexistence of rhombohedral and monoclinic phases, which indicates that the phase boundary is close to room temperature; the phase boundary for K0.30Na0.70NbO3 is found to be at approximately 180 K. (Source: Acta Crystallographica Section B)

Complex ceramic structures. I. Weberites

Acta Crystallographica Section B — Mon, 18 May 2009 23:00:00 +0100

This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interes...

Hantzsch 1,4-dihydropyridine esters and analogs: candidates for generating reproducible one-dimensional packing motifs

Acta Crystallographica Section B — Mon, 04 May 2009 07:05:19 +0100

Examination of the symmetric Hantzsch 1,4-dihydropyridine ester derivatives of the prototypical nifedipine molecule indicates the tendency of this class of molecule to form a common packing motif. Crystal structure analysis of 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic diesters and analogs reveals that they form extended chains, characterized as the C(6) packing motif, via intermolecular (amine) N—H...O=C (C3,C5 carbonyl) hydrogen bonds. In addition, all the prepared derivatives also satisfy the basic structural requirements for their high binding efficiency to the receptor. The reproducible C(6) packing motif observed among these compounds has a use in the design of solid-state materials. (Source: Acta Crystallographica Section B)

On the correlations between the polyhedron eccentricity parameters and the bond-valence sums for the cations with one lone electron pair. Addendum

Acta Crystallographica Section B — Sat, 02 May 2009 04:00:00 +0100

Systematic variations of the bond-valence sums calculated from the poorly determined bond-valence parameters [Sidey (2008), Acta Cryst. B64, 515–518] have been illustrated using a simple graphical scheme. (Source: Acta Crystallographica Section B)

Absolute structure determination as a reference for the enantiomeric resolution of racemic mixtures of cyclophosphazenes via chiral high-performance liquid chromatography

Acta Crystallographica Section B — Sat, 02 May 2009 04:00:00 +0100

Reversed-phase chiral high-performance liquid chromatography (HPLC) is a potentially powerful technique for the enantiomeric resolution of racemic mixtures, although the elution order of enantiomers is only relative and it is necessary to fully characterize reference systems for this method to provide absolute configurational information. The enantiomeric resolution of a series of racemic di-spiro cyclotriphosphazene derivatives, N3P3X2[O(CH2)3NH]2 (X = Cl, Ph, SPh, NHPh, OPh) [(1)–(5), respectively] was carried out by reversed-phase chiral HPLC on a commercially available Pirkle-type chiral stationary phase (R,R)-Whelk-01 using 85:15 (v/v) hexane–thf as the mobile phase. The absolute configurations of the resulting enantiomers of compounds (3) (X = SPh) and (5) (X = OPh) were determin...

Structures of the ZrZn22 family: suprapolyhedral nanoclusters, methods of self-assembly and superstructural ordering

Acta Crystallographica Section B — Sat, 02 May 2009 04:00:00 +0100

A combinatorial topological analysis is carried out by means of the program package TOPOS4.0 [Blatov (2006), IUCr Comput. Commun. Newsl. 7, 4–38] and the matrix self-assembly is modeled for crystal structures of the ZrZn22 family (space group Fd\bar 3m, Pearson code cF184), including the compounds with superstructural ordering. A number of strict rules are proposed to model the crystal structures of intermetallics as a network of cluster precursors. According to these rules the self-assembly of the ZrZn22-like structures was considered within the hierarchical scheme: primary polyhedral cluster → zero-dimensional nanocluster precursor → one-dimensional primary chain → two-dimensional microlayer → three-dimensional microframework (three-dimensional supraprecursor). The suprapolyhed...

Charge density and electrostatic potential analyses in paracetamol

Acta Crystallographica Section B — Sat, 02 May 2009 04:00:00 +0100

The electron density of monoclinic paracetamol was derived from high-resolution X-ray diffraction at 100 K. The Hansen–Coppens multipole model was used to refine the experimental electron density. The topologies of the electron density and the electrostatic potential were carefully analyzed. Numerical and analytical procedures were used to derive the charges integrated over the atomic basins. The highest charge magnitude (−1.2 e) was found for the N atom of the paracetamol molecule, which is in agreement with the observed nucleophilic attack occurring in the biological media. The electric field generated by the paracetamol molecule was used to calculate the atomic charges using the divergence theorem. This was simultaneously applied to estimate the total electrostatic force exerted...

Significant progress in predicting the crystal structures of small organic molecules – a report on the fourth blind test

Acta Crystallographica Section B — Mon, 16 Mar 2009 18:43:32 +0100

We report on the organization and outcome of the fourth blind test of crystal structure prediction, an international collaborative project organized to evaluate the present state in computational methods of predicting the crystal structures of small organic molecules. There were 14 research groups which took part, using a variety of methods to generate and rank the most likely crystal structures for four target systems: three single-component crystal structures and a 1:1 cocrystal. Participants were challenged to predict the crystal structures of the four systems, given only their molecular diagrams, while the recently determined but as-yet unpublished crystal structures were withheld by an independent referee. Three predictions were allowed for each system. The results demonstrate a drama...

