Acta Crystallographica Section B

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

Orosel, D. — 2012-01-18

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.

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

Schmidbauer, M. — 2012-01-06

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.

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

Gemmi, M. — 2012-01-06

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 (< 10 µm) of less than 100 µm in diameter. The crystal structure has been solved from and refined against electron diffraction of a microcrystal. Data sets have been measured by both a manual and an automated version of the new electron-diffraction tomography technique combined with the precession of the electron beam. The sarrabusite structure is monoclinic and consists of (010) layers of straight chains formed by alternating edge-sharing CuO4Cl2 and PbO8 polyhedra parallel to the c axis, which share corners laterally with two zigzag corner-sharing chains of PbO6Cl2 and PbO4Cl4 bicapped trigonal prisms. These blocks are linked together by SeO_3^{2-} flat-pyramidal groups.

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

Balachandran, P.V. — 2012-01-18

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 z = 0 for the AI site), is shown to classify apatite crystal chemistries based on site occupancy on the A, B and X sites. The classification is accomplished using a K-means clustering analysis.

Polymorphism of Li2Zn3

Pavlyuk, V. — 2012-01-18

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 were obtained after structure refinements. According to electronic structure calculations using the tight-binding–linear muffin-tin orbital–atomic spheres approximations (TB–LMTO–ASA) method, strong covalent Sn—Sn and Ga—Ga interactions were established in Li5Sn2 and Li5Ga4, but no similar Zn—Zn interactions were observed in Li2Zn3.

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

Kusz, J. — 2012-01-18

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 symmetry remains R\bar 3, despite a jump in lattice parameters. We use a special technique to analyse the disorder model of the slowly cooled samples, which consists of layered domains shifted in the hexagonal ab plane. The low-spin disordered phase was solved in a novel approach to accommodate the very unusual twinning and refined in the non-standard space group C\bar 1. In contrast to the ordered low-spin phase, the Fe ion is in a non-centrosymmetric coordination polyhedron and two of the six propyl groups change their conformation.

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

Molčanov, K. — 2012-01-06

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 Å).

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

Olmstead, M.M. — 2012-01-06

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.

Hydrogen-bridged chelate ring-assisted π-stacking interactions

Karabıyık, H. — 2012-01-06

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 derivatives in fully or partially NH tautomeric form, and aromaticity levels of certain fragments of SA derivatives have dramatic effects on their stacking arrangements. These interactions are distinguished from the usual π...π interactions by their formation character, i.e. both σ- and π-deficient and σ-deficient character of pure interactions is more pronounced than that of the hybrid ones.

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

Chan, E.J. — 2012-01-18

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 between these layers are present in the model for form (II) as opposed to form (I). An investigation into the peculiarities of librational disorder was also conducted, however, correlation values turn out to be so small that any structural information concerning librational correlation is inconclusive. The purpose of this experiment was to identify if the diffuse scattering features could provide further insight into understanding the physical reasoning behind the metastability of form (II). The form (II) → (I) phase transition is also not currently well understood and usually phase transitional information can be obtained from pronounced diffuse scattering features. Since the diffuse scattering is modelled adequately using harmonic potentials it is our conjecture that the `diffuse' is essentially thermal in origin and does not afford any extra information about the form (II) → (I) phase transition.

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

Solovyov, L.A. — 2012-01-18

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.

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

Röttger, K. — 2012-01-06

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.

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

Schmidt, H. — 2012-01-06

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

Notes for authors 2012

IUCr Editorial Office — 2012-01-18

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.

Ferraris, G. — 2012-01-18