Room-Temperature Superstructure of ZrV₂O₇

ZrV₂O₇ and related phases are of interest for their negative thermal expansion properties, but there has been no detailed report of their structures to date. We describe here the room-temperature 3 × 3 × 3 superstructure of ZrV₂O₇, which has been determined from single-crystal synchrotron X-ray data.

Pressure-Induced Order–Disorder Phase Transition of Spinel Single Crystals

Single crystals with the composition Mg_0.4Al_2.4O₄ show a pressure-induced phase transition at 10.7 GPa which is partially reversible on pressure release. The resulting compound reveals a disordering of the cations on the tetrahedral and octahedral sites of the spinel structure.

Ideal and Real Structure of Ti₅O₄(PO₄)₄: X-ray and HRTEM Investigations

Pentatitanium tetraoxide tetrakis(phosphate) belongs to the Fe₂O(PO₄) structure family. The relationship is indicated by the formula □₃Ti₅O₄(PO₄)₄. X-ray single-crystal and high-resolution transmission electron microscopy investigations reveal partial disorder of Ti⁴⁺ over eight available octahedral voids. The ideal structure consists of four (TiO)O₅, one TiO₆ and four slightly distorted PO₄ units.

Distribution of the M-M Distances in the Rare Earth Oxides

The cation arrays in rare earth oxides reproduce the topology and distances of the structures of the pure metals.

Equation of State of α-Al₂O₃ (Corundum) from Quasi-Harmonic Atomistic Simulations

An interatomic potential is developed for a static and vibrational simulation of α-Al₂O₃. Structural, elastic and thermal expansion data are predicted in the p,T domain from 0 to 40 GPa and from 300 to 1700 K.

Atomic Structure of PbZrO₃ Determined by Pulsed Neutron Diffraction

The atomic structure of lead zirconate, PbZrO₃, and its variation with temperature were determined by pulsed neutron diffraction using both Rietveld refinement and atomic pair distribution function analysis. The symmetry at low temperatures was shown to be Pbam and a new antiferroelectric structure was identified for the intermediate phase.

New Measure of Distortion for Coordination Polyhedra

Use of the centroid concept for coordination polyhedra enables comparison of distorted and ideal polyhedra with the same number of coordinated atoms. The relation between the volumes of the distorted and the ideal polyhedra can be used as a global measure of distortion.

Multipole Analysis of the Electron Density in Topaz Using X-ray Diffraction Data

The multipole deformation electron density, Laplacian of the electron density and (3,-1) bond critical point parameters are analyzed in topaz, Al₂[SiO₄]F₂, using accurate X-ray diffraction data.

Group-Theoretical Analysis of Octahedral Tilting in Perovskites

Simple tilting of the octahedral units in perovskite gives rise to a number of distinct, closely related structures. Fifteen possible space groups are listed along with group–subgroup relationships and an indication of which translations between the different structures are allowed to be continuous.

Isotope Effect in TlH₂PO₄ and TlD₂PO₄

The phase transitions observed in TlH₂PO₄ and TlD₂PO₄ with decreasing temperature are analysed.

Distortion of Crystal Structures of Some Co^III Ammine Complexes. III. Distortion of Crystal Structure of [Co(NH₃)₅NO₂]Cl₂ at Hydrostatic Pressures up to 3.5 GPa

The anisotropy of structural distortion of the title compound under pressure was shown to be qualitatively different to that on cooling. The role of non-covalent interactions, in particular hydrogen bonds, in the anisotropy of structural distortion is discussed.

Structures of Fluoroarsenates KAsF_6n(OH)_n, n = 0, 1, 2: Application of the Heavy-Atom Method for Modulated Structures

The structures of the fluoroarsenates KAsF_6-n(OH)_n, n = 0, 1, 2, depend on the number and type of hydrogen bonds; a standard structure, a superstructure and an incommensurate modulated structure are found for n = 0, 1 and 2, respectively. A heavy-atom method developed for modulated structures is used for the solution of the incommensurate structure.

Distribution and Topology of the Electron Density in an Aluminosilicate Compound from High-Resolution X-ray Diffraction Data: the Case of Scolecite

The Si-O and Al-O bonds in a natural zeolite, scolecite, are characterized by the topological features of the experimental electron density. The estimation of the atomic net charges in this compound is described according to data analysis by the statistical prediction matrix (leverages) method.