Polymorphism in iodotris(tri-p-tolylphosphine)silver(I)

Acta Crystallographica Section B — Sat, 21 Feb 2009 06:40:27 +0100

The reaction of silver(I) iodide with tri(p-tolyl)phosphine in MeCN solution in 1:3 molar ratio yields a polymorph of the complex of the formula [AgI{P(4-MeC6H4)3}3], with the Ag atom in a distorted tetrahedral environment. A polymorphic structure of this complex (a) is compared with previously published crystal structures (b), determined at different temperatures. The two polymorphs are compared using r.m.s. overlay calculations as well as half-normal probability plots. (Source: Acta Crystallographica Section B)

An aza-cyclophane stacked in racemic columnar assemblies: whole-molecule disorder in a two-dimensional solid solution

Acta Crystallographica Section B — Fri, 20 Feb 2009 05:00:00 +0100

The oxime derivative of [1.1.1]cyclophane cyclotriveratrylene (CTV) was ring expanded utilizing a Beckmann rearrangement to provide a ten-membered N-acetyl macrocyclic amide that crystallizes as a chloroform monosolvate in columnar assemblies manifesting an unusual disorder within the crystal. Columns made up of this structure consist of infinite columnar assemblies of alternating d and l enantiomers and therefore necessarily are made up of a racemate, yet the chiralities of individual molecules in adjacent columns are independent of one another, leading to the overall formation of a two-dimensional solid solution. The random arrangement of the columns within the structure leads to the emergence of a crystallographic mirror plane not reflected by the molecular symmetry, to a change of symm...

X-ray powder diffraction and electron diffraction studies of the thortveitite-related L phase, (Zn,Mn)2V2O7

Acta Crystallographica Section B — Fri, 20 Feb 2009 05:00:00 +0100

The phase designated γ-Zn3(VO4)2 reported as a minor second phase in zinc oxide-based varistor materials doped with vanadium oxide and manganese oxide is shown to be the L phase, (Zn1 − xMnx)2V2O7 (0.188 < x < 0.538), in the pseudo-binary Mn2V2O7–Zn2V2O7 system. Analysis of X-ray powder diffraction patterns and electron diffraction patterns of this phase shows that the previously published a, c and β values for this thortveitite-related phase are incorrect. Instead, Rietveld refinement of the X-ray powder pattern of the L phase shows that it has a monoclinic C lattice with Z = 6, with a = 10.3791 (1), b = 8.5557 (1), c = 9.3539 (1) Å and β = 98.467 (1)°. Although prior convergent-beam electron diffraction work of `γ-Zn3(VO4)2' confirmed the C Bravais lattice, th...

Ln3M1 − δTX7 – quasi-isostructural compounds: stereochemistry and silver-ion motion in the Ln3Ag1 − δGeS7 (Ln = La–Nd, Sm, Gd–Er and Y; δ = 0.11–0.50) compounds

Acta Crystallographica Section B — Fri, 20 Feb 2009 05:00:00 +0100

The crystal structures of the Ln3Ag1 − δGeS7 (Ln = La–Nd, Sm, Gd–Er, Y; δ = 0.11–0.50, space group P63) compounds were determined by means of X-ray single-crystal diffraction and the similarities among the crystal structures of all Ln3M1 − δTX7 (space group P63; Ln – lanthanide element, M – monovalent element; T – tetravalent element and X – S, Se) compounds deposited in the Inorganic Crystal Structure Database (ICSD) are discussed. Substitutions of each element in Ln3M1 − δTX7 result in a different structural effect. On the basis of the data deposited in the ICSD the large family of the Ln3M1 − δTX7 compounds was divided into three groups depending on the position of the monovalent element in the lattice. This position determines what kind of ste...

Trans-cinnamic acid and coumarin-3-carboxylic acid: experimental charge-density studies to shed light on [2 + 2] cycloaddition reactions

Acta Crystallographica Section B — Fri, 20 Feb 2009 05:00:00 +0100

As part of an ongoing series of experimental charge-density investigations into the intra- and intermolecular interactions present in compounds which undergo solid-state [2 + 2] cycloaddition reactions, the charge-density analyses of trans-cinnamic acid and coumarin-3-carboxylic acid are reported. Thus, high-resolution single-crystal X-ray diffraction data were recorded at 100 K for trans-cinnamic acid (sin θ/λmax = 1.03 Å−1) and coumarin-3-carboxylic acid (sin θ/λmax = 1.19 Å−1). In addition to the anticipated O—H...O hydrogen bonds weak C—H...O interactions were identified in both structures along with very weak intermolecular interactions between pairs of molecules that undergo solid-state [2 + 2] cycloaddition reactions upon irradiation. (Source: Acta Cr...

