Acta Crystallographica Section B via MedWorm.com

Structure of a new high-pressure–high-temperature modification of antimony(III) oxide, γ-Sb2O3, from high-resolution synchrotron powder diffraction data

Acta Crystallographica Section B — Sat, 21 Jan 2012 17:46:19 +0100

A quenchable new high-pressure–high-temperature modification of antimony(III) oxide, γ-Sb2O3, has been obtained at hydrostatic pressures of 9–11 GPa and temperatures of 573–773 K. Its crystal structure has been determined from high-resolution synchrotron powder diffraction data. γ-Sb2O3 consists of three-dimensionally cross-linked infinite chains of SbO3E units (E = lone pair) with the chains forming tetragonal rod-packing. The underlying topology of γ-Sb2O3 (3,3T8) is found very rarely in inorganic structures; it is realised only for the polyanion [Si4O4N6]10− that occurs in the Ce4(Si4O4N6)O structure type. The structural relation to the two previously known polymorphs of Sb2O3 at ambient pressure, valentinite and senarmontite is discussed. (Source: Acta Crystallographica ...

Structural Mineralogy and Inorganic Crystal Chemistry. By Sergey V. Krivovichev. St Petersburg University Press, 2009. Pp. 398. (Hardbound) Price unknown. ISBN 978-5-288-05004-6.

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

(Source: Acta Crystallographica Section B)

Notes for authors 2012

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

(Source: Acta Crystallographica Section B)

Revision of the Mg(ClO4)2·4H2O crystal structure

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

A revision is presented of the restrained Rietveld analysis of the crystal structure of magnesium perchlorate tetrahydrate, Mg(ClO4)2·4H2O, recently published by Robertson & Bish [(2010), Acta Cryst. B66, 579–584]. The actual symmetry of the material is shown to be C2/m. The corrected structure model is refined by the derivative difference method [Solovyov (2004). J. Appl. Cryst. 37, 743–749], anisotropically for all non-H atoms and isotropically for two independent H atoms without restraints. (Source: Acta Crystallographica Section B)

Study of the single-crystal X-ray diffuse scattering in paracetamol polymorphs

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

Single-crystal diffuse X-ray scattering from paracetamol polymorphs is successfully calculated with Monte Carlo (MC) models that are used to simulate the crystals. In order to obtain the correct model appropriate force constants are required that describe the interatomic potentials used in the MC algorithm. Coefficients for an empirical `Buckingham'-type formula are used to determine these force constants. These coefficients are subsequently refined using the least-squares method and are found to converge on similar values for both polymorphic forms. An investigation of the correlation space generated from each model provides what would be expected given that strong displacive correlations exist between the molecules comprising the densely hydrogen-bonded layers. More disordered motions be...

Structural phase transition to disorder low-temperature phase in [Fe(ptz)6](BF4)2 spin-crossover compounds

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

In the spin-crossover compound [Fe(ptz)6](BF4)2 (where ptz=1-n-propyltetrazole) six different phases are observed. When a single crystal is slowly cooled from high temperatures to those below 125 K, the reflections broaden into diffuse maxima and split into two maxima along the c* direction [Kusz, Gütlich & Spiering (2004). Top. Curr. Chem. 234, 129–153]. As both maxima are broad along the c* direction, the short-range order exists only along the c direction and in the ab plane the structure remains long-range ordered. In this disordered phase additional satellite reflections appear. Upon heating above 135 K, the diffuse maxima return to their previous shape and this process is completely reversible. Rapidly cooled samples, on the other hand, do not show such splitting and the symme...

Polymorphism of Li2Zn3

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

Crystal structures of low- and high-temperature modifications of the binary phase Li2Zn3 were determined by single-crystal X-ray diffraction techniques. The low-temperature modification is a disordered variant of Li5Sn2, space group R\bar 3m (No. 166). The high-temperature modification crystallizes as an anti-type to Li5Ga4, space group P\bar 3m1 (No. 164). Two polymorphs can be described as derivative structures to binary Li5Ga4, Li5Sn2, Li13Sn5, Li8Pb3, CeCd2 and CdI2 phases which belong to class 2 with the parent W-type in Krypyakevich's classification. All atoms in both polymorphs are coordinated by rhombic dodecahedra (coordination number CN = 14) like atoms in related structures. The Li2Zn2.76 (for the low-temperature phase) and Li2Zn2.82 (for the high-temperature phase) compositions...

Structure maps for AI4AII6(BO4)6X2 apatite compounds via data mining

Acta Crystallographica Section B — Wed, 18 Jan 2012 05:00:00 +0100

This paper describes a method to identify key crystallographic parameters that can serve as strong classifiers of crystal chemistries and hence define new structure maps. The selection of this pair of key parameters from a large set of potential classifiers is accomplished through a linear data-dimensionality reduction method. A multivariate data set of known AI4AII6(BO4)6X2 apatites is used as the basis for the study where each AI4AII6(BO4)6X2 compound is represented as a 29-dimensional vector, where the vector components are discrete scalar descriptors of electronic and crystal structure attributes. A new structure map, defined using the two distortion angles αAII (rotation angle of AII—AII—AII triangular units) and ψAIz = 0AI—O1 (angle the AI—O1 bond makes with the c axis when...

Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O. Corrigendum

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

The correspondence author in the paper by Schmidt et al. [(2011), Acta Cryst. B67, 467–475] is corrected. (Source: Acta Crystallographica Section B)

Lattice constants and thermal expansion of H2O and D2O Ice Ih between 10 and 265 K. Addendum

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

In a previous paper we reported the lattice constants and thermal expansion of normal and deuterated ice Ih [Röttger et al. (1994). Acta Cryst. B50, 644–648]. Synchrotron X-ray powder diffraction data were used to obtain the lattice constants and unit-cell volumes of H2O and D2O ice Ih in the temperature range 15–265 K. A polynomial expression was given for the unit-cell volumes. It turns out that the coefficients quoted have an insufficient number of digits to faithfully reproduce the volume cell data. Here we provide a table with more significant digits. Moreover, we also provide the coefficients of a polynomial fit to the previously published a and c lattice constants of normal and deuterated ice Ih for the same temperature range. (Source: Acta Crystallographica Section B)

Hydrogen-bridged chelate ring-assisted π-stacking interactions

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

A salicylideneaniline (SA) derivative, (6Z)-6-({[2-(hydroxymethyl)phenyl]amino}methylidene)-3,5-dimethoxycyclohexa-2,4-dien-1-one monohydrate, has an increased aromaticity within its hydrogen-bridged chelate ring owing to its NH character. In the reported crystal structure, nonconventional π-stacking interactions, which are referred to as hybrid π-stacking interactions, are observed between a quasiaromatic chelate ring, formed as a result of the resonance-assisted intramolecular hydrogen bond and ordinary aromatic rings. Besides, π-stacking interactions are also seen between two hydrogen-bridged quasiaromatic chelate rings, which are referred to as pure π-stacking interactions. A CSD search has revealed that both kinds of interactions are frequently observed in molecular crystals of SA...

An order–disorder phase transition in the structure of C60·4benzene

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

A reversible, broad, order–disorder phase transition in the structure of C60·4benzene has been observed to take place with an inflection point at 134 K. In the low-temperature structure fullerene disorder is removed, although the average atomic displacement parameters differ for the two C60 molecules in the asymmetric unit. Some benzene disorder persists, even at 13 K. (Source: Acta Crystallographica Section B)

Face-to-face stacking of quinoid rings of alkali salts of bromanilic acid

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

A series of alkali salts of hydrogen bromanilic acid trihydrates (K+, Rb+ and Cs+, potassium, rubidium and caesium 2,5-dibromo-4-hydroxy-3,6-dioxocyclohexa-1,4-dien-1-olate trihydrate), bromanilic acid tetrahydrate (Na+, disodium 2,5-dibromo-3,6-dioxocyclohexa-1,4-diene-1,4-diolate tetrahydrate) and bromanilic acid dihydrates (K+, Rb+ and Cs+, dipotasium, dirubidium and dicaesium 2,5-dibromo-3,6-dioxocyclohexa-1,4-diene-1,4-diolate dihydrate) were prepared and studied by single-crystal X-ray diffraction. Their crystal packings are dominated by quinoid ring stacking. The monoanionic quinoid rings pack face-to-face without offset and with short centroid separations (3.25–3.30 Å), while the dianionic rings form offset stacks (1.4–1.8 Å) with a larger centroid separation (3.8–4.1�...

Structure of the new mineral sarrabusite, Pb5CuCl4(SeO3)4, solved by manual electron-diffraction tomography

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

The new mineral sarrabusite Pb5CuCl4(SeO3)4 has been discovered in the Sardinian mine of Baccu Locci, near Villaputzu. It occurs as small lemon–yellow spherical aggregates of tabular crystals ( (Source: Acta Crystallographica Section B)

High-precision absolute lattice parameter determination of SrTiO3, DyScO3 and NdGaO3 single crystals

Acta Crystallographica Section B — Fri, 06 Jan 2012 05:00:00 +0100

The lattice parameters of three perovskite-related oxides have been measured with high precision at room temperature. An accuracy of the order of 10−5 has been achieved by applying a sophisticated high-resolution X-ray diffraction technique which is based on the modified Bond method. The results on cubic SrTiO3 [a = 3.905268 (98) Å], orthorhombic DyScO3 [a = 5.442417 (54), b = 5.719357 (52) and c = 7.904326 (98) Å], and orthorhombic NdGaO3 [a = 5.428410 (54), b = 5.498407 (55) and c = 7.708878 (95) Å] are discussed in view of possible systematic errors as well as non-stoichiometry in the crystals. (Source: Acta Crystallographica Section B)

Dynamic orientational disorder in crystals of fluoroelpasolites, structural refinement of (NH4)3AlF6, (NH4)3TiOF5 and Rb2KTiOF5

Acta Crystallographica Section B — Thu, 17 Nov 2011 20:56:46 +0100

Room-temperature crystal structures of triammonium hexafluoroaluminate, (NH4)3AlF6 (I), and triammonium oxopentafluorotitanate, (NH4)3TiOF5 (II), were refined, and the crystal structures of dirubidium potassium oxopentaflourotitanate, Rb2KTiOF5, at 297 K (III) and 218 K (IV) were determined using single-crystal X-ray diffraction techniques. In ammonium fluoroelpasolites [(I) and (II)], the ligand (O, F) atoms are located in the mixed 24e + 96j position of the Fm3m unit cell. The 24e position is occupied by the ligand atoms predominantly in (III) and fully in (IV). `Ordered' N1 and Rb atoms are tetrahedrally displaced from the 8c position into the 32f site, and the H atoms of the disordered ammonium group N2 are statistically distributed in the 96k and 32f positions. The Ti atoms in (II...

