The structures and phase transitions to incommensurate structures in Rb₂V₃O₈ and K₂V₃O₈ mixed-valence vanadate fresnoites are studied with synchrotron single-crystal diffraction at low temperatures and ambient pressure. Although mixed satellite reflections are absent, the modulated structure of K₂V₃O₈ below 115 K is better described in (3 + 2)- than in (3 + 1)-dimensional space. The geometries of the VO₄ and VO₅ building units are rigid and it is mainly slight rotations of these polyhedra and small variation of the intermediate K—O distances that are modulated. Prolonged exposure to the high-brilliance synchrotron beam suppresses the incommensurate phase. The previously postulated phase transition to the incommensurate phase in Rb₂V₃O₈ at 270 K was not observed. One of the reasons could be that the intense radiation also affects the modulation in this material. Strategies to collect and analyse single-crystal diffraction data measured with very intense synchrotron radiation using modern low-noise pixel area detectors are discussed.

The magnetic structure of chromium arsenide CrAs is studied with neutron powder diffraction at ambient pressure in the temperature range 1.5–300 K as well as with neutron single-crystal diffraction at 2 K and 0.12 GPa. The material undergoes an anti-isostructural phase transition at T _N = 267 K and atmospheric conditions, in which both orthorhombic phases have the same space-group symmetry (Pnma, Z = 4) but different distortions of the parent hexagonal structure of the NiAs type (P6₃/mmc, Z = 2). The magnetic structure below T _N is incommensurate with the propagation vector k = (0, 0, k _c). At ambient pressure, the component k _c decreases from k _c = 0.3807 (7) at 260 K to k _c = 0.3531 (6) at 50 K. Below this temperature, it is basically constant. With increasing pressure at 2 K, k _c is also constant within standard uncertainties [k _c = 0.353 (2)]. For the analysis of the magnetic structure, a group-theoretical approach based on the space group of the nuclear structure and its subgroups is used. To avoid falling into false minima in the refinements, a random search for magnetic moments in the models is implemented. In the literature, the magnetic structure has been determined on the basis of powder diffraction data as a double helix propagating along the c axis. Although this double-helical model leads to satisfactory agreement factors for our powder data, it does not reproduce the intensities of the magnetic satellite reflections measured on single-crystal data in a satisfactory way and can therefore be discarded. Instead, several other models are found that lead to better agreement. Each of them is spiral-like with directional components in all three directions and with no spin-density wave character that would cause a non-constant magnetic moment. In all these models, the ordering of the spins is neither a pure helix nor a pure cycloid. Instead, the unit vectors of the spin rotation planes make an angle α, 0° < α < 90°, with respect to the c* direction. The model in superspace group P2₁.1′(α0γ)0s yields the best agreement factors in the refinements of the neutron single-crystal and powder diffraction data. This model is unique as it is the only one in which all the magnetic moments rotate with the same chirality.

The discovery of an in-situ method to grow flexible single crystals of methanol solvated cocrystals of caffeine and 4-chloro-3-nitrobenzoic acid by desolvation-resolvation of the solvent-grown material is reported. The in-situ obtained crystals are isomorphous to the starting Form with orthorhombic space group symmetry Fdd2, show similar habit and demonstrate macroscopic flexibility but possess greater unit cell volume. Crystal structures of the in-situ Form obtained employing synchrotron radiation suggest that the solvent channels are enlarged by an additional ~½ methanol molecule which leads to elongation of the unit cell axis parallel to the crystal growth direction. The new in-situ Form conserves the hydrogen bond geometries of the solvent grown material, while caffeine–coformer dimers along π···π stacks parallel to methanol channels are re-oriented by increased intermolecular distances and tilt of their mean planes. The most striking impact of the greater density of solvent molecules is in the thermal expansion of the in-situ crystals which is ~ 73 % lower than that of the solvent grown Form. The present results demonstrate the strength of in-situ techniques which not only depicts an unprecedented “breathing property” of these elastic crystals similar to zeolites, MOFs and COFs but also the potential of tuning their thermal response.

Relations between structural properties and their variations due to phase transitions under variable thermodynamic conditions is of central interest in material science. More than a decade ago, the discovery of elastic bending in single crystals of caffeine cocrystal solvates opened up a new area of research in mechanical properties of molecular crystals. Here we report structural transition at T > 320 K that is accompanied by negative area expansion of the plane normal to the bending axis and non-linear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles from 360 K ≤ T ≤ 380 K to T = 100 K, three structurally irreversible states are discovered that are characterized by hysteresis in the area of the planes and intermolecular distances. All the phase transformations are associated with significant crystal degradation as suggested by increasing mosaicity but not decomposition. These results suggest that the loss of elastic flexibility above room temperature is governed by increased crystal defects rather than previously proposed loss of solvent molecules from the structure. During these various thermal processes, the acid-base dimers rotate as well as move. The ratio (η) between variation in rotations and distances reveals a striking similarity in its order in the high T phase as well as those of irreversible structural states to that calculated from reported deformations during mechanical bending at T = 100 K.

The pressure dependence of structural behavior in the orthorhombic (Pccn, PI) and monoclinic (P2₁/c, PII) polymorphs of the compound [Fe(PM-BiA)₂(NCS)₂], where PM–BiA = (N–(2′–pyridylmethylene)–4-amino–bi–pheynyl), is studied with synchrotron single-crystal X-ray diffraction and vibrational spectroscopy. Both polymorphs are stable up to ∼1.5 GPa, with a spin state transition occurring only in polymorph PII under hydrostatic conditions as documented by single-crystal synchrotron diffraction. The diffraction data also provide evidence of the formation of superstructures for both PI, with a doubled c axis, and PII, with a doubled b axis, on applying pressures above 2 GPa. The LS and HS states seem to coexist at high-pressures for both polymorphs studied with synchrotron infrared spectroscopy at quasi-hydrostatic conditions. Such results indicate that the occurrence of spin-crossover transformations in [Fe(PM-BiA)₂(NCS)₂] might strongly depend on the stress in the sample.