Compounds with a `stuffed' anti-bixbyite-type structure, analysed in terms of the Zintl–Klemm and coordination-defect concepts

Acta Crystallographica Section B — Sat, 20 Dec 2008 17:42:09 +0100

The bixbyite structure (Mn2O3) (Ia\bar 3) is often described as a distorted face-centered cubic (f.c.c.) array of Mn atoms, with O atoms occupying 3/4 of the tetrahedral holes. The empty M4 tetrahedra are centred at 16c. In anti-bixbyite structures (Mg3N2), cation vacancies are centred in empty N4 tetrahedra. If 16 hypothetical atoms were located at this site they would form the structure of γ-Si. This means that anti-bixbyite structures are ideally prepared to accommodate Si(Ge) atoms at these holes. Several compounds (Li3AlN2 and Li3ScN2) fully satisfy this expectation. They are really anti-bixbyites `stuffed' with Al(Sc). The presence of these atoms in 16c is illuminated in the light of the extended Zintl–Klemm concept (EZKC) [Vegas & García-Baonza (2007). Acta Cryst. B63, 339–345...

Alternative presentation of the Brown–Wu bond-valence parameters for some s2 cation/O2− ion pairs

Acta Crystallographica Section B — Sat, 20 Dec 2008 05:00:00 +0100

Using the least-squares refinement procedure, high-performance Brown–Wu bond-valence parameters, r0 and n, reported for the Sn2+/O2−, Sb3+/O2−, Te4+/O2− and I5+/O2− ion pairs have been converted into the commonly used bond-valence parameters, r0 and b. The r0 and b values obtained can be recommended as a replacement for the conventional bond-valence parameters based on the `universal constant' b = 0.37 Å. (Source: Acta Crystallographica Section B)

Structural study of KxNa1 − xNbO3 (KNN) for compositions in the range x = 0.24–0.36

Acta Crystallographica Section B — Sat, 20 Dec 2008 05:00:00 +0100

The structure of the A-site substituted perovskite KxNa1 − xNbO3, x = 0.24–0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293 K and in P4mm at 523 K. The Glazer tilt system of the room-temperature monoclinic phase is a0b+c0, which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase. (Source: Acta Crystallogr...

High-pressure crystal structure of the non-linear optical compound BiB3O6 from two-dimensional powder diffraction data

Acta Crystallographica Section B — Sat, 20 Dec 2008 05:00:00 +0100

Our recently proposed method for automatic detection, calibration and evaluation of Debye–Scherrer ellipses using pattern-recognition techniques and advanced signal filtering was applied to the two-dimensional powder diffraction data of the non-ferroelectric, non-centrosymmetric non-linear optical (NLO) compound α-BiB3O6 as a function of pressure. At ambient conditions, α-BiB3O6 crystallizes in the space group C2 (phase I). In the pressure range between P = 6.09 and 6.86 GPa, it exhibits a first-order phase transition into a structure with the space group C1 (P1) [phase II at P = 8.34 GPa: a = 7.4781 (6), b = 3.9340 (4), c = 6.2321 (6) Å, α = 93.73 (1), β = 102.93 (1), γ = 90.76 (1)°, and V = 178.24 (3) Å3]. Non-linear compression behaviour over the entire p...

Planar packing of tetrachlorodicyanobenzenes. II

Acta Crystallographica Section B — Sat, 20 Dec 2008 05:00:00 +0100

Equimolar mixtures of ortho- and meta-, or ortho- and para-, or meta- and para-tetrachlorodicyanobenzene, C6Cl4(CN)2, form structures with disordered layers containing both kinds of molecules in rhombohedral or pseudo-rhombohedral cells. These structures are similar to the structure of the rhombohedral form of the meta isomer. The guiding force appears to be an intermolecular synthon, in which a nitrile group in one molecule interacts with Cl atoms in two adjacent molecules with N...Cl distances of approximately 3.1 Å. (Source: Acta Crystallographica Section B)

Disorder in crystals of dioxofluorotungstates, (NH4)2WO2F4 and Rb2WO2F4

Acta Crystallographica Section B — Tue, 25 Nov 2008 14:48:27 +0100

Dioxotetrafluorotungstates (NH4)2WO2F4 [(I) at 297 K and (II) at 133 K] and Rb2WO2F4 (III) were synthesized in a single-crystal form and their structures were determined by X-ray diffraction. Two independent states of the cis-WO2F4 octahedron are characteristic of static and dynamic disorder in structure (I). Dynamically disordered W2 is displaced from the symmetry axis producing four possible orientations of anion that permits O and F atoms to be identified in separate orientations owing to the inherent differences between W—O and W—F bonding. After the phase transition at lower temperature (201 K), (I) transforms into the twin structure (II) with complete O/F ordering. Structure (III) is characterized by full O/F static disorder without any phase transitions at lower temperatur...