Experimental and theoretical electron-density study of three isoindole derivatives: topological and Hirshfeld surface analysis of weak intermolecular interactions

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

A combined experimental and theoretical study of three isoindole derivatives was made on the basis of a topological analysis of their electron-density distributions. Experimental electron densities were determined from high-resolution X-ray diffraction data sets measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using DFT methods at the B3LYP\6-311++G(3df,3pd) level of approximation. Both experimental and theoretical models are in good agreement with each other. Since the analysed structures possess a variety of hydrogen-bonding interactions, weak intermolecular contacts of C—H...C(π), C,N(π)...C,N(π) and H...H types were subject to our special interest and are discussed in detail. They were characterized quantitatively and qualitatively by...

The whole range of hydrogen bonds in one crystal structure: neutron diffraction and charge-density studies of N,N-dimethylbiguanidinium bis(hydrogensquarate)

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

N,N-Dimethylbiguanidinium bis(hydrogensquarate) features an impressive range of hydrogen bonds within the same crystal structure: neighbouring anions aggregate to a dianionic pair through two strong O—H...O interactions; one of these can be classified among the shortest hydrogen bonds ever studied. Cations and anions in this organic salt further interact via conventional N—H...O and nonclassical C—H...O contacts to an extended structure. As all these interactions occur in the same sample, the title compound is particularly suitable to monitor even subtle trends in hydrogen bonds. Neutron and high-resolution X-ray diffraction experiments have enabled us to determine the electron density precisely and to address its properties with an emphasis on the nature of the X—H...O interaction...

Towards crystal structure prediction of complex organic compounds – a report on the fifth blind test

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

Following on from the success of the previous crystal structure prediction blind tests (CSP1999, CSP2001, CSP2004 and CSP2007), a fifth such collaborative project (CSP2010) was organized at the Cambridge Crystallographic Data Centre. A range of methodologies was used by the participating groups in order to evaluate the ability of the current computational methods to predict the crystal structures of the six organic molecules chosen as targets for this blind test. The first four targets, two rigid molecules, one semi-flexible molecule and a 1:1 salt, matched the criteria for the targets from CSP2007, while the last two targets belonged to two new challenging categories – a larger, much more flexible molecule and a hydrate with more than one polymorph. Each group submitted three prediction...

Structural study of piracetam polymorphs and cocrystals: crystallography redetermination and quantum mechanics calculations

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

Pharmaceutical compounds are mostly developed as solid dosage forms containing a single-crystal form. It means that the selection of a particular crystal state for a given molecule is an important step for further clinical outlooks. In this context, piracetam, a pharmaceutical molecule known since the sixties for its nootropic properties, is considered in the present work. This molecule is analyzed using several experimental and theoretical approaches. First, the conformational space of the molecule has been systematically explored by performing a quantum mechanics scan of the two most relevant dihedral angles of the lateral chain. The predicted stable conformations have been compared to all the reported experimental geometries retrieved from the Cambridge Structural Database (CSD) coverin...

[Ni(H2O)6](NO3)2·(15-crown-5)·2H2O: three phase transitions and an intermediate modulated phase stable over a range of ca 40 K

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

In the range 308–90 K single crystals of [Ni(H2O)6](NO3)2·(15-crown-5)·2H2O continue to diffract well as they undergo three phase transitions. Structures have been determined at 28 different temperatures; data were collected at six more but could not be refined satisfactorily. The transitions identified structurally are in good agreement with those found by thermal analysis. Phase I (above ca 292 K; I2/m, Z = 2, Z′ = ¼) is disordered because the 15-crown-5 molecule is located on a 2/m site. Phase II (ca 292–248 K; P21/m, Z = 2, Z′ = ½) is less disordered. Phase III (ca 248–208 K; B21, Z = 28, Z′ = 7 in a commensurate approximation) is modulated with some regions resembling phase II and the rest resembling the fully ordered phase IV (P21/c, Z = 4, Z′ = 1) that is s...

Stacking faults and superstructures in a layered brownmillerite

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

Single crystals of Ca4Fe2Mn0.5Ti0.5O9 have been synthesized using a flux method. The structural characterization using single-crystal X-ray diffraction revealed the space group Amma and unit-cell dimensions of a = 5.3510 (6), b = 26.669 (3), c = 5.4914 (6) Å. The structure is isotypic with Sr3NdFe3O9 [Barrier et al. (2005). Chem. Mater. 17, 6619–6623] and exhibits separated brownmillerite-type layers. One-dimensional diffuse scattering shows that the unit cell is doubled along c by alternating the intra-layer order of tetrahedral chains, causing stacking faults along the b direction. A computer simulation was performed, proving that the observed intensity variations along the diffuse scattering rods originates from two different local structures depending on the configuration of t...

Water channel structure of bassanite at high air humidity: crystal structure of CaSO4·0.625H2O

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

Structure analysis using single-crystal diffraction was carried out as a contribution to the dispute about the nature of the water channel structure of bassanite (CaSO4·0.5H2O). A recent result of Weiss & Bräu (2009) for the crystal structure of bassanite (monoclinic, space group C2) at ambient conditions of air humidity was confirmed. In the presence of high relative air humidity the crystal structure of bassanite transformed due to the incorporation of additional water of hydration. The crystal structure of CaSO4·0.625H2O was solved by single-crystal diffraction at 298 K and 75% relative air humidity. The experimental results provided an insight into both crystal structures. A model explaining the phase transition from CaSO4·0.625H2O to CaSO4·0.5H2O was derived. The monoclinic cel...

The not so simple cubic structure of PbZr1 − xTixO3 (PZT): complex local structural effects in perovskites

Acta Crystallographica Section B — Thu, 17 Nov 2011 05:00:00 +0100

High-resolution neutron diffraction on the important piezoelectric lead zirconate titanate (PZT) has found that oxygen disorder exists well into the cubic phase. This unexpected result shows that within this phase there persists a remnant of the tilted oxygen octahedra present within the room-temperature ferroelectric phase. The result is that the cubic phase, far from having a simple crystal structure, exhibits a more complex local structure than had hitherto been thought. (Source: Acta Crystallographica Section B)

Crystal packing and melting temperatures of small oxalate esters: the role of C—H...O hydrogen bonding

Acta Crystallographica Section B — Thu, 13 Oct 2011 07:43:42 +0100

The simple dialkyl oxalates are generally liquids at room temperature except for dimethyl and di-tert-butyl oxalate which melt at 327 and 343 K. The crystal structures of diethyl, di-iso-propyl, di-n-butyl, di-tert-butyl and methyl ethyl oxalates were determined. The liquid esters were crystallized using the cryocrystallization technique. A comparison of the intermolecular interactions and packing features in these crystal structures was carried out. The crystal structure of dimethyl oxalate was redetermined at various temperatures. The other compounds were also studied at several temperatures in order to assess the attractive nature of the hydrogen bonds therein. A number of moderate to well defined C—H...O interactions account for the higher melting points of the two solid esters. Ad...

Diffuse scattering resulting from macromolecular frustration

Acta Crystallographica Section B — Thu, 13 Oct 2011 04:00:00 +0100

Distinctive diffuse scattering in the form of diffuse rings around Bragg positions has been observed in the diffraction patterns of a crystal of the N-terminal fragment of the Gag protein from Feline Foamy Virus. It is shown that these are caused by geometric frustration as molecules try to pack on the triangular b–c mesh of the space group P6122. In order to explain the strong diffuse scattering it is necessary for the crystal to contain occupational disorder such that each unit cell contains one or other of two different molecular arrangements, A and B. The frustration arises because the nearest-neighbour packing prefers neighbouring cells to be AB or BA, which cannot be achieved on all three sides of a triangle simultaneously. To explain the observation that reciprocal sections hk5n, ...

Determining the structure of a benzene7.2-silicalite-1 zeolite using a single-crystal X-ray method

Acta Crystallographica Section B — Thu, 13 Oct 2011 04:00:00 +0100

A simple method for preparing orthorhombic single crystals of benzene-silicalite-1 was developed. A silicalite-1 crystal was pressed with a weight of 2 g along the +c and −c crystallographic axes while the temperature was increased to 473 K. The temperature was then slowly reduced to 313 K, and these heating and cooling steps were repeated three times. After the orthorhombic single crystals adsorbed benzene, the crystal structure of the resulting benzene-silicalite-1 was determined. There were two kinds of benzene molecules in the asymmetric unit. One was located at the intersection of the straight channels and the sinusoidal channels with the benzene ring parallel to the ac plane. The other benzene was located in the middle of the straight channel. (Source: Acta Crystallographica ...

Molecular origins of commercial laser dye functionality in azacoumarins and 2-quinolones: LD 425, LD 489 and LD 473

Acta Crystallographica Section B — Thu, 13 Oct 2011 04:00:00 +0100

The molecular structures of three compounds, LD 425 (C13H14N2O3) (1), LD 489 (C15H15F3N2O2) (2) and LD 473 (C17H19F3N2O) (3), are determined by single-crystal X-ray diffraction (XRD) at 180 K. Azacoumarins (1) and (2) possess para-quinoidal bond-length patterns in their benzene rings due to intramolecular charge transfer (ICT) from these rings to the adjoining rings. In contrast, substitution of O with N within the coumarin heterocycle, to form a 2-quinolone, results in the suppression of this ICT effect. Instead, charge transfer within the heterocycle is shown to become more pronounced. Resonance theory is employed to discuss these bond pattern differences and characteristic spectral blue shifts in relation to their coumarin analogues. The application of this theory offers an intuitive ...

Structure of LaTi2Al9O19 and reanalysis of the crystal structure of La3Ti5Al15O37

Acta Crystallographica Section B — Thu, 13 Oct 2011 04:00:00 +0100

The non-perovskite compound LaTi2Al9O19 was synthesized and structurally characterized by conventional X-ray powder diffraction and shown to be isostructural with SrTi3Al8O19, as confirmed by bond-valence sum calculations. The dielectric properties of LaTi2Al9O19 at 1 MHz were measured. The crystal structure of La3Ti5Al15O37, which is referred to as the most complex structure solved ab initio from X-ray powder diffraction (XRPD) to date, is shown to be incorrect. (Source: Acta Crystallographica Section B)

Crystallization of the chalcogenide compound Sb8Te3

Acta Crystallographica Section B — Mon, 19 Sep 2011 20:38:10 +0100

The crystallization of a sputtered Sb8Te3 film was examined in an X-ray powder diffraction experiment. An as-sputtered, amorphous Sb8Te3 film crystallized during heating into a structure of Sb–Te homologous series modulated along the stacking direction. During heating the lattice parameters and the modulation period γ were found to change significantly and continuously; this observation suggests a continuous change in the stacking sequence. A superspace analysis revealed that with heating the modulation period γ increased to a value that seemed to be determined by the atomic composition. Once γ reached this value it remained unchanged with cooling. A three-dimensional projection of the converged four-dimensional superspace structure corresponded to the homologous Sb8Te3 structure. (So...

