Villamaninite, a case of noncubic pyrite-type structure

A single-crystal X-ray study was carried out on two villamaninite samples from Villamanín (León, Spain). The systematic structural refinement over subgroups of the pyrite Pa-3 space group leads to a monoclinic model, space group P12₁1, for this mineral.

Synthèse et caractérisation structurale d'un phosphate d'indium In₃P₂O₈ présentant des paires In–In

The crystal structure of the new indium phosphate In₃P₂O₈ is characterized by a three-dimensional arrangement of isolated PO₄³⁻ tetrahedra forming the anionic network, while the electroneutrality is ensured by (In-In)⁴⁺ cationic pairs.

Structures and isomerization in diamminedi­chloropalladium(II)

Three forms (cis-, trans-and β-trans-) of [Pd(NH₃)₂Cl₂] were investigated by X-ray powder diffraction. The crystal structures obtained form the basis for considering the possible mechanisms of isomerization.

A novel intergrowth structure between ReO₃-type and tetragonal tungsten bronze-type in the Zr/Nb/W/O system

Electron diffraction and high-resolution transmission electron microscopy investigations reveal the structure of Zr_nNb_8-2nW_12+nO₅₆ (0.5<n<1). The characteristic structural features are double pentagons composed of edge-sharing five-membered rings of octahedra.

Atomic displacement parameters for Ni(ND₃)₄(NO₂)₂ from 9 K X-ray and 13 K time-of-flight neutron diffraction data

Excellent agreement is obtained between both positional and thermal parameters derived separately from very low-temperature X-ray and time-of-flight neutron diffraction data. This gives confidence in the deconvolution of thermal motion from the X-ray data in a subsequent calculation of the static electron density distribution.

Synthesis and structure refinement of a new type of layered intergrowth phase: Sb_3+xNb_3−xTiO₁₄ and Sb_3+xTa_3−xTiO₁₄

The isomorphous phases Sb_3+xNb_3−xTiO₁₄ and Sb_3+xTa_3−x-TiO₁₄ have been discovered and their structures solved. They have layered structures related to those found in simpler oxides of Sb^III and Sb^V.

An extension of the Voronoi analysis of crystal structures

A new methodology is defined for discriminating between different crystal structures on the basis of their Voronoi polyhedra.

A re-examination of the relationship between lattice strain, octahedral tilt angle and octahedral strain in rhombohedral perovskites

The relationship between lattice strain and BO₆ octahedral tilt angle is examined in rhombohedral perovskites, ABO₃. Consideration is also given to factors governing the observed space group.

Structure of UMoO₅ studied by single-crystal X-ray diffraction and high-resolution transmission electron microscopy

A combination of X-ray diffraction and high-resolution transmission electron microscopy has been used to study the crystal structure of UMoO₅, obtained by hydrothermal and ceramic methods. The structure is related to that of UVO₅.

Structural relationship of various squarates

The structural relationship between three types of metal squarates and the ligand squarate in its acid form and in its monoanion form is illustrated. It relies heavily on intra- and intermolecular hydrogen bonds.

Temperature dependence of crystal structure of NaK_0.72(NH₄)_0.28(+)-C₄H₄O₆.4H₂O in the paraelectric phase

X-ray investigations reveal details of structural change with temperature in the paraelectric phase of Rochelle salt-ammonium Rochelle salt mixed crystal.

Ab initio powder structure determination and thermal behavior of a new lead(II) phenylphosphonate, Pb(O₃PC₆H₅)

The structure of layered Pb(O₃PC₆H₅) has been determined ab initio from powder diffraction data by minimizing the preferred orientation. MAS-NMR has been used to confirm the triclinic structure with two independent structural units.

Inclusion compounds of thiourea and peralkylated ammonium salts. III. Hydrogen-bonded host lattices built of thiourea and nitrate ions

The present series of thiourea-quaternary ammonium nitrate complexes exhibit two distinctly different types of thiourea-anion lattices depending on the stoichiometric ratio of thiourea to nitrate ion. Compouds (1) and (2) (3:1) are novel channel inclusion compounds, whereas (3), (4) and (5) (1:1) are each characterized by an anionic composite ribbon composed of a zigzag arrangement of thiourea molecules with nitrate ions attached to both sides.

Structures of L-valyl-L-glutamine and L-glutamyl-L-valine

The title compounds were compared with other mixed hydrophobic-hydrophilic dipeptides. Emphasis is placed on crystal packing patterns and hydrogen-bond preferences.

Structures and photochemistry of dibenzobarrelene mono­amides

The photochemistry of 9,10-ethenoanthracene-11-monoamides has been studied and correlated with the crystal structures determined for two derivatives; photoproduct structures have been established from a crystal structure analysis of one of the products and from NMR correlations. Mechanisms are derived for the formation of three types of photoproduct: (i) the well known di-i-methane reaction [(2M)-type photoproduct]; (ii) a hydrogen abstraction process [(2H)]; (iii) β-lactam formation [(2L)].

Methyl group conformation-determining intermolecular C–H⋯O hydrogen bonds: structure of N-methyl-2-pyrrolidone

In crystalline N-methyl-2-pyrrolidone (NMP) the N-methyl group takes part in two intermolecular C-H⋯O hydrogen bonds, which cause a notable deviation from its usually preferred conformation with one C-H bond being almost eclipsed to the adjacent N-C amide bond. This is substantiated by force-field and ab initio calculations on isolated NMP molecules.

Molecular electric moments and electric field gradients from X-ray diffraction data: model studies

Model X-ray data sets are computed from ab initio molecular electron densities for 12 different molecular crystals. By the use of several multipole models, molecular dipole and second moments, as well as electric field gradients at each nuclear site, are computed and compared with those from the original ab initio wavefunctions. The results provide valuable insight into the reliability of these properties extracted in the same way from experimental X-ray data.

Hexa­methylene­tetramine suberate, a strongly anharmonic modulated structure

The incommensurate structure of hexamethylenetetramine suberate is characterized by large non-sinusoidal displacements of up to 1 Å. The modulation results from the constrained packing of the aliphatic chains with layers of hexamine.

Symmetry constraints, molecular recognition and crystal engineering. Comparative structural studies of urea–butanedioic and urea–E-butanedioic acid (2:1) cocrystals

The crystal structure of two urea-α,ω-dicarboxylic acid cocrystals (2:1) are determined. The geometry and the conformation of the comolecular units and their parent molecules are studied with respect to the three-dimensional network organization. The effective molecular recognition in crystals is demonstrated to be not only dynamically selective and functionally complementary, but also a toposelective and symmetry-constrained process.

Prediction of ferroelectricity in recent inorganic crystal structure database entries under space group pba2. Erratum

Correction to Acta Cryst. (1996), B52, 806-809.

Molecular modeling of inorganic compounds edited by P. Comba and T. W. Hambley

Fundamental principles of molecular modeling by W. Gans, A. Amann and J. C. A. Boeyens

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