Polymorphism in cyclohexanol. Corrigendum

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

A replacement Fig. 6 to the paper by Ibberson et al. (2008), Acta Cryst. B64, 573–582 is given. (Source: Acta Crystallographica Section B)

Chiral carboxylic acids and their effects on melting-point behaviour in co-crystals with isonicotinamide

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

The crystal structures of co-crystals of two systems of chiral carboxylic acids, optically active and racemic 2-phenylpropionic acid and 2-phenylbutyric acid, with isonicotinamide are reported to investigate the effects of the chirality of the chiral carboxylic acids on the melting point of the co-crystal complexes. It was found that the racemic co-crystal has a higher melting point than the optically active co-crystal, which correlates with the denser packing arrangement inherent in centrosymmetric space groups. (Source: Acta Crystallographica Section B)

Five more phases of the structural family [M(H2O)2(15-crown-5)](NO3)2

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

The structures of five more phases of the structural family of compounds [M(H2O)2(15-crown-5)](NO3)2 have been determined. All of these phases are stable at room temperature or above, but transform to other phases if cooled slowly. All five phase transitions take place without significant damage to the single crystal. The M = Co phase, which has a three-dimensional hydrogen-bonding pattern, is a disordered version of the structure known at room temperature; Z′ changes from 2 to ½. The other four structures have a two-dimensional hydrogen-bonding pattern, are all modulated variants of the same basic cell (or subcell) and are at least mostly ordered. For M = Mg and Zn the structure found somewhat above room temperature is the Z′ = 8 variant found previously for M = Fe. For M = Cu and Ni...

The elusive [Ni(H2O)2(15-crown-5)]2+ cation and related co-crystals of nickel(II) hydrates and 15-crown-5

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

Initial attempts to make [Ni(H2O)2(15-crown-5)](NO3)2, i.e. to insert the Ni2+ ion into the 15-crown-5 macrocycle, gave the mono- (two polymorphs) and dihydrate of a co-crystal of [Ni(H2O)6](NO3)2 and 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane = 15C5). Synthetic routes designed to restrict the water available to the Ni2+ cation gave three additional compounds, [Ni(H2O)6](NO3)2-trans-[Ni(H2O)4(MeOH)2](NO3)2·2(15C5), cis-[Ni(H2O)4(NO3)2]-trans-[Ni(H2O)4(NO3)2]·2(15C5) and [Ni(H2O)2(MeCN)(NO3)2]·15C5·MeCN. All five compounds contain Ni2+ ions with two trans aqua ligands. In all six structures these aqua ligands make hydrogen bonds to the 15C5 molecules to form stacks in which the Ni complexes and 15C5 molecules alternate. The structures are surprisingly complicated: all are co-cryst...

Structural characterization of RE10W22O81 rare-earth tungstates (RE = Ce, Nd)

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

The single-crystal diffraction study of Ce10W22O81 and powder X-ray diffraction (XRD) experiments on Ce10W22O81 and Nd10W22O81 show that the true space group of these phases is Pbnm, contrary to the previous literature reports of the space groups Pbcn and Pbcm for the rare-earth tungstates (RE = La, Ce, Pr, Nd) with this general formula. The structure contains rare-earth cations in seven-, eight- and ninefold coordination. W atoms are found in corner-sharing WO5 groups, and also in WO6 and WO7 polyhedra which share edges and corners to form W6O17 units. The new model obtained from our single-crystal work gives better agreement with powder XRD data on both Ce10W22O81 and Nd10W22O81, and gives more plausible coordination environments and bond-valence sums. (Source: Acta Crystallographica Sec...

Formability of ABX3 (X = F, Cl, Br, I) halide perovskites

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

In this study a total of 186 complex halide systems were collected; the formabilities of ABX3 (X = F, Cl, Br and I) halide perovskites were investigated using the empirical structure map, which was constructed by Goldschmidt's tolerance factor and the octahedral factor. A model for halide perovskite formability was built up. In this model obtained, for all 186 complex halides systems, only one system (CsF–MnF2) without perovskite structure and six systems (RbF–PbF2, CsF–BeF2, KCl–FeCl2, TlI–MnI2, RbI–SnI2, TlI–PbI2) with perovskite structure were wrongly classified, so its predicting accuracy reaches 96%. It is also indicated that both the tolerance factor and the octahedral factor are a necessary but not sufficient condition for ABX3 halide perovskite formability, and a lowe...