Intermolecular interactions and unexpected isostructurality in the crystal structures of the dichlorobenzaldehyde isomers

Acta Crystallographica Section B — Fri, 16 Sep 2011 04:00:00 +0100

The crystal structures of the six dichlorobenzaldehyde isomers, four of them newly determined, are analyzed in terms of the geometry and energies of their intermolecular interactions, quantified using the semi-classical density sums (SCDS-PIXEL) method. A consistent feature in all six structures is molecular stacks propagating along a short crystallographic axis of ca 3.8 Å. The stacks have a closely comparable geometry in each isomer, but the interaction energies between stacked molecules are variable on account of the differing relative positions of the Cl substituents. In the majority of the isomers the stacking interactions are the most stabilizing in the structure. Exceptions are the 2,4- and 3,5-isomers, where more stabilizing interactions are made between stacks. In general, the ...

On the transferability of QTAIMC descriptors derived from X-ray diffraction data and DFT calculations: substituted hydropyrimidine derivatives

Acta Crystallographica Section B — Fri, 16 Sep 2011 04:00:00 +0100

The combined study of electron-density features in three substituted hydropyrimidines of the Biginelli compound family has been fulfilled. Results of the low-temperature X-ray diffraction measurements and density functional theory (DFT) B3LYP/6-311++G** calculations of these compounds are described. The experimentally derived atomic and bonding characteristics determined within the quantum-topological theory of atoms in molecules and crystals (QTAIMC) were demonstrated to be fully transferable within chemically similar structures such as the Biginelli compounds. However, for certain covalent bonds they differ significantly from the theoretical results because of insufficient flexibility of the atom-centered multipole electron density model. It was concluded that currently analysis of the t...

Hidden superlattice in Tl2(SC6H4S) and Tl2(SeC6H4Se) solved from powder X-ray diffraction

Acta Crystallographica Section B — Fri, 16 Sep 2011 04:00:00 +0100

The crystal structures of the isostructural title compounds poly[(μ-benzene-1,4-dithiolato)dithallium], Tl2(SC6H4S), and poly[(μ-benzene-1,4-diselenolato)dithallium], Tl2(SeC6H4Se), were solved by simulated annealing from high-resolution synchrotron X-ray powder diffraction. Rietveld refinements of an initial structure with one formula unit per triclinic cell gave satisfactory agreement with the data, but led to a structure with impossibly close non-bonded contacts. A disordered model was proposed to alleviate this problem, but an alternative supercell structure leads to slightly improved agreement with the data. The isostructural superlattice structures were confirmed for both compounds through additional data collection, with substantially better counting statistics, which revealed the...

Charge-density distribution in hydrogen methylphosphonates of calcium and lithium

Acta Crystallographica Section B — Sun, 04 Sep 2011 01:27:58 +0100

Two new crystal structures, calcium bis(hydrogen methylphosphonate), Ca(CH3PO3H)2, and lithium hydrogen methylphosphonate, Li(CH3PO3H), have been obtained, and the experimental and theoretical charge densities, as well as their topological properties, are reported. Both compounds display layered structures. Each hydrogen methylphosphonate anion coordinates three metal cations in the calcium compound and four in the lithium one. Weak polarization of oxygen lone pairs is observed, with lithium showing somewhat stronger polarization strength than calcium. The reported topological properties from the density functional theory (DFT) and X-ray approach are consistent with each other. In both structures the P—O bonds have a significant share of ionic character. The hyperconjugation effects with...

Effects of the [OC6F5] moiety upon structural geometry: crystal structures of half-sandwich tantalum(V) aryloxide complexes from reaction of Cp*Ta(NtBu)(CH2R)2 with pentafluorophenol

Acta Crystallographica Section B — Tue, 30 Aug 2011 23:00:00 +0100

The synthesis, chemical and structural characterization of a series of pentamethylcyclopentadienyl (Cp*) tantalum imido complexes and aryloxide derivatives are presented. Specifically, the imido complexes Cp*Ta(NtBu)(CH2R)2, where R = Ph [dibenzyl(tert-butylamido) (η5-pentamethylcyclopentadienyl)tantalum(IV) (1)], Me2Ph [tert-butylamido)bis(2-methyl-2-phenylpropyl) (η5-pentamethylcyclopentadienyl)tantalum(IV) (2)], CMe3 [(tert-butylamido)bis(2,2-dimethylpropyl) (η5-pentamethylcyclopentadienyl)tantalum(IV) (3)], are reported. The crystal structure of (3) reveals α-agostic interactions with the Ta atom. The resulting increase in the tantalum core coordination improves electronic stability. As such it does not react with pentafluorophenol, in contrast to the other two reported imido compl...

Neutron powder diffraction refinement of PbZr1 − xTixO3

Acta Crystallographica Section B — Tue, 30 Aug 2011 23:00:00 +0100

Rietveld refinement of different structural models using neutron diffraction data for a series of powders of lead zirconate titanate, PbZr1 − xTixO3 (PZT), is described. It is found that at all the compositions and temperatures studied the best results include contributions from more than one phase in order to fit the data. Consequently a new phase diagram is proposed. (Source: Acta Crystallographica Section B)

Structural building principles of complex face-centered cubic intermetallics

Acta Crystallographica Section B — Thu, 21 Jul 2011 23:00:55 +0100

Fundamental structural building principles are discussed for all 56 known intermetallic phases with approximately 400 or more atoms per unit cell and space-group symmetry F\bar{4}3m, Fd\bar{3}m, Fd\bar{3}, Fm\bar{3}m or Fm\bar{3}c. Despite fundamental differences in chemical composition, bonding and electronic band structure, their complex crystal structures show striking similarities indicating common building principles. We demonstrate that the structure-determining elements are flat and puckered atomic {110} layers stacked with periodicities 2p. The atoms on this set of layers, which intersect each other, form pentagon face-sharing endohedral fullerene-like clusters arranged in a face-centered cubic packing (f.c.c.). Due to their topological layer structure, all these crystal structures...

R-free factor and experimental charge-density analysis of 1-(2′-aminophenyl)-2-methyl-4-nitroimidazole: a crystal structure with Z′ = 2

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

The experimental charge-density distribution was determined for 1-(2′-aminophenyl)-2-methyl-4-nitro-1H-imidazole crystals. An anharmonic model was applied to the N atoms of both amino groups and to one nitro group in order to account for high residual peaks after harmonic multipole refinement and to obtain a better charge-density model. Free R-factor calculations [Brünger (1992). Nature, 355, 472–475] with restrained models implemented in MoPro were used to determine the degree of similarity of the two symmetry-independent molecules in the unit cell. The results are compared with 1-phenyl-4-nitroimidazole in order to analyze the influence of the amine and methyl functional groups. The asymmetric unit contains two symmetry-independent molecules giving rise to a dimer connected via stro...

Determination of the crystal structure of nifedipine form C by synchrotron powder diffraction

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

The crystal structure of the metastable form C polymorph of nifedipine [C17H18N2O6, 3,5-dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate] was determined by means of direct-space techniques applied to high-resolution synchrotron powder diffraction data. The polymorph crystallizes in the space group P\bar 1 and exhibits a molecular packing significantly different from that of the stable modification, with molecules aligned in an orthogonal configuration inside the unit cell. The molecular conformation, on the other hand, remains substantially unmodified between the two polymorphs. Additionally, in situ thermal characterization of nifedipine crystallization behaviour was performed, confirming the nucleation of another metastable polymorph (form B) prior to the com...

Deducing chemical structure from crystallographically determined atomic coordinates

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

An improved algorithm has been developed for assigning chemical structures to incoming entries to the Cambridge Structural Database, using only the information available in the deposited CIF. Steps in the algorithm include detection of bonds, selection of polymer unit, resolution of disorder, and assignment of bond types and formal charges. The chief difficulty is posed by the large number of metallo-organic crystal structures that must be processed, given our aspiration that assigned chemical structures should accurately reflect properties such as the oxidation states of metals and redox-active ligands, metal coordination numbers and hapticities, and the aromaticity or otherwise of metal ligands. Other complications arise from disorder, especially when it is symmetry imposed or modelled w...

d-Orbital orientation in a dimer cobalt complex: link to magnetic properties?

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

The experimental charge-density distribution of the dinuclear cobalt(II) complex [Co2(sym-hmp)2](BPh4)2·2H2O·2C3H6O was determined at 100 K. When decreasing the temperature, the magnetic susceptibility of this complex deviates from Curie law because of anti-ferromagnetic exchange interactions, but the susceptibility increases sharply at low temperature (< 20 K). To explain this magnetic behaviour a tilt angle between the Co-atom environments was previously theoretically predicted. The structure and experimental charge density determined in this study show a tilt angle. The calculated value, based on the 100 K experimental d-orbital model, is in agreement with the theoretical one. (Source: Acta Crystallographica Section B)

Bis(μ2-2-(dimethylamino)ethoxo-N,O,O)-di(phenolato-O)ditin(II): a high-resolution single-crystal X-ray diffraction and quantum chemical study

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

Experimental and calculated electron density functions ρ(r) of the title compound in the crystal were obtained. These were compared with ρ(r) for an isolated dimer. Application of the `Atoms in Molecules' theory allowed the visualization of the electron lone pair (Lp) of tin(II) and the calculation of some bond energies and all atomic charges. The stereochemical activity of the Lp was demonstrated and its volume was estimated to be approximately 10 Å3. The energies of N→Sn and Sn—O bonds were found to be 13–18 and 25–52 kJ mol−1. According to the experimental, AM05-PW and PBE0/6-311G(d,p) calculation data, µ2-2-(dimethylamino)ethoxoate accepts 0.68, 0.45 and 0.40 e from the Sn atom. Using the example of µ2-2-(dimethylamino)ethoxoates and complexes of bis(n-butyl)tin(...

Octahedral tilts, symmetry-adapted displacive modes and polyhedral volume ratios in perovskite structures

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

The structures of tilted perovskites in each of the 15 tilt systems have been decomposed into the amplitudes of symmetry-adapted modes in order to provide a clear and unambiguous definition of the tilt angles. A full expression in terms of the mode amplitudes for the ratio of the volumes of the two polyhedra within the perovskite structure for each of the 15 tilt systems is derived, along with more general expressions in terms of either mode amplitudes or tilt angles that can be used to estimate this ratio when the distortions of the octahedra are small. (Source: Acta Crystallographica Section B)

CaSeO4·0.625H2O – water channel occupation in a bassanite related structure

Acta Crystallographica Section B — Sun, 17 Jul 2011 23:00:00 +0100

Calcium selenate subhydrate, CaSeO4·0.625H2O, was prepared by hydrothermal conversion of CaSeO4·2H2O at 463 K. From the single crystals obtained in the shape of hexagonal needles, 50–300 µm in length, the crystal structure could be solved in a trigonal unit cell with space group P3221. The cell was confirmed and refined by high-resolution synchrotron powder diffraction. The subhydrate was characterized by thermal analysis and Raman spectroscopy. (Source: Acta Crystallographica Section B)

Charge-density analysis of 1-nitroindoline: refinement quality using free R factors and restraints. Corrigendum

Acta Crystallographica Section B — Wed, 08 Jun 2011 23:00:00 +0100

The De (dissociation energy) values in Table 6 of the article by Zarychta et al. [(2011). Acta Cryst. B67, 250–262] are corrected. (Source: Acta Crystallographica Section B)

Conformational preference of chlorothioformate species: molecular structure of ethyl chlorothioformate, ClC(O)SCH2CH3, in the solid phase and NBO analysis

Acta Crystallographica Section B — Wed, 08 Jun 2011 23:00:00 +0100

The molecular structure of ethyl chlorothioformate, ClC(O)SCH2CH3, has been investigated in the solid phase by X-ray diffraction analysis at low temperature using a miniature zone-melting procedure and IR laser radiation. The crystalline solid consists exclusively of molecules with the synperiplanar conformation with respect to the C=O double bond and the S—C single bond, and gauche orientation of the ethyl group (syn-gauche). These results coincide with previous studies devoted to gas-phase conformational properties. The conformational preference for the ClC(O)SY (Y = Cl, CF3, CH3 and CH2CH3) series of molecules was rationalized using the natural bond orbital (NBO) scheme. It was found that both resonance (mesomeric) and anomeric (hyperconjugation) intermolecular charge-transfer interac...

