Approaches to crystal structure landscape exploration

Three approaches to the exploration of the crystal structure landscape are outlined. These are crystal structure prediction, non-ambient crystallography and charge density analysis.

Ascertaining the correctness of the assigned central 3d transition metal in coordination complexes

A commentary is given on the article by Dittrich et al. [Acta Cryst. (2017), B73, 794–804] in this issue.

Qu­antitative analysis of solid-state diversity in tri­fluoro­methyl­ated phenylhydrazones

The crystal structures of six tri­fluoro­methyl­ated phenyl­hydrazones have been studied in detail, particularly in terms of the solid-state diversity (such as polymorphism and isostructurality) associated with different supramolecular architectures with H/CF₃ substitution in the absence of any strong hydrogen bonding. The most stabilizing molecular building blocks have a higher dispersion contribution due to the greater overlap between molecular surfaces, leading to an increase in the magnitude of van der Waals interactions.

Using invariom modelling to distinguish correct and incorrect central atoms in `duplicate structures' with neighbouring 3d elements

Aspherical atom refinement with conventional data sets is now possible for coordination compounds. As example structures, a number of pairs of published structures, where the element-type assignment of the metal was unclear, have been re-investigated. Identification of which structure is correct can be made from the deposited Bragg intensities alone.

Qu­antitative characterization of new supramolecular synthons involving fluorine atoms in the crystal structures of di- and tetrafluorinated benzamides

New supramolecular synthons in crystal structures involving organic fluorine have been observed and characterized. The nature, energetics and topological properties of these contacts derived from electron-density studies establish their bonding character. The enhanced acidic nature of the participating H atoms is responsible for short, strong and highly directional contacts with fluorine. Their hydrogen-bonding character is thus established unequivocally, with a pronounced modification of the features associated with crystal packing.

Structural changes of relaxor ferroelectric Sr_0.52Ba_0.48Nb₂O₆ (SBN52) on quenching and reheating

Quenching enhances the disorder of the cation distribution in Sr_0.52Ba_0.48Nb₂O₆ crystals which leads to higher spontaneous electric polarization. The amplitudes of the incommensurate modulations are also increased.

Evolution of the α-BaMg(CO₃)₂ low-temperature superstructure and the tricritical nature of its α–β phase transition

Using a high-sensitivity silicon detector and a micro-focus source, the norsethite [BaMg(CO₃)₂] low-temperature superstructure has been tracked across the temperature-driven α–β phase transition using single-crystal diffraction experiments. Systematic analysis of physical potential order parameters reveals the structure-related driving force and that the transition is tricritical in nature.

Incommensurately modulated structure of morpholinium tetra­fluoroborate and configurational versus chemical entropies at the incommensurate and lock-in phase transitions

The incommensurately modulated crystal structure of [C₄H₁₀NO]⁺[BF₄]⁻ involves partial ordering of [BF₄]⁻ along with conformational changes of the morpholinium cation. Both features are essential for the phase transition. A pure order–disorder transition is excluded on the basis of computations of the configurational entropy of the incommensurate structure.

Phase transition sequences in tetra­methyl­ammonium tetra­chloro­metallates by X-ray diffraction and spectroscopic measurements

Structures and phase sequences for two tetra­methyl­ammonium tetra­chloro­metallate compounds have been determined as a function of temperature. Crystal-packing analysis and first-principles calculations were used to rationalize the experimentally determined sequence.

A commensurately modulated structure of parabutlerite, Fe^IIISO₄(OH)·2H₂O

The structure of the ferric sulfate mineral parabutlerite was found to be commensurately modulated and was refined using a superspace approach as a commensurate (3 + 1)-dimensional case. Modulations are connected with accommodation of the best achievable geometry for the hydrogen bonds.

Effect of cationic substitution on the double-well hydrogen-bond potential in [K_1−x(NH₄)_x]₃H(SO₄)₂ proton conductors: a single-crystal neutron diffraction study

Single-crystal neutron diffraction was performed on [K_1−x(NH₄)_x]₃H(SO₄)₂ in order to obtain the exact hydrogen-bonding network. A correlation between the kinetics of the superprotonic phase transition in the crystalline state and hydrogen-bond strength is described.

Crystal structure of cobalt hydroxide carbonate Co₂CO₃(OH)₂: density functional theory and X-ray diffraction investigation

The crystal structure of cobalt carbonate hydroxide Co₂CO₃(OH)₂, a solid important in materials and environmental science, is investigated using density functional theory (DFT) simulations and powder X-ray diffraction (PXRD) measurements.

A note on the distortion theorem

Theoretical analysis of the bond length–bond valence correlation yielded an extension of the distortion theorem.

Temperature-induced order–disorder structural phase transitions of two-dimensional isostructural hexa­methyl­ene­tetramine co-crystals

This paper describes the single-crystal-to-single-crystal phase transitions triggered by molecular rotator ordering and structural order–disorder transformation in the two-dimensionally isostructural but energetically similar co-crystals, which are hexa­methyl­enetetramine–benzoic acid (1/2) and hexa­methyl­enetetramine–4-methyl­benzoic acid (1/2) at the transition temperatures of 257.5 (5) and 265.5 (5) K, respectively.