Structural behaviour of synthetic Co2SiO4 at low temperatures

Acta Crystallographica Section B — Fri, 14 Nov 2008 00:00:00 +0100

Synthetic Co2SiO4 has an olivine structure with isolated SiO4 groups (space group Pnma) and shows magnetic ordering below 50 K. Single-crystal neutron diffraction was applied to determine precise crystal structure parameters at low temperatures. No structural phase transition was revealed in the temperature range 2.5–300 K. Lattice parameters were determined by high-resolution X-ray powder diffraction between 15 and 300 K. There is a clear evidence of an anomalous thermal expansion related to the magnetic phase transition which can be attributed to magnetostriction. (Source: Acta Crystallographica Section B)

Structures of mono-unsaturated triacylglycerols. V. The β′1-2, β′-3 and β2-3 polymorphs of 1,3-dilauroyl-2-oleoylglycerol (LaOLa) from synchrotron and laboratory powder diffraction data

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

The crystal structures of the \beta ^{\prime}_1-2, the β′-3 and the β2-3 polymorphs of 1,3-dilauroyl-2-oleoylglycerol have been solved from powder diffraction data. The packing of the β2-3 polymorph is similar to that of other cis mono-unsaturated triacylglycerols. Both the β′ polymorphs are crystallized in a novel type of packing in which one of the saturated lauroyl chains is packed side-by-side with part of the unsaturated oleoyl chain. (Source: Acta Crystallographica Section B)

Bitter sweeteners: tetrazole derivatives of arylsulfonylalcanoids – synthesis, structure and comparative study

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

Within a research project aimed at the design of new sweeteners, the tetrazole moiety was introduced to arylsulfonylalkanoic acids (ASA) as a bioisostere of the carboxyl group. The crystal structures of four newly synthesized tetrazole derivatives and one intermediate product of the reaction were determined in order to explain the bitter taste of these compounds. Three chiral compounds crystallize as racemic mixtures in centrosymmetric space groups of the monoclinic system, whereas the non-chiral compound, with a higher dipole moment, crystallizes in the polar space group Cc. Intermolecular N—H...N hydrogen bonds between tetrazole moieties were observed in all four structures and are compared with the analogous interactions observed in tetrazole derivatives deposited in the Cambridge Str...

Improvement of anisotropic displacement parameters from invariom-model refinements for three l-hydroxylysine structures

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

Three l-hydroxylysine structures have been determined at 100 K by single-crystal X-ray diffraction. High-resolution data using either a laboratory or synchrotron source were collected and subjected to invariom- and independent atom-model (IAM) refinements. Anisotropic displacement parameters (ADPs) obtained from invariom refinement were compared (i) with results from a full multipole and (ii) with an IAM high-order refinement. Differences were visualized with the program PEANUT and were complemented by quantitative results from a Hirshfeld test. Influences of scale factor differences, and of refinement against F^2 versus F, have been investigated. Systematic errors were observed in the IAM, especially when only low-order data were available. Although these errors were reduced in high-ord...

Modular crystals as modulated structures: the case of the lillianite homologous series

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

The use of the superspace formalism is extended to the description and refinement of the homologous series of modular structures with two symmetry-related modules with different orientations. The lillianite homologous series has been taken as a study case. Starting from a commensurate modulated composite description with two basic subsystems corresponding to the two different modules, it is shown how a more efficient description can be achieved using so-called zigzag modulation functions. These linear zigzag modulations, newly implemented in the program JANA2006, have very large fixed amplitudes and introduce in the starting model the two orientations of the underlying module sublattices. We show that a composite approach with this type of function, which treats the cations and anions as t...

Original disorder–order transition related to electronic and magnetic properties in the thermoelectric misfit phase [Ca2CoO3][CoO2]1.62

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

A structural phase transition is shown around 400 K for the thermoelectric lamellar misfit cobalt oxide [Ca2CoO3][CoO2]1.62. This transition is related to a rearrangement of the central [CoO] layer of the [Ca2CoO3] slab of this structure, characterized by a commensurate intrinsic modulation q2 = 2\over 3a* − 1\over 3c*. The partial residual disorder related to split Co and O atomic sites along the misfit b direction disappears and one can describe this layer with its triple chains as a modulated configuration with a regular and not distorted periodicity along b. This phase transition is associated with small changes observed in the transport and magnetic properties as a function of temperature. (Source: Acta Crystallographica Section B)

Order–disorder character and twinning in the structure of a new synthetic titanosilicate: (Ba,Sr)4Ti6Si4O24·H2O

Acta Crystallographica Section B — Sat, 08 Nov 2008 00:00:00 +0100

Prismatic crystals of the title compound, up to 100 µm long and {001} twinned by metric merohedry, were obtained as a side-product of a hydrothermal run devoted to synthesizing the heterophyllosilicate lamprophyllite. Single-crystal X-ray diffraction data were collected on a Bruker-AXS Smart Apex diffractometer from a crystal with approximate composition (Ba0.80Sr0.20)4Ti6Si4O24·H2O. The structure was solved and refined as a disordered structure in the space group Cmmm [ao = 5.906 (2), bo = 20.618 (8), co = 16.719 (6) Å, R = 0.089 for 682 reflections with Io > 2σ(Io)], and then deciphered by the order–disorder (OD) theory as an ordered structure and refined in the space group P2/c [a = 5.906, b = 16.719, c = 10.724 Å, β = 105.99°, R = 0.083 for 1090 reflections with Io...