Frank H. Herbstein (1926–2011)

Acta Crystallographica Section B — Fri, 13 May 2011 23:00:00 +0100

(Source: Acta Crystallographica Section B)

Charge-density analysis of 1-nitroindoline: refinement quality using free R factors and restraints

Acta Crystallographica Section B — Fri, 13 May 2011 23:00:00 +0100

Nitramines and related N-nitro compounds have attracted significant attention owing to their use in rocket fuel and as explosives. The charge density of 1-nitroindoline was determined experimentally and from theoretical calculations. Electron-density refinements were performed using the multipolar atom formalism. In order to design the ideal restraint strategy for the charge-density parameters, R-free analyses were performed involving a series of comprehensive refinements. Different weights were applied to the charge-density restraints, namely the similarity between chemically equivalent atoms and local symmetry. Additionally, isotropic thermal motion and an anisotropic model calculated by rigid-body analysis were tested on H atoms. The restraint weights which resulted in the lowest values...

Intermolecular interactions and charge transfer in the 2:1 tetrathiafulvalene bromanil complex, (TTF)2-BA

Acta Crystallographica Section B — Fri, 13 May 2011 23:00:00 +0100

The crystal structure of the 2:1 charge-transfer complex of tetrathiafulvalene [2,2′-bis(1,3-dithiolylidene)] and bromanil (tetrabromo-1,4-benzoquinone) [(TTF)2-BA, (C6H4S4)2–C6Br4O2] has been determined by X-ray diffraction at room temperature, 100 and 25 K. No structural phase transition occurs in the temperature range studied. The crystal is made of TTF–BA–TTF sandwich trimers. A charge-transfer estimation between donor and acceptor (0.2 e) molecules is proposed in comparison to the molecular geometries of TTF–BA and TTF and BA isolated molecules. Displacement parameters of the molecules have been modeled with the TLS formalism. (Source: Acta Crystallographica Section B)

Structure analysis of titanate nanorods by automated electron diffraction tomography

Acta Crystallographica Section B — Fri, 13 May 2011 23:00:00 +0100

A hitherto unknown phase of sodium titanate, NaTi3O6(OH)·2H2O, was identified as the intermediate species in the synthesis of TiO2 nanorods. This new phase, prepared as nanorods, was investigated by electron diffraction, X-ray powder diffraction, thermogravimetric analysis and high-resolution transmission electron microscopy. The structure was determined ab initio using electron diffraction data collected by the recently developed automated diffraction tomography technique. NaTi3O6(OH)·2H2O crystallizes in the monoclinic space group C2/m. Corrugated layers of corner- and edge-sharing distorted TiO6 octahedra are intercalated with Na+ and water of crystallization. The nanorods are typically affected by pervasive defects, such as mutual layer shifts, that produce diffraction streaks along ...

Modulated anharmonic ADPs are intrinsic to aperiodic crystals: a case study on incommensurate Rb2ZnCl4

Acta Crystallographica Section B — Fri, 13 May 2011 23:00:00 +0100

A combination of structure refinements, analysis of the superspace MEM density and interpretation of difference-Fourier maps has been used to characterize the incommensurate modulation of rubidium tetrachlorozincate, Rb2ZnCl4, at a temperature of T = 196 K, close to the lock-in transition at Tlock-in = 192 K. The modulation is found to consist of a combination of displacement modulation functions, modulated atomic displacement parameters (ADPs) and modulated third-order anharmonic ADPs. Up to fifth-order Fourier coefficients could be refined against diffraction data containing up to fifth-order satellite reflections. The center-of-charge of the atomic basins of the MEM density and the displacive modulation functions of the structure model provide equivalent descriptions of the displaci...

The effect of pressure on the crystal structure of bianthrone

Acta Crystallographica Section B — Wed, 13 Apr 2011 03:59:16 +0100

Bianthrone [10(10-oxoanthracen-9-ylidene)anthracen-9-one] consists of two tricyclic anthraceneone units connected by a carbon–carbon double bond. Crystals of the form obtained under ambient conditions are yellow and contain folded centrosymmetric conformers in which the central ring of the anthraceneone unit is non-planar. When hydrostatic pressure is applied the crystals assume a red colouration which gradually deepens as pressures increases. The colour change is limited in extent to the surface of the crystals, the bulk remaining yellow. Comparison of high-pressure, single-crystal UV–vis spectra and powder diffraction data demonstrate that the colour change is associated with the formation of a polymorph containing a conformer in which the tricyclic fragments are planar and the molec...

A simplified empirical model for approximation of the `bond valence–bond length' correlation for H—O bonds

Acta Crystallographica Section B — Tue, 12 Apr 2011 23:00:00 +0100

The `bond valence (s)–bond length (r)' correlation reported earlier for H—O bonds [Brown (2009). Chem. Rev. 109, 6858–6919] has been closely approximated using the function s = (α − βr)1/γ, where α = 1.185 (10), β = 0.321 (8) and γ = 0.36. (Source: Acta Crystallographica Section B)

Determination of the local structure of a cage with an oxygen ion in Ca12Al14O33

Acta Crystallographica Section B — Tue, 12 Apr 2011 23:00:00 +0100

The crystal structure of mayenite (12CaO·7Al2O3) has been investigated by single-crystal synchrotron diffraction with high resolution and accuracy, using a four-circle diffractometer equipped with an avalanche photodiode detector (APD) detector installed at PF14A in Tsukuba, Japan. Analysis revealed random displacements of ions by the electrostatic force of the O2− ion (O3) clathrated in two out of 12 cages. O3 ions are located at general positions close to the \bar 4 site at the centre of each cage. The difference-density map revealed two large peaks corresponding to displaced Ca ions. The positive ions close to O3 are displaced and one-to-one correspondence was found between one of the four equivalent O3 ions and the displaced ions. When an O3 ion is present in the cage the Al ion at ...

Solid-state and gas-phase structures of two conformers of N-(4-methylphenyl)-N′,N′′-bis(morpholinyl) phosphoric triamide; insights from X-ray crystallography and DFT calculations

Acta Crystallographica Section B — Tue, 12 Apr 2011 23:00:00 +0100

A phosphoric triamide with the formula (4-CH3—C6H4NH)P(O)(NC4H8O)2 has been synthesized and characterized. X-ray crystallography at 120 K reveals that the title compound is composed of two symmetrically independent molecules in the solid state. Density functional theory (DFT) calculations reveal that two conformers A and B are very close to each other from an energy point of view. Thus there is equal chance that the presence of two conformers in the lattice may lead to hydrogen-bonded chains with an ABABAB arrangement. Hydrogen bonds of the type OP...H—N (OP being the phosphoryl O atom) are established between the two conformers with binding energies of −18.8 and −20.3 kJ mol−1 (at B3LYP/6–31+G*). The electronic delocalization LP(OP) → σ*(N—H), LP(OP) being the lone ...

Seven hexamethylenetetramine (HMTA) complexes with mono- and dicarboxylic acids: analysis of packing modes of HMTA complexes in the literature

Acta Crystallographica Section B — Thu, 10 Mar 2011 00:00:00 +0100

The crystal structures of seven hexamethylenetetramine (HMTA) complexes, or co-crystals, with carboxylic acid donor molecules are reported to explain the link between the molecular structure of HMTA and the crystal structure of the co-crystals, i.e. the dimension and shape of their hydrogen-bonded assembly. A comprehensive and detailed literature survey of HMTA complexes (38), be they neutral co-crystals or salts, with molecules containing carboxylic acid and phenol functional groups reveals that in general two N acceptors are used for strong O—H...N interactions. Owing to the relative arrangement of two of the four N atoms, the most common type of assembly features one-dimensional zigzag chains. Weak interactions of the C—H...N type are formed by N atoms not involved in strong interac...

Solid-state transformations in the β-form of chlorpropamide on cooling to 100 K

Acta Crystallographica Section B — Thu, 10 Mar 2011 00:00:00 +0100

A single-crystal X-ray diffraction study of the effect of cooling down to 100 K on the β-form of chlorpropamide, 4-chloro-N-(propylaminocarbonyl)benzenesulfonamide, has revealed reversible phase transitions at ∼ 257 K and between 150 and 125 K: β (Pbcn, Z′ = 1)  βII (P2/c, Z′ = 2)  βIII (P2/n, a′ = 2a, Z′ = 4); the sequence corresponds to cooling. Despite changes in the space group and number of symmetry-independent molecules, the volume per molecule changes continuously in the temperature range 100–300 K. The phase transition at ∼ 257 K is accompanied by non-merohedral twinning, which is preserved on further cooling and through the second phase transition, but the original single crystal does not crack. DSC (differential scanning calorimetry) and X-r...

Charge-density analysis in polymorphs of urea–barbituric acid co-crystals

Acta Crystallographica Section B — Thu, 10 Mar 2011 00:00:00 +0100

High-resolution single-crystal X-ray diffraction measurements at 100 K were performed for the two polymorphs of urea–barbituric acid co-crystals: (I) P21/c and (II) Cc. Experimental and theoretical charge density and its properties were analysed for (I) and (II) in order to confirm the previous observation that in the polymorphs studied the barbituric acid molecules adopt different mesomeric forms, leading to different hydrogen-bond systems. Koch and Popelier criteria were applied to distinguish between hydrogen bonds and van der Waals interactions in the structures presented. (Source: Acta Crystallographica Section B)

Detailed study of the phase transition in [Ni(H2O)6](NO3)2·(15-crown-5)·H2O and analysis in terms of mean-field theory

Acta Crystallographica Section B — Thu, 10 Mar 2011 00:00:00 +0100

The transition between 190 and 200 K in [Ni(H2O)6](NO3)2·(15-crown-5)·H2O has been followed by determining the structure at 22 temperatures in the range 90–273 K. The structural change is a zone-boundary transition with a critical point at (½, 0, ½) in the Brillouin zone of the high-temperature phase; both phases have space-group symmetry P21 but the volume of the unit cell is halved when a crystal is heated through the transition. The only obvious disorder in the high-temperature phase is of the lattice water molecule, which occupies two sites; some disorder persists below the transition. The greatest changes in the structure below the transition are the rotations of one of the two 15-crown-5 molecules and of one of the two nitrate ions; above the transition the two molecules ar...

