On the crystal chemistry of olivine-type germanate compounds, Ca_1 + xM_1 − xGeO₄ (M²⁺ = Ca, Mg, Co, Mn)

The variation of structural and distortional parameters in synthetic olivine-type germanate compounds is studied, and a comparison is made with silicate olivine.

Vacancy-ordering effects in AlB₂-type ErGe_{2 − x} (0.4 < x ≤ 0.5)

Vacancy ordering in AlB₂-type ErGe_{2 − δ} is described within a unified superspace approach. The observed powder diffraction data are indexed in the monoclinic superspace group X2m(α0γ)0s, X representing the centring vector (½,½,0,½). The q vector is shown to be a direct measure of the germanium occupancy as ErGe_{2 − α}, with α in the range to ½.

Structural analysis of perovskite LaCr_1 − xNi_xO₃ by Rietveld refinement of X-ray powder diffraction data

The crystal structure of perovskite LaCr_1 − xNi_xO₃ (0 ≤ x ≤ 1.0) at room temperature has been investigated by means of Rietveld refinement of X-ray diffraction data.

Coherence length of the KY(WO₄)₂ single crystal

The coherence scattering lengths of a KY(WO₄)₂ single crystal in the directions parallel and perpendicular to the growth directions were estimated from the broadening of X-ray reflections. Qualitative analysis of the shape of the diffracted peaks and the intensity distribution in reciprocal space maps indicated the presence of dislocations and volume defects, which introduced a negative dilation into the crystal host.

Structure and electron density of oxysulfide Sm₂Ti₂S₂O_4.9, a visible-light-responsive photocatalyst

We report the crystal structure, electron density, electronic band dispersion and electronic density of states of samarium titanium oxysulfide (Sm₂Ti₂S₂O_4.9) photocatalyst obtained through the Rietveld analysis, maximum-entropy method (MEM) and MEM-based pattern fitting of the high-resolution synchrotron powder diffraction data taken at 298.7 K, in combination with the density-functional theory calculations.

A bond-valence investigation of two series of isostructural lanthanide compounds

The variation of the bond-valence sums (BVS) of the lanthanide atoms with the element number Z_Ln in isostructural garnet-type compounds Ln₃Te₂Li₃O₁₂ is strikingly similar to the variation of the third ionization energy of Ln with Z_Ln. This means that BVS does not represent the integer-number stoichiometric valence, but a non-integer structure-dependent property for which the term structural valence is proposed.

Orientational disorder in crystals of (NH₄)₃MoO₃F₃ and (NH₄)₃WO₃F₃

In highly dynamically disordered structures of (NH₄)₃MoO₃F₃ and (NH₄)₃WO₃F₃, it is possible to identify O and F atoms on a local scale using X-ray diffraction techniques. The true geometry of the distorted [MoO₃F₃]³⁻ octahedron, which is rather far from C_3v, explains the character of the observed vibrational spectra of the compounds.

Solid-state structures of the covalent hydrides germane and stannane

The low-temperature phase III structures of perdeuterogermane and -stannane have been determined by high-resolution neutron powder diffraction at 5 K. Germane has an orthorhombic structure comprising molecules with C₁ site symmetry and the structure of stannane is monoclinic with molecules located on twofold rotation axes.

Five-coordinate copper(II) complexes: crystal structures, spectroscopic properties and new extended structural pathways of [Cu(chelate)₂X]Y, where chelate = dpyam, phen and bipy; X = pseudohalide ligands

The four new distortion isomers of the five-coordinate [Cu(chelate)₂X]⁺ cation have been compared by scatterplot analysis with 25 [Cu(chelate)₂X]Y complexes of known crystal structure. Three of the present complexes are found to lie on an extended structural pathway towards the square-based pyramid.

Disorder and pseudo-symmetry in octa­kis­(tri­vinylsilyl)octasilicate

Crystal structures of two structural variants of octakis(trivinylsilyl)octasilicate, Si₁₆O₂₀C₄₈H₇₂, are reported. The paper illustrates both pseudo-symmetry and how the structure of a well ordered molecular compound can be used to determine the average structure of its disordered variant.

Hydrogen-bonding properties of galanthamine: an investigation through crystallographic database observations and computational chemistry

The hydrogen-bonding interactions of galanthamine, an anti-Alzheimer agent, have been analysed experimentally through observations in the Cambridge Structural Database and the Protein Data Bank and theoretically from ab initio and density functional theory calculations. Galanthamine interacts to a large degree through (i) multicentered hydrogen bonds involving its O atoms and (ii) polarized CH groups appearing as significant hydrogen-bond donors.

Conformational variability of molecules in different crystal environments: a database study

A method is described for the automated conformational clustering of complete molecules that occur in different crystal environments in the Cambridge Structural Database, by identifying all occurrences of a specific molecule either as the sole crystal component or co-crystallized with other component molecules or ions. The overall results of the methodology are tabulated and analysed in terms of molecular conformational diversity.

Comparative experimental electron density and electron localization function study of thymidine based on 20 K X-ray diffraction data

From a low-temperature (T = 20 K) high-resolution X-ray data set (sin θ/λ = 1.1 Å⁻¹) the electron-density distribution of thymidine was derived by a classical multipole approach and by two invariom applications which considered the nearest and next-nearest neighbors for the heteroaromatic thymine ring. An electron localization function application yielded electron populations in the disynaptic valence basins which could be related to bond orders.

Monitoring structural transformations in crystals. 12. Course of an intramolecular [4 + 4] photo­cycloaddition in a crystal

Variations in crystal and molecular structures, brought about by an intramolecular photochemical reaction of bi(anthracene-9,10-dimethylene), are described and explained.

Synthesis and structural characteristics of lithocholate triads: steroid-type channels occupied by spacer fragments

X-ray studies of three steroidal triads, A—B—A, reveal their wavy shape, inclusion properties and close resemblance to the crystal structures of cholic acids, and point to the special role of carboxylic acid/pyridine strong hydrogen bonds, and pyridine stacking interactions in the construction of one-dimensional polymeric tapes. The lithocholic acid triad:pyridine H:G crystals undergo a symmetry-lowering phase transition below 180 K.

Dipolar C N⋯C N interactions in organic crystal structures: database analysis and calculation of interaction energies

Database analysis shows that cyano groups form dipolar interactions in crystal structures, with the antiparallel dimer as the dominant motif. Ab initio calculations show this motif to have an attractive interaction energy similar to that of a medium-strength hydrogen bond.

Experimental electron density study of ethylene oxide at 100 K

Weak intermolecular C—H⋯O interactions influence the electron density of ethylene oxide. This is visible on the Hirshfeld surface and in the polarization of the electrostatic potential.

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

In organic carboxylic acid crystals the single and double carbon–oxygen bond lengths are variable and tend to be almost equal in aromatic acids. Reasons are tentatively discussed.