Structure, phase transitions and ionic conductivity of K₃NdSi₆O₁₅·xH₂O. I. α-K₃NdSi₆O₁₅·2H₂O and its polymorphs

Reanalysis of the structure of the potassium ion conductor α-K₃NdSi₆O₁₅·xH₂O reveals it to be a hydrate. Structural features of its high-temperature polymorphs are described.

Structure, phase transitions and ionic conductivity of K₃NdSi₆O₁₅·xH₂O. II. Structure of β-K₃NdSi₆O₁₅

The new compound β-K₃NdSi₆O₁₅ is a phyllosilicate with a silicate sheet topology that is distinct from that of the essentially isocompositional compound α-K₃NdSi₆O₁₅·2H₂O. The compactness of its structure results in a low ionic conductivity.

Modulated structure of Ag₂SnO₃ studied by high-resolution electron microscopy

A model structure of Ag₂SnO₃ with a one-dimensional modulation has been elucidated by high-resolution electron microscopy, which is also supported by the `disorder' of the X-ray determined structure. The modulation occurs in 10–100 nm hexagonally twinned domains, which explains the apparent modulation in the three directions in electron diffraction patterns.

Structures and physical properties of ∊-FeSi-type and CsCl-type RuSi studied by first-principles pseudopotential calculations

The relative stability of the two known polymorphs of RuSi, which have the ∊-FeSi and CsCl structures, has been investigated by first-principles pseudopotential calculations. It appears that RuSi having a CsCl-type structure will be the thermodynamically most stable phase for pressures greater than 3.6 GPa. The calculated bulk moduli are 202 and 244 GPa for the ∊-FeSi and CsCl phases, respectively.

Symmetry and twins in the monophosphate tungsten bronze series (PO₂)₄(WO₃)_2m (2 ≤ m ≤ 14)

A structural law based on the mutual arrangement of WO₃-type layers is proposed to explain the symmetries and twins encountered in the series of monophosphate tungsten bronzes with pentagonal tunnels.

Structure of ν-Al_80.61Cr_10.71Fe_8.68, a giant hexagonal approximant of a quasicrystal determined by a combination of electron microscopy and X-ray diffraction

The structure of the hexagonal ν-Al_80.61Cr_10.71Fe_8.68 [a = 40.68 (7), c = 12.546 (1) Å with 131 unique atoms] was solved by direct methods based on the structural model of a large icosahedral cluster derived from high-resolution electron microscopic images.

Structures and phase transitions of the A₇PSe₆ (A = Ag, Cu) argyrodite-type ionic conductors. II. β- and γ-Cu₇PSe₆

The crystal structures of β- and γ-Cu₇PSe₆, two of the three polymorphic forms of the argyrodite Cu₇PSe₆ compound, heptacopper phosphorus hexaselenide, are determined from single-crystal X-ray diffraction data. The copper-ion diffusion paths in the high-temperature form and the copper distribution in the second phase are studied by means of a Gram–Charlier non-harmonic development of the Debye–Waller factor. The analysis reveals a partial localization of copper, at the phase transition, in different low-coordination selenium sites, stressing its relative stability in such selenium environments.

A new structure type in the hexagonal perovskite family; structure determination of the modulated misfit compound Sr_9/8TiS₃

The structure of a new hexagonal perovskite phase, Sr_9/8TiS₃, was determined from single-crystal X-ray diffraction data, using the (3 + 1)-dimensional formalism; various original crenel functions were used to describe the S and Sr atoms accurately. A new TiS₆ polyhedron was observed in the TiS₃ chains, differentiating the hexagonal perovskite sulfides from the oxide counterparts.

Structures of [Pd(NH₃)₂X₂] and its chemical transformation in the solid state

The crystal structures of five diammine-dihalide palladates are determined using an X-ray powder diffraction technique. The mechanism of the phase transition is discussed.

The tetragonal phase of Na_0.5Bi_0.5TiO₃ – a new variant of the perovskite structure

The structure of the tetragonal phase of the A-site substituted perovskite sodium bismuth titanate, Na_0.5Bi_0.5TiO₃, is determined by neutron powder diffraction at 698 K. In-phase tilts accompanied by anti-parallel cation displacements parallel to the polar c axis, space group P4bm, are observed. This unusual combination results in a new variant of the perovskite structure.