Disorder of (NH4)3H(SO4)2 in the high-temperature phase I

Acta Crystallographica Section B — Thu, 10 Mar 2011 00:00:00 +0100

The highly disordered crystal structure of triammonium hydrogen disulfate, (NH4)3H(SO4)2, in the high-temperature phase I was studied using single-crystal neutron diffraction. It is known that the O atom involved in hydrogen bonding between neighbouring SO4 tetrahedra is disordered and takes a split-atom position, building a two-dimensional hydrogen-bond network in the (001) plane. The H atoms in these SO4–H—SO4 hydrogen bonds are disordered and hence refined with a split-atom model. Moreover, from the much larger anisotropic mean-square displacements of ammonium protons the NH_4^+ groups were refined with a reasonable split-atom model, and their motional behaviour was also analysed by rigid-body treatment. Finally, careful consideration was given to show possible supplementary proton ...

Structural phase transitions in tetra(isopropylammonium) decachlorotricadmate(II), [(CH3)2CHNH3]4Cd3Cl10, crystal with a two-dimensional cadmium(II) halide network

Acta Crystallographica Section B — Sun, 20 Feb 2011 00:28:38 +0100

Single crystals of tetra(isopropylammonium) decachlorotricadmate(II) as a rare example of a two-dimensional cadmium(II) halide network of [Cd3Cl10]_n^{4 - } have been synthesized and characterized by means of calorimetry and X-ray diffraction. The crystals exhibit polymorphism in a relatively narrow temperature range (three phase transitions at 353, 294 and 259 K). Our main focus was to establish the mechanism of these successive transformations. The crystal structure was solved and refined in the space group Cmce at 375 K (Phase I), Pbca at 320 K (Phase II) and P212121 (Phase III) at 275 K in the same unit-cell metric. The structure is composed of face-sharing polyanionic [Cd3Cl10]4− units which are interconnected at the bridging Cl atom into four-membered rings forming a unique...

Predicted and experimental crystal structures of ethyl-tert-butyl ether

Acta Crystallographica Section B — Sat, 19 Feb 2011 00:00:00 +0100

Possible crystal structures of ethyl-tert-butyl ether (ETBE) were predicted by global lattice-energy minimizations using the force-field approach. 33 structures were found within an energy range of 2 kJ mol−1 above the global minimum. Low-temperature crystallization experiments were carried out at 80–160 K. The crystal structure was determined from X-ray powder data. ETBE crystallizes in C2/m, Z = 4, with molecules on mirror planes. The ETBE molecule adopts a trans conformation with a (CH3)3C—O—C—C torsion angle of 180°. The experimental structure corresponds with high accuracy to the predicted structure with energy rank 2, which has an energy of 0.54 kJ mol−1 above the global minimum and is the most dense low-energy structure. In some crystallization experiments a s...

High-pressure experimental study on Rb2S: antifluorite to Ni2In-type phase transitions

Acta Crystallographica Section B — Sat, 19 Feb 2011 00:00:00 +0100

The high-pressure behaviour of dirubidium sulfide, Rb2S, with antifluorite-type structure under room conditions (space group Fm\bar 3 m) has been studied up to 8 GPa at room temperature using angle-dispersive X-ray powder diffraction in a diamond–anvil cell (DAC). X-ray measurements have allowed us to completely characterize two phase transitions upon compression: (i) to an anticotunnite-type structure (Pnma) at some pressure between 1 bar and 0.7 GPa, and (ii) to a Ni2In-type structure (P63/mmc) at 2.6 GPa. A gradual transition from the Pnma to the P63/mmc structures seems to occur between 2.6 and 4.5 GPa. These results are in excellent agreement with previous theoretical predictions. Strong luminescence is observed above 2.6 GPa (band maximum at 703 nm) when the transitio...

Unifying cluster-based structure models of decagonal Al–Co–Ni, Al–Co–Cu and Al–Fe–Ni

Acta Crystallographica Section B — Sat, 15 Jan 2011 23:05:07 +0100

The geometrical building principles of Al-based decagonal quasicrystals and their approximants are discussed from a cluster-based approach. Our investigations cover 11 modifications with two- or four-layer periodicity in the systems Al–Co–Ni, Al–Co–Cu and Al–Fe–Ni. We identified a cluster that leads to a unifying view of all these phases. This unit cluster has ∼ 20 Å diameter, four-layer periodicity along its tenfold axis and rod symmetry group p \overline{10}2m. The models obtained are in agreement with all the electron-density maps and electron-microscopy images available. (Source: Acta Crystallographica Section B)

Failures of fractional crystallization: ordered co-crystals of isomers and near isomers

Acta Crystallographica Section B — Sat, 15 Jan 2011 00:00:00 +0100

A list of 270 structures of ordered co-crystals of isomers, near isomers and molecules that are almost the same has been compiled. Searches for structures containing isomers could be automated by the use of IUPAC International Chemical Identifier (InChI™) strings but searches for co-crystals of very similar molecules were more labor intensive. Compounds in which the heteromolecular A...B interactions are clearly better than the average of the homomolecular A...A and B...B interactions were excluded. The two largest structural classes found include co-crystals of configurational diastereomers and of quasienantiomers (or quasiracemates). These two groups overlap. There are 114 co-crystals of diastereomers and the same number of quasiracemates, with 71 structures being counted in both group...

Temperature-dependent analysis of thermal motion, disorder and structures of tris(ethylenediamine)zinc(II) sulfate and tris(ethylenediamine)copper(II) sulfate

Acta Crystallographica Section B — Sat, 15 Jan 2011 00:00:00 +0100

The crystal structures of the title compounds have been determined in the temperature range 140–290 K for the zinc complex, and 190–270 K for the copper complex. The two structures are isostructural in the trigonal space group P{\bar{3}1c} with the sulfate anion severely disordered on a site with 32 (D3) symmetry. This sulfate disorder leads to a disordered three-dimensional hydrogen-bond network, with the N—H atoms acting as donors and the sulfate O atoms as acceptors. The displacement parameters of the N and C atoms in both compounds contain disorder contributions in the out-of-ligand plane direction owing to ring puckering and/or disorder in hydrogen bonding. In the Zn compound the vibrational amplitudes in the bond directions are closely similar. Their differences show no sig...

Structures of two new high-pressure forms of AlPO4 by X-ray powder diffraction and NMR spectroscopy

Acta Crystallographica Section B — Sat, 15 Jan 2011 00:00:00 +0100

This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures. (Source: Acta Crystallographica Section B)

Modulated structure of nepheline

Acta Crystallographica Section B — Sat, 15 Jan 2011 00:00:00 +0100

The incommensurately modulated structure of a natural nepheline of composition K0.54Na3.24Ca0.03Al3.84Si4.16O16 has been determined in superspace. The compound crystallizes in the trigonal centered superspace group X3(00γ)0 with γ = 0.2048 (10), X = (0, 0, 0, 0), (1/3, 2/3, 0, 2/3), (2/3, 1/3, 0, 1/3), a = 17.2889 (8) and c = 8.3622 (10) Å. The structure is characterized by a framework of corner-connected (Al,Si)O4 tetrahedra. The additional cations are incorporated in two different types of channels of the framework. All atoms in the structure are displacively modulated with amplitudes below 0.1 Å. The modulation can be well described taking into account harmonics of first order only. Atomic positions in the smaller channels of the framework are fully occupied by Na+. Cation...

Structural analysis and multipole modelling of quercetin monohydrate – a quantitative and comparative study

Acta Crystallographica Section B — Mon, 20 Dec 2010 09:08:23 +0100

The multipolar atom model, constructed by transferring the charge-density parameters from an experimental or theoretical database, is considered to be an easy replacement of the widely used independent atom model. The present study on a new crystal structure of quercetin monohydrate [2-(3,4-dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one monohydrate], a plant flavonoid, determined by X-ray diffraction, demonstrates that the transferred multipolar atom model approach greatly improves several factors: the accuracy of atomic positions and the magnitudes of atomic displacement parameters, the residual electron densities and the crystallographic figures of merit. The charge-density features, topological analysis and electrostatic interaction energies obtained from the multipole models based ...

Notes for authors 2011

Acta Crystallographica Section B — Sat, 18 Dec 2010 00:00:00 +0100

(Source: Acta Crystallographica Section B)

Conformation and geometry of cyclopropane rings having π-acceptor substituents: a theoretical and database study

Acta Crystallographica Section B — Sat, 18 Dec 2010 00:00:00 +0100

The 3e′ orbitals of cyclopropane have the correct symmetry to interact with low-lying unoccupied orbitals of π-acceptor substituents and maximum overlap occurs when the two orbital systems are parallel, i.e. when the π-acceptor bisects the ring in projection down the substituent bond. Since the cyclopropyl group is a common component of active pharmaceutical and agrochemical ingredients, it is important that these strong conjugative interactions are well modelled by computational techniques, and clearly represented in experimental crystal structures. Here we show that torsion angle distributions derived from crystal structure data in the Cambridge Structural Database are in excellent correspondence with torsional energy profiles computed using density functional theory (DFT) for a rang...

Investigation of the structure and phase transitions of the polymeric inorganic–organic hybrids: [M(Im)4V2O6]∞; M = Mn, Co, Ni, Im = imidazole

Acta Crystallographica Section B — Sat, 18 Dec 2010 00:00:00 +0100

The polymeric isomorphous hybrid inorganic–organic vanadium oxide compounds [M(Im)4V2O6]∞, M = Mn, Co, Ni, Im = imidazole, were investigated at various temperatures between 100 and 295 K by single-crystal X-ray diffraction. The crystals all contain two-dimensional polymeric sheets packed perpendicular to c* and are 1:1 disordered in the space group P42/n (Z = 8) at 295 K. The disordered phase is reversibly transformed to an I41/a ordered phase (Z = 32) below 281 K for the Mn compound and below 175 K for the Co compound. Within a localized region of the I41/a phase eight imidazoles are in close proximity and seven of these are hydrogen bonded to framework O atoms. The hydrogen-bond connectivity of six of these ligands is unchanged by the phase transition that allows an inversion...