Isosymmetrical phase transition in α-YbV4O8

Acta Crystallographica Section B — Fri, 10 Oct 2008 18:36:44 +0100

The structure of YbV4O8 is related to the CaFe2O4 structure type. VO6 octahedra form a three-dimensional framework with tunnels in which the Yb3+ ions are incorporated. Two different polymorphs α and β are known and differ mainly in the arrangement of the Yb ions within the framework. We studied the structure and magnetic properties of α-YbV4O8 as a function of temperature. At approximately 70 K α-YbV4O8 undergoes a first-order isosymmetrical phase transition (P21/n → P21/n). While in the high-temperature α phase the three V3+ and one V4+ are disordered over the four symmetrically independent octahedral sites, in the low-temperature α′ phase complete charge ordering is observed. The transition is accompanied by a paramagnetic–paramagnetic anomaly in the magnetic susceptibilit...

Synchrotron powder diffraction characterization of the zeolite-based (p-N,N-dimethylnitroaniline–mordenite) guest–host phase

Acta Crystallographica Section B — Fri, 10 Oct 2008 04:00:00 +0100

The crystal structure of a new phase consisting of the inclusion of the hyperpolarizable molecule p-N,N-dimethylnitroaniline (dimethyl-para-nitroaniline or dmpNA) in the large-pore zeolite mordenite (MOR) has been determined from high-resolution synchrotron powder diffraction at 300 and 90 K. The unit-cell parameters and space group at 300 K are similar to those of as-synthesized mordenite. The crystallographic study indicates that the MOR straight channels are almost fully loaded with molecules that are disordered over eight symmetry-related sites. As expected, the molecules are located in the large 12-membered ring channel, at the intersection with the secondary eight-membered channel with which they might form hydrogen bonds. The elongation axes (and then the dipole moments) of the ...

Structure of Ga2O3(ZnO)6: a member of the homologous series Ga2O3(ZnO)m

Acta Crystallographica Section B — Tue, 16 Sep 2008 16:10:13 +0100

The structure of Ga2O3(ZnO)6 was determined using single-crystal X-ray diffraction techniques in the space group Cmcm. The metal ion sublattice resembles some of the Zn ions in the wurtzite ZnO structure. The oxygen ion sublattice in Ga2O3(ZnO)6 also resembles some of the O ions in ZnO. Structural relationships between Ga2O3(ZnO)6 and ZnO are discussed, illustrating the process for obtaining the centrosymmetric Ga2O3(ZnO)6 structure from the noncentrosymmetric ZnO. Structures of phases in the homologous series Ga2O3(ZnO)m are predicted on the basis of the structural data for Ga2O3(ZnO)6. The structures of even m are constructed by simply extending the structure units seen in Ga2O3(ZnO)6, while those of odd m consist of structure units which are of different types from those used for even m...

Orientational disorder and phase transitions in crystals of (NH4)2NbOF5

Acta Crystallographica Section B — Sat, 06 Sep 2008 14:33:44 +0100

Ammonium oxopentafluoroniobate, (NH4)2NbOF5, was synthesized in a single-crystal form and the structures of its different phases were determined by X-ray diffraction at three temperatures: phase (I) at 297 K, phase (II) at 233 K and phase (III) at 198 K. The distorted [NbOF5]2− octahedra are of similar geometry in all three structures, with the central atom shifted towards the O atom. The structure of (I) is disordered, with three spatial orientations of the [NbOF5]2− octahedron related by a jump rotation around the pseudo-threefold local axis such that the disorder observed is of a dynamic nature. As the temperature decreases, the compound undergoes two phase transitions. The first is accompanied by full anionic ordering and partial ordering of the ammonium groups (phase II). Th...

Polymorphism in cyclohexanol

Acta Crystallographica Section B — Sat, 06 Sep 2008 04:00:00 +0100

The crystal structures and phase behaviour of phase II and the metastable phases III′ and III of cyclohexanol, C6H11OH, have been determined using high-resolution neutron powder, synchrotron X-ray powder and single-crystal X-ray diffraction techniques. Cyclohexanol-II is formed by a transition from the plastic phase I cubic structure at 265 K and crystallizes in a tetragonal structure, space group P\bar 42_1c (Z′ = 1), in which the molecules are arranged in a hydrogen-bonded tetrameric ring motif. The structures of phases III′ and III are monoclinic, space groups P21/c (Z′ = 3) and Pc (Z′ = 2), respectively, and are characterized by the formation of hydrogen-bonded molecular chains with a threefold-helical and wave-like nature, respectively. Phase III crystallizes at 195 K fr...