Redetermination of the crystal structures of the `Arppe compound' Bi₂₄O₃₁Cl₁₀ and the isomorphous Bi₂₄O₃₁Br₁₀

The crystal structures of Bi₂₄O₃₁X₁₀ (X = Cl, Br) have been redetermined; the previous structure model of simple construction has to be replaced by a more complicated one.

Bonding effects and the crystal structures of (NH₄)₂[Cu(H₂O)₆](SO₄)₂ and its H₂¹⁸O substituted form at 9.5 K

The crystal structures of (NH₄)₂[Cu(H₂O)₆](SO₄)₂ and of its form with isotopically substituted H₂¹⁸O are reported. They are very similar but differ in detail from the structure of the compound produced using deuterium oxide. This suggests that the influence of deuterium on the crystal structure is associated with the hydrogen-bonding interactions in the lattice rather than with its effect on the Jahn–Teller coupling through the increase in mass of the water ligand.

Structure correlation study of four-coordinate copper(I) and (II) complexes

The geometries of four-coordinate Cu^I and Cu^II complexes in the Cambridge Structural Database have been analysed systematically and compared using symmetry-deformation coordinates and principal component analysis.

The assignment and validation of metal oxidation states in the Cambridge Structural Database

A methodology for the semi-automatic assignment and checking of metal oxidation states for metallo-organic complexes in the Cambridge Structural Database, which uses both chemical connectivity and bond-length data via ligand donor group templates and bond-valence sums, is described. This method has been used to validate the oxidation states in 743 four-coordinate copper complexes with a ca 99% success rate.

Automated assignment of graph-set descriptors for crystallographically symmetric molecules

Algorithms for the automatic assignment of graph-set notation for intermolecular networks have been extended to molecules having internal crystallographic symmetry, for patterns up to the second level, and applied to a number of example molecules.

Terephthalate salts: salts of monopositive cations

The crystal structure of six salts of terephthalic acid (three of which were determined ab initio using powder data) indicate that electrostatic interactions among the anions are important in determining the crystal packing.

Metastable β-phase of benzo­phenone: independent structure determinations via X-ray powder diffraction and single crystal studies

Benzophenone was first identified as a polymorphic substance more than a century ago. In this paper the crystal structure of the metastable β-phase is presented for the first time and compared in detail with that of the well known stable phase.

Novel infinite three-dimensional network of neutral fullerene molecules in (C₆₀)₈(twin-TDAS)₆

The crystal structure of (C₆₀)₈(C₄S₆N₄)₆ has been redetermined in the correct space group (I23) and reveals a novel type of extended, neutral, fullerene network. Tetrahedral holes are identified with a stoichiometry of five tetrahedral holes for eight C₆₀ molecules.

Molecular coordination numbers in crystal structures of organic compounds

The calculation of molecular coordination numbers (MCNs) in the crystal structures of 23 067 organic compounds shows that Kitaigorodskii's close-packing model, assuming the predominance of MCN = 12, works correctly in only a few cases, whereas MCN = 14 is the most frequent. To explain this fact the close-packing model is extended with the model of the thinnest space covering by deformable molecules.

Crystal chemistry of some synthetic 2-oxa-steroids: conformation, packing motifs and isostructurality

The crystal structures of six 2-oxa-steroids are analysed in terms of O—H⋯O and C—H⋯O hydrogen bonding. The first example of O—H⋯O/C—H⋯O interaction mimicry and binary solid solution is reported.

On the inclusion of solvent molecules in the crystal structures of organic compounds

Cocrystallization of solvent molecules in the structures of organic and metalloorganic compounds has increased tremendously in the past 30 years. Relevant statistical data are presented, with special focus on structures that include two or more different types of solvent.

Structure predictions allowing more than one molecule in the asymmetric unit

In crystal structure prediction it is usually assumed that only one molecule is present in the asymmetric unit. The number of possible crystal structures increases enormously if more than one independent molecule is allowed. The possibilities and limitations of theoretical prediction for such structures are investigated.

Reactivity of molecular solids