Electron density studies on hydrogen bonding in two chromone derivatives

Acta Crystallographica Section B — Wed, 17 Nov 2010 00:00:00 +0100

The experimental electron densities of two chromone derivatives have been determined from X-ray synchrotron diffraction data at low temperature (100 K). Topological analysis of the electron density has been used to analyze the formation of resonance-assisted hydrogen bonds (RAHBs). Geometrical and topological parameters confirm π-electron delocalization within the hydrogen-bonded ring. In addition, weak C—H...O interactions were identified in both structures. Hydrogen-bond energies allowed medium and weak hydrogen bonds to be distinguished. (Source: Acta Crystallographica Section B)

Structures of cyclic dipeptides: an X-ray and computational study of cis- and trans-cyclo(Pip-Phe), cyclo(Pro-Phe) and their N-methyl derivatives

Acta Crystallographica Section B — Wed, 17 Nov 2010 00:00:00 +0100

The crystal structures of eight cyclodipeptides are determined, incorporating pipecolic acid or proline and phenylalanine or N-methyl phenylalanine. This set of structures allows the evaluation of the effects on molecular conformation and crystal packing of imino acid ring-size, relative configuration of the two amino acids, and N-methylation. In the non-methylated compounds, hydrogen-bonding interactions form one-dimensional motifs that dominate the packing arrangement. Three compounds have more than one symmetry-independent molecule in the asymmetric unit (Z′ > 1), indicative of a broad and shallow molecular energy minimum. Density functional theory calculations reveal the interplay between inter- and intramolecular factors in the crystals. Only for the N-methylated compounds do simula...

Crystal chemistry of transition metal diarsenates M2As2O7 (M = Mn, Co, Ni, Zn): variants of the thortveitite structure

Acta Crystallographica Section B — Wed, 17 Nov 2010 00:00:00 +0100

The structures of the 3d divalent transition-metal diarsenates M2As2O7 (M = Mn, Co, Ni, Zn) can be considered as variants of the monoclinic (C2/m) thortveitite [Sc2Si2O7] structure type with a ≃ 6.7, b ≃ 8.5, c ≃ 4.7 Å, α ≃ 90, β ≃ 102, γ ≃ 90° and Z = 2. Co2As2O7 and Ni2As2O7 are dimorphic. Their high-temperature (β) polymorphs adopt the thortveitite aristotype structure in C2/m, whereas their low-temperature (α) polymorphs are hettotypes and crystallize with larger unit cells in the triclinic crystal system in space groups P\bar 1 and P1, respectively. Mn2As2O7 undergoes no phase transition and likewise adopts the thortveitite structure type in C2/m. Zn2As2O7 has an incommensurately modulated crystal structure [C2/m(α,0,γ)0s] with q = [0.3190 (1), 0, 0.3717 (1)...

The modulated structure of the calcium aluminate Ca6(AlO2)12·Bi2O3

Acta Crystallographica Section B — Wed, 17 Nov 2010 00:00:00 +0100

Bismuth calcium aluminate, Bi2Ca6Al12O27, has been prepared as a ceramic and a single crystal. Analysis of reciprocal space using both electron and X-ray diffraction show an R-centred hexagonal unit cell: a = b = 17.3892 (4), c = 6.986 (1) Å. Additional weak reflections are observed; they require the introduction of a modulation wavevector q = 0.0453 (2)c* for indexing. The modulated structure has been solved using the superspace formalism [superspace group X\overline{3}(00\gamma)0]. A framework of corner-sharing AlO4 tetrahedra forms corrugated sixfold rings and uncommon triple rings. The Ca2+ cations exhibit an eightfold coordination sphere; edge-sharing CaO8 polyhedra form intertwinned zigzagging rows along c creating a three-dimensional net. Bi atoms are located in large hexag...

Determination of the crystal structure of magnesium perchlorate hydrates by X-ray powder diffraction and the charge-flipping method

Acta Crystallographica Section B — Wed, 10 Nov 2010 15:36:03 +0100

X-ray powder diffraction (XRD) data were used to solve the crystal structures of phases in the magnesium perchlorate hydrate system, Mg(ClO4)2·nH2O (n = 4, 2). A heating stage and humidity generator interfaced to an environmental cell enabled in-situ XRD analyses of dehydration reactions under controlled temperatures and partial pressures of H2O (P_{{\rm H}_2{\rm O}}). The crystal structures were determined using an ab initio charge-flipping method and were refined using fundamental-parameter Rietveld methods. Dehydration of magnesium perchlorate hexahydrate to tetrahydrate (348 K) results in a decrease in symmetry (space group = C2), where isolated Mg2+ cations are equatorially coordinated by four H2O molecules with two [ClO4]− tetrahedra at the apices. Further dehydration to the dih...

Thermal motion of tert-butyl groups III. tert-Butyl substituents in aromatic hydrocarbons, the view from the bottom of the well

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

The rigidity of the tert-butyl group (TBG) as a substituent in aromatic hydrocarbons is investigated, with a modified Hirshfeld test of anisotropic displacement parameters (ADPs) as a primary criterion. Four new structures are analyzed, along with low-temperature studies of a previously published crowded supermesityl dimer; three of the five structures meet the primary test. Most of the TBGs meet the Hirshfeld test at 100 K, and the ADPs are improved by omitting low-order data in the final refinement. The three most precise structures yield a wide variation in libration amplitudes (and in estimated rotation barriers) for 13 unique TBGs. A similar range of values is found in analyses of structures in the Cambridge Crystallographic Database. The libration amplitudes are calculated with the...

Solid solution of two diastereomers of [3a(R,S),7a(R,S)]-3-[(1′R)-1-phenylethyl]perhydro-1,3-benzothiazol-2-iminium chloride

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

A mixture of two diastereomers with the configurations (3aS,7aS,1′R) and (3aR,7aR,1′R) forms co-crystals in which there is one unique molecule in the asymmetric unit, but the molecule displays disorder which is a result of the presence of the two diastereomers at the same crystallographic site. Theoretical calculations carried out by the DFT method with the 6-311++G(2df,p) basis set allowed for the estimation of the energy difference between the two diastereomers both in the isolated and the solid state, while the natural bond orbital (NBO), Mulliken, natural population (NPA) and CHelpG analyses helped to establish the electronic structure of the thazolidin-2-imine fragment. (Source: Acta Crystallographica Section B)

Diffuse scattering study of aspirin forms (I) and (II)

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

Full three-dimensional diffuse scattering data have been recorded for both polymorphic forms [(I) and (II)] of aspirin and these data have been analysed using Monte Carlo computer modelling. The observed scattering in form (I) is well reproduced by a simple harmonic model of thermally induced displacements. The data for form (II) show, in addition to thermal diffuse scattering (TDS) similar to that in form (I), diffuse streaks originating from stacking fault-like defects as well as other effects that can be attributed to strain induced by these defects. The present study has provided strong evidence that the aspirin form (II) structure is a true polymorph with a structure quite distinct from that of form (I). The diffuse scattering evidence presented shows that crystals of form (II) are es...

Direct observation of various reaction pathways of arylnitrenes in different crystal environments caused by acid–base complex formation

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

The structures of reaction intermediates, arylnitrenes and their final products have been successfully analyzed by X-rays using acid–base complex formation. The acid–base complexes of 2-azidobenzoic acid (2a), 3-azidobenzoic acid (3a) and 4-azidobenzoic acid (4a) were made with dibenzylamine (db), N-benzyl-2-phenylethylamine (bp) and dicyclohexylamine (dc). For the complex crystals of (3a) and db (3a-db), and (4a) and db (4a-db) two forms of (I) and (II) were obtained. Eight types of complex crystals, (2a-db), (3a-db-I), (3a-db-II), (3a-dc), (4a-db-I), (4a-db-II), (4a-bp) and (4a-dc), suitable for X-ray analysis were obtained. When the crystals were irradiated with UV light at low temperatures, the reactions proceeded keeping the single-crystal form in the five crystals (2a-db), (3a-db...

Direct observation of arylnitrene formation in the photoreaction of arylazide crystals

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

Seven crystal structures of arylazides, 2-azidobiphenyl (2), 4-(4-azidophenyl)butanoic acid (3), 3-azidobenzoic acid (4), N-(4-azidophenyl)acetamide (5), 2,4,6-trichlorophenyl azide (6), 2,5-dibromophenyl azide (7) and 2,4,6-tribromophenyl azide (8), have been analyzed by X-rays. When the crystals were irradiated with UV light at ≃ 80 K, only 2-azidobiphenyl gradually changed its cell dimensions with the retention of the single-crystal form. The crystal structure after photo-irradiation was analyzed by X-rays under the same conditions as those before photo-irradiation. Approximately 20% of the 2-azidobiphenyl molecule was converted to the triplet 2-biphenylnitrene and dinitrogen molecules. The existence of the triplet nitrene was confirmed by ESR and IR measurements. Although the str...

Structure and electrical properties of the new pyrochlore-type protonic solid electrolyte K0.88Nb2O7.58H4.28

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

Single-crystal, synchrotron powder X-ray diffraction and neutron powder diffraction studies of the novel pyrochlore-type compound with the structural formula K0.88(OH)0.54H1.66(H2O)1.04Nb2O6 suggests that the water molecules are located in 32e sites, and the hydroxide ions and potassium ions are located in 16d sites with a significant amount of `free' protons in 96g sites. The total weight loss at temperatures up to 773 K is only about 8%, suggesting the oxygen escape from 48f sites can be excluded and `free' protons must be preserved in the structure. The bulk conductivity in ambient air reaches 10−2 S cm−1 at 623 K. Owing to the extended stability range and resistance to water solubility, the compound can be considered as a candidate for intermediate temperature solid-oxide f...

Structure of N-(5-ethyl-[1,3,4]-thiadiazole-2-yl)toluenesulfonamide by combined X-ray powder diffraction, 13C solid-state NMR and molecular modelling

Acta Crystallographica Section B — Wed, 10 Nov 2010 00:00:00 +0100

The crystal structure solution of the title compound is determined from microcrystalline powder using a multi-technique approach that combines X-ray powder diffraction (XRPD) data analysis based on direct-space methods with information from 13C solid-state NMR (SSNMR), and molecular modelling using the GIPAW (gauge including projector augmented-wave) method. The space group is Pbca with one molecule in the asymmetric unit. The proposed methodology proves very useful for unambiguously characterizing the supramolecular arrangement adopted by the N-(5-ethyl-[1,3,4]-thiadiazole-2-yl)toluenesulfonamide molecules in the crystal, which consists of extended double strands held together by C—H...π non-covalent interactions. (Source: Acta Crystallographica Section B)

Validation of experimental molecular crystal structures with dispersion-corrected density functional theory calculations

Acta Crystallographica Section B — Fri, 10 Sep 2010 23:00:00 +0100

This paper describes the validation of a dispersion-corrected density functional theory (d-DFT) method for the purpose of assessing the correctness of experimental organic crystal structures and enhancing the information content of purely experimental data. 241 experimental organic crystal structures from the August 2008 issue of Acta Cryst. Section E were energy-minimized in full, including unit-cell parameters. The differences between the experimental and the minimized crystal structures were subjected to statistical analysis. The r.m.s. Cartesian displacement excluding H atoms upon energy minimization with flexible unit-cell parameters is selected as a pertinent indicator of the correctness of a crystal structure. All 241 experimental crystal structures are reproduced very well: the ave...