Redetermination of the structure and dielectric properties of bis(thiourea) pyridinium iodide – a new ferroelectric inclusion compound

Acta Crystallographica Section B — Sat, 06 Sep 2008 04:00:00 +0100

The crystal structure of bis(thiourea) pyridinium iodide (T2PyI) was previously determined at 295 and 110 K [Prout, Heyes, Dobson, McDaid, Maris, Mueller & Seaman (2000). Chem. Mater. 12, 3561–3569] and the two phases were described in the space groups Cmcm and P21cn, respectively. Because differential scanning calorimetry revealed two phase transitions, at 161 and 141 K, a redetermination of the structure of T2pyI at 295, 155 and 110 K has been undertaken, and the following sequence of space groups obtained: Cmcm (I) → C2cm (II) → P21cn (III). The high- (I) and low-temperature (III) phases confirmed the results reported in the previous study. In the new intermediate phase II, the mirror plane perpendicular to the x axis vanishes and the crystal structure loses the centre of sy...

Hydrogen-bond patterns and the structures of 1,4-cyclohexanediol: 2:1 cis:trans-1,4-cyclohexanediol

Acta Crystallographica Section B — Sat, 06 Sep 2008 04:00:00 +0100

The structure of a co-crystal with both trans and cis isomers of 1,4-cyclohexanediol (1,4-CHD) is reported. The intermolecular hydrogen-bond patterns are described and compared with those of the all trans structure, using the graph-set model. A second crystal with possible cis/trans disorder is also described. The results of molecular modeling of the simple isomers and conformers are compared with the known structures. (Source: Acta Crystallographica Section B)

Incommensurate structure of InAl1 − xTixO3 + x/2[x = 0.701 (1)]: comparison between modulated and composite models

Acta Crystallographica Section B — Tue, 22 Jul 2008 16:58:15 +0100

The structure of the monoclinic phase of the compound InAl1 − xTixO3 + x/2 with x = 0.701 (1) has been analyzed within the (3 + 1)-dimensional superspace formalism. Two different models were refined describing the structure as an incommensurate modulated layer and modulated composite, respectively. Both models include the same composition–structure relation. In the composite approach it is derived from the mismatching between the two subsystems. In the incommensurate modulated system, it is derived from a closeness condition between O atomic domains. The distribution and coordination of the cations is discussed and compared with previously proposed models for similar compounds. (Source: Acta Crystallographica Section B)

Pressure-freezing with conformational conversion of 3-aminopropan-1-ol molecules

Acta Crystallographica Section B — Thu, 10 Jul 2008 17:45:24 +0100

3-Aminopropan-1-ol, NH2(CH2)3OH, was pressure-frozen and its structure determined at 0.2, 0.9 and 1.31 GPa by single-crystal X-ray diffraction. The freezing pressure of 0.13 GPa at 296 K was measured by ruby fluorescence in the diamond–anvil cell and from compressibility measurement in the piston-and-cylinder reaction press. The molecules assume an extended conformation in the crystalline state, different from the pseudo-ring conformers, with the terminal groups linked by an intramolecular hydrogen bond, present in the gaseous and liquid states. The polar arrangement in the 3-aminopropan-1-ol crystals is explained in terms of the pattern of intermolecular hydrogen bonds. (Source: Acta Crystallographica Section B)

On the correlations between the polyhedron eccentricity parameters and the bond-valence sums for the cations with one lone electron pair

Acta Crystallographica Section B — Thu, 10 Jul 2008 04:00:00 +0100

Applicability of the Wang–Liebau polyhedron eccentricity parameter in the bond-valence model [Wang & Liebau (2007). Acta Cryst. B63, 216–228] has been found to be doubtful: the correlations between the values of the polyhedron eccentricity parameters and the bond-valence sums calculated for the cations with one lone electron pair are probably an artifact of the poorly determined bond-valence parameters. (Source: Acta Crystallographica Section B)

Interaction geometries and energies of hydrogen bonds to C=O and C=S acceptors: a comparative study

Acta Crystallographica Section B — Thu, 10 Jul 2008 04:00:00 +0100

The occurrence, geometries and energies of hydrogen bonds from N—H and O—H donors to the S acceptors of thiourea derivatives, thioamides and thiones are compared with data for their O analogues – ureas, amides and ketones. Geometrical data derived from the Cambridge Structural Database indicate that hydrogen bonds to the C=S acceptors are much weaker than those to their C=O counterparts: van der Waals normalized hydrogen bonds to O are shorter than those to S by ∼ 0.25 Å. Further, the directionality of the approach of the hydrogen bond with respect to S, defined by the C=S...H angle, is in the range 102–109°, much lower than the analogous C=O...H angle which lies in the range 127–140°. Ab initio calculations using intermolecular perturbation theory show good agreement wi...