Comparison of the crystal structures and thermochemistry of a novel soluble guanylate cyclase stimulator riociguat and its solvates

Intermolecular interactions and molecular conformations are compared for riociguat molecules in the solvent-free drug [form (II)] and three different solvate structures. Desolvation of solvates result in form (I) of the riociguat.

Crystal structure of the OH-dominant gadolinite-(Y) analogue (Y,Ca)₂(Fe,□)Be₂Si₂O₈(OH,O)₂ from Heftetjern pegmatite, Norway

A hydroxyl-dominant analogue of gadolinite-(Y) has been discovered in the Heftetjern granitic pegmatite, southern Norway. Data are provided here on the chemical composition, physical properties and crystal structure of this mineral, as well as its relationships with other members of the gadolinite–datolite group.

Crystal chemistry and the role of ionic radius in rare earth tetrasilicates: Ba₂RE₂Si₄O₁₂F₂ (RE = Er³⁺–Lu³⁺) and Ba₂RE₂Si₄O₁₃ (RE = La³⁺–Ho³⁺)

Multiple structure types were found in a systematic study of barium rare earth tetrasilicates. The ionic radius of the rare earth element and the presence of fluorine in the crystal structures direct both the phase stability and the structural dimensionality.

Markedly different molecular formation in DPP-based small-molecule solar cells probed by grazing-incidence wide-angle X-ray scattering

Two small molecules having different alkyl side chains showed considerably different crystalline structures, resulting in different device performances.

Low-temperature behaviour of K₂Sc[Si₂O₆]F: determination of the lock-in phase and its relationships with fresnoite- and melilite-type compounds

The commensurate lock-in phase of K₂Sc[Si₂O₆]F has been solved and refined from X-ray single-crystal diffraction data collected at 25 K. The low-temperature structure is characterized by octagonal clusters closely resembling those observed in the incommensurate structure of fresnoite- and melilite-type compounds.

The structure-directing amine changes everything: structures and optical properties of two-dimensional thio­stannates

The structures of two semiconducting thio­stannates (R₂Sn₃S₇, R is an organic cation) were investigated in detail using single-crystal X-ray diffraction, solid-state NMR spectroscopy and electron microscopy. The atomic level structures of the compounds are discussed in relation to their different light absorption properties.

Non-order–disorder allotwinning of the rhenium pincer complex cis-Re[(PNP^CH₂-iPr)(CO)₂Cl]

Crystals of cis-Re[(PNP^CH₂-iPr)(CO)₂Cl] are made up of two distinct non-order–disorder polytypes.

Multidimensional structural variation in the cyano­trichite family of merotypes: camerolaite-3b-F

This is the first refined example of a cyano­trichite-group mineral in which long-range order of interlayer anions produces a superstructure.

Empirical Lewis acid strengths for 135 cations bonded to oxygen

Lewis acid strengths are listed for 135 cations bonded to oxygen for use with published Lewis base strengths, and a new correlation between Lewis acid strength and ionization energy is shown.

Predicting displacements of octahedral cations in ferroelectric perovskites using machine learning

A machine learning approach is developed to rapidly predict the displacements of previously unexplored octahedral cations in PbTiO₃-based ferroelectric perovskites. Thus, expanding the knowledge base on ionic displacement data, which are important for the rational design of novel ferroelectric perovskites.

Non-covalent interactions in the multicomponent crystal of 1-amino­cyclo­pentane carb­oxy­lic acid, oxalic acid and water: a crystallographic and a theoretical approach

Single-crystal X-ray diffraction, PBE-DFT optimization under periodic boundary conditions, and Atoms In Molecules and electron localization function theories were used to examine in detail non-covalent interactions in the [2:1:1] multicomponent crystal of 1,1-amino­cyclo­pentane­carb­oxy­lic acid:oxalic acid:water. Interactions were scaled in magnitude, visualized within the crystal with Non-Covalent Index isosurfaces and compared with those rendered by X-ray diffraction geometrical analysis.

3-(Pyridin-4-yl)acetyl­acetone: a donor ligand towards mercury(II) halides and a versatile linker for complex materials

3-(Pyridin-4-yl)acetyl­acetone (HacacPy) may be used as an N-donor ligand in coordination chemistry and in thermal degradation. After deprotonation of the acetyl­acetone moiety in HacacPy, its metal complexes, Fe(acacPy)₃ and Al(acacPy)₃, can act as poly(pyridine) bridges between hard and soft metal cations.

Investigations of the phase relations among e1, e2 and C structures of Na-rich plagioclase feldspars: a single-crystal X-ray diffraction study

Structure refinement of several e-plagioclase crystals reveals the ordering sequence and phase relations of Na-rich plagioclase feldspars, pushing one step closer towards solving the plagioclase phase diagram.

First-line antituberculosis drug, pyrazinamide, its pharmaceutically relevant cocrystals and a salt

Pharmaceutical cocrystals of the antituberculosis drug pyrazinamide with substituted hy­droxy­acids (2,4-di­hydroxy­benzoic acid; 2,6-di­hydroxy­benzoic acid; 3,5-di­hydroxy­benzoic acid and ferulic acid) and a molecular salt with p-toluene­sulfonic acid have been synthesized using both mechanical grinding as well as solution crystallization. This is the first report of the pyrazinamide salt.

Symmetry relationships between crystal structures

Imperfections in crystalline solids