Conformational polymorphism in a Schiff-base macrocyclic organic ligand: an experimental and theoretical study

Acta Crystallographica Section B — Fri, 10 Sep 2010 23:00:00 +0100

Polymorphism in the highly flexible organic Schiff-base macrocycle ligand 3,6,9,17,20,23-hexa-azapentacyclo(23.3.1.111,15.02,6.016,20)triaconta-1(29),9,11,13,15(30),23,25,27-octaene (DIEN, C24H30N6) has been studied by single-crystal X-ray diffraction and both solid-state and gas-phase density functional theory (DFT) calculations. In the literature, only solvated structures of the title compound are known. Two new polymorphs and a new solvated form of DIEN, all obtained from the same solvent with different crystallization conditions, are presented for the first time. They all have P\bar 1 symmetry, with the macrocycle positioned on inversion centres. The two unsolvated polymorphic forms differ in the number of molecules in the asymmetric unit Z′, density and cohesive energy. Theoretical ...

Experimental and predicted crystal structures of Pigment Red 168 and other dihalogenated anthanthrones

Acta Crystallographica Section B — Fri, 10 Sep 2010 23:00:00 +0100

The crystal structures of 4,10-dibromo-anthanthrone (Pigment Red 168; 4,10-dibromo-dibenzo[def,mno]chrysene-6,12-dione), 4,10-dichloro- and 4,10-diiodo-anthanthrone have been determined by single-crystal X-ray analyses. The dibromo and diiodo derivatives crystallize in P21/c, Z = 2, the dichloro derivative in P\bar 1, Z = 1. The molecular structures are almost identical and the unit-cell parameters show some similarities for all three compounds, but the crystal structures are neither isotypic to another nor to the unsubstituted anthanthrone, which crystallizes in P21/c, Z = 8. In order to explain why the four anthanthrone derivatives have four different crystal structures, lattice-energy minimizations were performed using anisotropic atom–atom model potentials as well as using the semi-c...

Bond catastrophes in rhodium complexes: experimental charge-density studies of [Rh(C7H8)(PtBu3)Cl] and [Rh(C7H8)(PCy3)Cl]

Acta Crystallographica Section B — Fri, 10 Sep 2010 23:00:00 +0100

Rhodium complexes have potential uses in both catalysis and promoting the cleavage of C—C bonds. In order to further our understanding of these species and their potential applications, it is vital to obtain insight into the bonding within the species, particularly the Rh—C interactions, and to this end experimental charge-density studies have been undertaken on the title complexes. High-resolution single-crystal datasets to sin θ/λ = 1.06 Å−1 were obtained at 100 K and analysed using Bader's `Atoms in Molecules' (AIM) approach. The results of the studies have provided unique insights into the bonding involving rhodium and highlight the importance of undertaking such investigations for transition metal compounds. (Source: Acta Crystallographica Section B)

Symmetry of platelet defects in diamond: new insights with synchrotron light

Acta Crystallographica Section B — Sat, 04 Sep 2010 08:15:01 +0100

Mapping of reciprocal space for Ia-type diamond single crystals with synchrotron radiation has uncovered a variety of diffuse scattering features, some of them have not been observed before. The main component of diffuse scattering in the form of diffuse rods corresponds to a set of platelets which join together blocks of diamond structure. The platelets are ordered structural entities with lattice periodicity 8^{1/2} a_0 \times (1/2)^{1/2} a_0 \times 0.55a_0, where a_0 is a unit-cell dimension of diamond. Intensity distribution along the rods has been measured and used for recognition of symmetry elements of the platelet structure. These findings, together with previously reported transmission electron microscopy (TEM) observations, provide strong constraints for atomistic modelling of th...

Validation of experimental charge densities: refinement of the macrolide antibiotic roxithromycin

Acta Crystallographica Section B — Fri, 03 Sep 2010 23:00:00 +0100

We report an investigation of the detailed experimental electron-density distribution (EDD) of roxithromycin, a macrolide antibiotic consisting of 134 atoms. Although the experimental multipole refinement on high-resolution synchrotron data converged smoothly, validation of the electron density by calculation of an `experiment minus invariom' difference density revealed conformational disorder of the H atoms. Hydrogen disorder is shown to affect the EDD, the electrostatic potential and atomic properties as defined by Bader's quantum theory of atoms in molecules. A procedure to obtain the electron density distribution in the presence of disorder is proposed. (Source: Acta Crystallographica Section B)

Single-crystal neutron diffraction investigation on crystals belonging to the langasite family: a comparative study

Acta Crystallographica Section B — Fri, 03 Sep 2010 23:00:00 +0100

Crystals of the langasite family are of interest as they are piezoelectric in different devices. The properties of these classes of crystals can be modified within certain limits by isomorphous substitution. Single-crystal neutron diffraction studies were carried out for LGT (La3Ga5.5Ta0.5O14), LGST (La3Ga5.25Ta0.25Si0.5O14) and LGZrT (La3Ga5.25Ta0.25Zr0.5O14) as the neutron study gives a better average picture of the crystal properties over a macroscopic region of the grown crystal. The effect of small substitutions at various sites on the piezoelectric properties of the crystal was studied. (Source: Acta Crystallographica Section B)

Charge density distribution in aminomethylphosphonic acid

Acta Crystallographica Section B — Fri, 03 Sep 2010 23:00:00 +0100

The experimental charge density distribution in aminomethylphosphonic acid has been determined from X-ray diffraction and its topological features have been analyzed. The results have shown that the P—O bonds are highly polarized, moreover the P—OH bond is weaker than the bonds to unprotonated O atoms. These facts have been confirmed by theoretical density functional theory (DFT) calculations, which have shown that the single, strongly polarized bonds within the phosphonate group are modified by hyperconjugation effects. (Source: Acta Crystallographica Section B)

An experimental charge density of HEPES

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

We report the experimental charge density of HEPES [4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid], which is a common buffering agent. The structure was refined using the Hansen–Coppens formalism. The ability of the HEPES molecule to form stable intermolecular interactions and intermolecular hydrogen bonds in the crystal structure is discussed in terms of its buffering properties. The protonation mode observed in the crystal structure is different from that expected in solution, suggesting that additional factors must be taken into consideration in order to explain the solution properties of the compound. As ordered HEPES molecules are found in the active sites of proteins in several protein crystal structures, our results will allow for quantitative analysis of the electrostatic po...

Structures of alkyl-substituted Tröger's base derivatives illustrate the importance of Z′ for packing in the absence of strong crystal synthons

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

Crystal structures of Tröger's base (5,11-methano-2,8-dimethyl-5,6,11,12-tetrahydrodibenzo[b,f][1,5]diazocine) analogues with the methyl groups replaced by ethyl, iso-propyl and tert-butyl groups were studied. The incidence of Z′ > 1 structures increases to rather conspicuous levels. The reasons behind this trend are expanded upon, and a possible explanation is given in the flexibility of the alkyl substituents and van der Waals stabilization. In combination these effects allow for an additional stabilization of the packing by small changes in the molecular conformations, thus expanding the size of the asymmetric unit. (Source: Acta Crystallographica Section B)

Pressure-induced phase transitions in l-alanine, revisited

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

The effect of pressure on l-alanine has been studied by X-ray powder diffraction (up to 12.3 GPa), single-crystal X-ray diffraction, Raman spectroscopy and optical microscopy (up to ∼ 6 GPa). No structural phase transitions have been observed. At ∼ 2 GPa the cell parameters a and b become accidentally equal to each other, but without a change in space-group symmetry. Neither of two transitions reported by others (to a tetragonal phase at ∼ 2 GPa and to a monoclinic phase at ∼ 9 GPa) was observed. The changes in cell parameters were continuous up to the highest measured pressures and the cells remained orthorhombic. Some important changes in the intermolecular interactions occur, which also manifest themselves in the Raman spectra. Two new orthorhombic phases could...

Structural polymorphism of pyrazinium hydrogen sulfate: extending chemistry of the pyrazinium salts with small anions

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

Two polymorphs (α, β) of pyrazinium hydrogen sulfate (pyzH+HSO_4^-, abbreviated as PHS) with distinctly different hydrogen-bond types and topologies but close electronic energies have been synthesized and characterized for the first time. The α-polymorph (P212121) forms distinct blocks in which the pyzH+ and HSO_4^- ions are interconnected through a network of NH...O and OH...O hydrogen bonds. The β-form (P\bar 1) consists of infinite chains of alternating pyzH+ and HSO_4^- ions connected by NH...O and OH...N hydrogen bonds. Density functional theory (DFT) calculations indicate the possible existence of a hypothetical polar P1 form of the β-polymorph with an unusually high dipole moment. (Source: Acta Crystallographica Section B)

On the sequence of three related phases of [Ni(H2O)2(15-crown-5)](HSO4)2 in the temperature range 110–295 K

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

Attempts to prepare the compound [Ni(H2O)2(15-crown-5)](X)2 were eventually successful with X = NO_3^- provided that a synthetic route aimed at restricting water was followed. Application of this method was extended to make the analogous compound with X = HSO_4^-, for which three symmetry-related phases were isolated between 295 and 110 K: a room-temperature phase with Z′ = ½ [phase (I)], an intermediate-temperature phase with Z′ = 1 [phase (II)] and a low-temperature phase with Z′ = 2 [phase (III)]. The phases are related by two reversible solid–solid phase transitions, and both transitions take place without a significant loss of crystallinity. In the phase sequence (I) ↔ (II) ↔ (III) (Z′: ½ ↔ 1 ↔ 2), the crystal packing remains remarkably similar but the degree of ...

Polymorphism and piezochromicity in the three-dimensional network-based phosphate RbCuPO4

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:29 +0100

Rubidium copper phosphate, RbCuPO4, forms two room-temperature polymorphs that have been investigated with neutron powder diffraction. Polymorph (II) can be converted quantitatively into (I) by grinding the material or by pelletization, and the phase transition is accompanied by a significant colour change from very pale green to sky blue. Polymorph (II) can be obtained essentially free of (I) by quenching from 723 K. Each polymorph shows two unique Cu atoms: in (I) both sites are four-coordinate in a 2:1 ratio, whereas in (II) the atoms are four- and five-coordinate in a 1:1 ratio. In each case these sites are linked by phosphate tetrahedra to form three-dimensional frameworks based on the 42638-a four-connected net. The Rb atoms are hosted in the six- and eight-ring channels that are s...