Special features of intermolecular bonding A...D(A = Si, Ge and D = nucleophile) in crystal structures

Acta Crystallographica Section B — Thu, 10 Jul 2008 04:00:00 +0100

An investigation of the crystal structures of 3346 silicon- and 718 germanium-containing compounds was carried out with a geometrical–topological analysis. The descriptors of inter- and intramolecular contacts A...D are given. Information on the descriptors of T—A...D (where T = leaving group) interactions in terms of a stereoatomic model of crystal structures was described as being helpful in modelling SN2 reactions. It was established that the formation of multiple intermolecular contacts in crystal structures is unfavorable. The dependence of the descriptors of intermolecular contacts T—A...D was studied as a function of the nature of T (leaving group in substitution reactions) and D (nucleophile atom or group of atoms) groups, and the coordination number of an A atom. The constan...

Incommensurate structure of Ca2Al2O5 at high temperatures – structure investigation and Raman spectroscopy

Acta Crystallographica Section B — Thu, 10 Jul 2008 04:00:00 +0100

A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca2Al2O5 transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca2Al2O5 were synthesized in an end-loaded piston cylinder press at 2.5 GPa and 1273 K. The diffraction pattern observed at 1090 (10) K by in situ single-crystal diffraction experiments can be indexed by an I-centred orthorhombic cell and a modulation wavevector of q = 0.595 (1)c*. A (3 + 1)-dimensional model in superspace group Imma(00γ)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO6] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO4] tetrahedra running along a. The modu...

Incommensurate structure of InAl1 − xTixO3 + x/2 [x = 0.701 (1)]: comparison between modulated and composite models

Acta Crystallographica Section B — Thu, 10 Jul 2008 04:00:00 +0100

Conformational richness and multiple Z′ in salt co-crystal of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate

Acta Crystallographica Section B — Sat, 14 Jun 2008 04:00:00 +0100

The co-crystal structure of N-methylpiperidine betaine with N-methylpiperidine betaine hexafluorosilicate represents an unusual case of a salt co-crystal with a high Z′ value (3), unexpected conformational variability, and with nearly 50% of its contents disordered. The betaine units from the salt and co-crystal formers are paired into several homoconjugated dimers via very short, linear O−...H+...O− bridges. These hydrogen bonds are the dominating interactions in the co-crystal structure, in variance with the simple hexafluorosilicate salt, which has a structure governed by COOH...F hydrogen bonds. The SiF^{2-}_6 anion in the co-crystal structure has only C—H...F interactions with the betaine units. The zwitterion:cation:anion stoichiometry is 3:3:1.5. Some of the betaine units di...

Orientational ordering in the low-temperature stable phases of deuterated thiophene

Acta Crystallographica Section B — Sat, 14 Jun 2008 04:00:00 +0100

The stable structures of deuterated thiophene C4D4S were investigated at 155 (phase III), 115 (phase IV), 100 and 1.5 K (phase V) by neutron powder diffraction. At 155 K, thiophene is orthorhombic with space group Pbnm. Although there is some degree of in-plane orientational disorder, molecules begin to order along two symmetrically equivalent main orientations. At 115 K the structure is incommensurate, with a wavevector q ≃ 0.55a*. At 100 K and below, there is a doubling of the a cell parameter and the structure space group is P21. For the first time, it is shown that, unlike C4H4S, phase V of C4D4S is not an orientational glass: thiophene molecules are perfectly ordered and are oriented within the molecular plane along two alternating directions, corresponding to the two main o...

X-ray diffuse scattering from HMTA: analysis via a Monte Carlo model

Acta Crystallographica Section B — Sat, 14 Jun 2008 04:00:00 +0100

Hexamethylenetetramine (HMT, C6H12N4, also referred to as urotropin) and azelaic acid [A, HOOC—(CH2)7—COOH] form a co-crystal or adduct (HMTA, also referred to as urotropin azelate) which exhibits several structural phases as a function of temperature. At room temperature, the structure is orthorhombic, but shows substantial disorder. Here, this disorder is explored by analyzing the diffuse scattering from single crystals of HMTA via Monte Carlo simulation. The disorder is in part occupational, with two orientations of azelaic acid occurring, and in part thermally induced, which is to say dynamic. The occupational disorder can be thought of as a combination of limited-range in-plane (bc plane) negative correlations combined with effectively zero correlation between planes (along a), ra...

On the crystal chemistry of olivine-type germanate compounds, Ca1 + xM1 − xGeO4 (M2+ = Ca, Mg, Co, Mn)

Acta Crystallographica Section B — Sun, 01 Jun 2008 04:00:00 +0100

Germanate compounds, CaMGeO4 with M2+ = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298 K and adopt the olivine structure. The M2 site is exclusively occupied by Ca2+, while on M1 both Ca2+ and M2+ cations are found. The amount of Ca2+ on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1 atoms per formula unit) and the largest in the Mn compound (0.20 atoms per formula unit), while in Ca2GeO4, also with olivine structure, both sites are completely filled with Ca2+. When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b ha...

Hydrogen bond strength and bond geometry in cyclic dimers of crystalline carboxylic acids