Carbides Ln₁₀Ru₁₀C₁₉ (Ln = Y, Gd–Lu): Crystal Structure of their Subcells and the Superstructures of Er₁₀Ru₁₀C₁₉

Subcell and superstructures of Er₁₀Ru₁₀C₁₉ are described where various stacking variants arise from an ordered arrangement of single C atoms and C₂ pairs which are situated in trigonal Ru prisms. The Ru and C atoms of the three-dimensionally infinite polyanionic network seem to obey the 18- and 8-electron rules, respectively.

Net Intensities: Accuracy Improvement Through a Bayesian Perspective on the Measuring Strategy and Their Persistent Lack of Precision. An Illustration

The standard error on an intensity can be reduced below counting statistical error margins when, in a fixed time experiment, a repetition of fast profile scans replaces the traditional slow, but single scan. The decrease in intensity errors is not followed by a similar reduction in parameter errors, because the latter are dictated by the randomization of systematic intensity errors.

Solid-State Modeling. VI. 2,3-Diketopiperazine: the Integration of Crystallographic and Spectroscopic Evidence

The crystallographic and spectroscopic analyses of 2,3-diketopiperazine are combined with ab initio calculations for both the gas phase and solid state. The calculated IR shifts for the C=O stretching frequencies, which are hydrogen-bonding dependent, fit with the experimental evidence.

Oxime–Carboxyl Hydrogen Bonds: the Preferred Interaction Determining Crystal Packing of `Carboxyoximes'

A statistical analysis of hydrogen-bond interactions formed by oximes and carboxylic acids is presented. Ab initio quantum-chemical calculations have been used to rationalize the observed preference for the oxime–carboxyl interaction compared with carboxyl–carboxyl and oxime–oxime interactions.

Energetic Study of the Disordered Solvent in the Crystal Structure of an Isoxazole Derivative

Packing potential energy calculations for host–guest model crystals account for the disorder of ethyl acetate solvent molecules in the crystal state.

Topological Description of Pseudosymmetries in trans-2,3,4,5,5a,6,7,8,9,9a-Deca­hydro-2-methyl-4-phenylamino­carbonylbenzo[f][1,4]oxazepin-3-one and Other Structures which Crystallize in Space Group P-1 with Z = 8

Symmetry analysis of numerous triclinic structures retrieved from the Cambridge Structural Database with P-1 and Z = 8 revealed a rich variety of *2₁/*c pseudosymmetry, which may exist in the oblique unit cells. The migrations of these symmetries are accompanied by the irregular translations of their complementary operators, which may lead even to transitional space-group formations.

Structure of Ammonium Hydrogen Succinate at 80 and 20 K

The temperature dependence of the geometries of the hydrogen bonds before and after the second-order phase transition (T_c = 170 K) is studied. The O⋯O distances of two O-H⋯O hydrogen bonds involving the hydrogen succinate ions increase as the temperature decreases below T_c and the N⋯O distance of a weak bifurcated hydrogen bond involving the ammonium ion decreases below T_c.

Structures and Photoreactivities of 2,4,6-Triiso­propylbenzo­phenones

Crystal structures of eleven 2,4,6-triisopropylbenzophenones have been determined to show that the molecular structures are essentially the same, independent of the substituted groups. The photostability of 4^'-COOMe and 4^'-COCl derivatives in the solid state is attributed to the narrow reaction cavity around the triisopropylphenylcarbonyl moiety.

Crystal-to-Crystal Diastereoselective Transformation of 2,4,6-Triiso­propyl-4'-(S)-phenylalanino­carbonylbenzo­phenone Methyl Ester

The diastereoselective photocyclization of the title compound was directly observed as a crystal-to-crystal transformation. The crystal structure after 100% conversion has been determined, revealing that the carbonyl C and the isopropyl central C atoms approach each other to form a C-C bond.

Charge Density Study of 2-Pyridone

Experimentally determined properties of 2-pyridone include the total electron density and its Laplacian, the electrostatic potential and the dipole moment. These are used to characterize the N-H⋯O, C-H⋯O and C-H⋯π interactions.

Arrays with Local Centers of Symmetry in Space Groups Pca2₁ and Pna2₁

Nearly 200 examples of local centers of symmetry have been found in each of the space groups Pca2₁ and Pna2₁ for Z > 4. These are concentrated about x=1/8, y=1/4 and x=1/8, y=0, respectively. Implications for the refinement process are discussed.

Concerning the κ Phase of Al–Cr–Ni

The original refinement of this structure, in space group P6₃, was restrained so as to yield a presumed composition of approximately Al₇₆Cr₁₈Ni₆. Revision of the space group, to P6₃/m, permits unrestrained refinement which indicates no more than a trace amount of Ni.

X-ray charge densities and chemical bonding by P. Coppens

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