Structural investigation of GeSb6Te10 and GeBi6Te10 intermetallic compounds in the chalcogenide homologous series

Acta Crystallographica Section B — Sat, 10 Jul 2010 07:19:28 +0100

The crystal structures of GeSb6Te10 and GeBi6Te10 were scrutinized using an X-ray powder diffraction method, which revealed that these compounds crystallize in trigonally distorted cubic close-packed structures with a 51-layer period (R\bar 3m). Each layer consists of a triangular atomic net; Te atoms occupy their own specific layers, whereas Ge, Sb and Bi atoms are located in the other layers. In these pseudobinary compounds, random atomic occupations of Ge and Sb/Bi are observed and the layers form two kinds of elemental structural blocks by their successive stacking along the c axis. These compounds can be presumed to be isostructural. It is known that the chemical formula of the chalcogenide compounds with the homologous structures found in these pseudobinary systems can be written as ...

Syntheses, spectroscopic study and X-ray crystallography of some new phosphoramidates and lanthanide(III) complexes of N-(4-nitrobenzoyl)-N′,N′′-bis(morpholino)phosphoric triamide

Acta Crystallographica Section B — Fri, 18 Jun 2010 07:25:10 +0100

New phosphoramidates with the formula RC(O)N(H)P(O)X2, R = 2-NO2—C6H4, 3-NO2—C6H4 and 4-NO2—C6H4, X = N(CH2CH3) (1)–(3), NC4H8 (4)–(6), and NC4H8O (7)–(9) were synthesized and characterized by 1H, 13C, 31P NMR and IR spectroscopy, and elemental analysis. The reaction of (9) with hydrated lanthanide(III) nitrate leads to ten- or nine-coordinated complexes, (10)–(13). The crystal structure has been determined for (3), (5), (9), (10) and (13). In contrast to all of the previously reported similar phosphoramidate compounds, the —C(O)—N(H)—P(O) skeleton in the free ligand (9) shows a cisoid conformation, with the C=O and P=O double bonds adopting a nearly syn conformation. Quantum chemical calculations were applied for clarifying this exceptional conformational behavior. The...

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3

Acta Crystallographica Section B — Wed, 16 Jun 2010 23:00:00 +0100

Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc21, a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) Å. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional the...

Structure of pyrrhotite 5C (Fe9S10)

Acta Crystallographica Section B — Sat, 15 May 2010 14:39:29 +0100

The distribution of vacancies throughout the underlying NiAs structure of pyrrhotite 5C was analysed through the application of vacancy avoidance and the closeness condition in conjunction with order–disorder layering. Two crystallographically equivalent structure solutions (chiral enantiomers) were produced consisting of layers containing one vacancy in every eight iron sites broken by a fully occupied layer every fifth iron layer, and best described by monoclinic statistical models. The statistical 5C structures were verified using synchrotron powder diffraction data as well as published electron-diffraction patterns. An order–disorder structure description is proposed for the intermediate pyrrhotites of which pyrrhotite 5C is an end-member. (Source: Acta Crystallographica Section B)

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at ∼ 135 K with a wide hysteresis of 15 K. Dielectric measurements confirm the transition at ∼ 127 K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and β angle change abruptly and remarkably at 129 (2) K, indicating that the system undergoes a first-order transition at Tc = 129 K. The crystal structures determined at 103 and 298 K are all monoclinic in P21/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are...

Bond lengths in organic and metal-organic compounds revisited: X—H bond lengths from neutron diffraction data

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

This paper is part of a short series that celebrates the addition of the 500 000th crystal structure to the Cambridge Structural Database in November 2009.The number of structures in the Cambridge Structural Database (CSD) has increased by an order of magnitude since the preparation of two major compilations of standard bond lengths in mid-1985. It is now of interest to examine whether this huge increase in data availability has implications for the mean bond-length values published in the late 1980s. Those compilations reported mean X—H bond lengths derived from rather sparse information and for rather few chemical environments. During the intervening years, the number of neutron studies has also increased, although only by a factor of around 2.25, permitting a new analysis of X—H b...

Charge-density analysis of the ground state of a photochromic 1,10-phenanthroline zinc(II) bis(thiolate) complex

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

The charge density of the title compound was determined at 90 K, using a spherical crystal of 150 µm diameter. The proper treatment of the Zn atom in the pseudo-tetrahedral environment is considered in detail. A satisfactory refinement is only obtained when anharmonic Gram–Charlier parameters are included as variables in the refinement. A successful combined anharmonic/multipole refinement indicates a small polarization of the 4s shell in the anisotropic environment. One of the two toluenethiols is approximately π-stacked with the phenanthroline ligand. A bond path is found connecting the two ligands. In addition the Zn—S bond to this ligand is slightly extended compared with the same bond to the second toluenethiol. A separate photocrystallographic and theoretical study indicate...

Structures of relevant ammonium salts in fertilizers

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

The crystal structures of two double salts of ammonium nitrate (AN) and ammonium sulfate (AS) are reported. The double salts 2NH4NO3·(NH4)2SO4 (2AN·AS) and 3NH4NO3·(NH4)2SO4 (3AN·AS) show a very similar crystal structure packing with alternating layers of anions and cations. The solid-state ionic distribution is controlled by an extensive hydrogen-bonding network with ammonium groups as the donors and O atoms acting as the acceptors. Crystallographic studies were conducted at both room temperature (293 K) and 100 K. Increasing the temperature involves shortening the b axis in the case of the 3AN·AS salt. Quantification of fertilizer mixtures using the Rietveld method was also carried out by means of the structural models reported in this paper for both salts. (Source: Acta Crystal...

Determination of zinc incorporation in the Zn-substituted gallophosphate ZnULM-5 by multiple wavelength anomalous dispersion techniques

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

The location of isomorphously substituted zinc over eight crystallographically different gallium sites has been determined in a single-crystal study of the gallophosphate ZnULM-5, Ga_{(16-x)}Zn_x(PO4)14(HPO4)2(OH)2F7, [H3N{CH2}6NH3]4, 6H2O, in an 11 wavelength experiment, using data from Station 9.8, SRS Daresbury. The measurement of datasets around the K edges of both Ga and Zn, as well as two reference datasets away from each absorption edge, was utilized to selectively exploit dispersive differences of each metal atom type in turn, which allowed the major sites of Zn incorporation to be identified as the metal 1 and 3 sites, M1 and M3. The preferential substitution of Zn at these sites probably arises because they are located in double four-ring (D4R) building units which can relax to a...

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi2Ga4O9, was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O...

X-ray atomic orbital analysis of 4f and 5d electron configuration of SmB6 at 100, 165, 230 and 298 K

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

Accurate electron-density measurement of SmB6 at 100, 165, 230 and 298 K, and X-ray atomic orbital (XAO) analysis were carried out. The 4f-electron density around Sm and 5d electron density at ∼ 1 Å from Sm were analysed by XAO analysis. The 5d electron density is due to the electrons of the 5dJ = 5/2Γ8 orbitals which stem from the eg orbitals in the strong field approximation. The change in electron populations of the 5d5/2Γ8 orbitals with temperature is similar to that of the resistivity. Since the conduction band consists of 5d5/2Γ8 and B-2p orbitals according to band theory, this indicates that the larger populations of the 5d5/2Γ8 orbitals correspond to the larger number of localized electrons and are correlated to the resistivity of SmB6. The occupation of the bulky 5d5/...

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure

Acta Crystallographica Section B — Fri, 14 May 2010 23:00:00 +0100

We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc0.5Ta0.5O3 (PST) and Pb0.78Ba0.22Sc0.5Ta0.5O3 (PST-Ba). At ambient pressure the symmetry of the average structure for both compounds is Fm\bar{3}m as a result of partial ordering of the Sc and Ta cations on the octahedral sites. At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections. Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure...

Symmetry analysis of extinction rules in diffuse-scattering experiments

Acta Crystallographica Section B — Thu, 06 May 2010 15:13:44 +0100

Structured diffuse-scattering intensities, whether of compositional or of pure displacive origin, static or dynamic, contain important information about the symmetry of the individual compositional and/or displacive modes responsible for the observed intensities. However, the interpretation of the experimental data is very often impeded by the lack of a symmetry-based approach to the analysis of the structured diffuse-scattering distributions. Recently, we have demonstrated the existence of systematic phonon selection rules for diffuse scattering that depend on the symmetries of the mode and the scattering vector, and not on the specific structure. Here, we show that such symmetry analysis can be successfully extended and also applied to structure-dependent diffuse scattering associated wi...

Towards a generalized vision of oxides: disclosing the role of cations and anions in determining unit-cell dimensions

Acta Crystallographica Section B — Wed, 05 May 2010 23:00:00 +0100

Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures (fcc: face-centred cubic and sc: simple cubic, respectively) exhibit charge concentration zones which coincide with the positions occupied by the O atoms in their oxides. Similar features, also observed for the pairs Ca/CaF2 and BaSn/BaSnO3, are supported by recent high-pressure experiments as well as electron-localization function (ELF) calculations, carried out on elemental K. At very high pressures, the elemental K adopts the hP4 structure, topologically identical to that of the K atoms in high-pressure K2S and high-temperature α-K2SO4. Moreover, the ELF for the hP4 structure shows charge concentration (∼ 2 electrons) at t...

On the accurate bond-valence parameters for the Sb3+/O2− ion pair

Acta Crystallographica Section B — Wed, 05 May 2010 23:00:00 +0100

The improved values of the bond-valence parameters for the Sb3+/O2– ion pair, r0 = 1.927 Å and b = 0.446 Å, have been deduced from the crystal structures of the α and β polymorphs of Sb2O3 and from the set of precisely determined complex structures containing [Sb3+On] coordination shells. (Source: Acta Crystallographica Section B)

The lines-of-force landscape of interactions between molecules in crystals; cohesive versus tolerant and `collateral damage' contact

Acta Crystallographica Section B — Wed, 05 May 2010 23:00:00 +0100

A quantitative analysis of relative stabilities in organic crystal structures is possible by means of reliable calculations of interaction energies between pairs of molecules. Such calculations have been performed by the PIXEL method for 1108 non-ionic and 98 ionic organic crystals, yielding total energies and separate Coulombic polarization and dispersive contributions. A classification of molecule–molecule interactions emerges based on pair energy and its first derivative, the interaction force, which is estimated here explicitly along an approximate stretching path. When molecular separation is not at the minimum-energy value, as frequently happens, forces may be attractive or repulsive. This information provides a fine structural fingerprint and may be relevant to the mechanical prop...

Polymorphism in the nitrate salt of the [Mn(acetylacetonate)2(H2O)2]+ ion

Acta Crystallographica Section B — Wed, 05 May 2010 23:00:00 +0100

The crystallization of [Mn(acac)2(H2O)2]+ from solutions containing excess nitrate leads to the formation of four polymorphs. All polymorphs contain two different types of complex ions, one containing essentially coplanar acac ligands and the other in which the two acac ligands together assume a chair conformation. Molecular modelling using DFT (density-functional theory) calculations shows that the coplanar conformation is the electronically stable one. The hydrogen bonding between the trans-water molecules and the nitrate ion produces a one-dimensional chain of 12-membered rings, which are further organized into a two-dimensional network via a lattice water molecule. Lattice-energy calculations have been carried out to compare the stabilities of the four polymorphs. (Source: Acta Crystal...

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