The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN4 tetrahedra remain relatively rigid, albeit less so than SiO4 tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.

We use a combination of neutron and X-ray total scattering measurements together with pair distribution function (PDF) analysis to characterise the variation in local structure across the orbital order-disorder transition in LaMnO3. Our experimental data are inconsistent with a conventional order-disorder description of the transition, and reflect instead the existence of a discontinuous change in local structure between ordered and disordered states. Within the orbital-ordered regime, the neutron and X-ray PDFs are best described by a local structure model with the same local orbital arrangements as those observed in the average (long-range) crystal structure. We show that a variety of meaningfully-different local orbital arrangement models can give fits of comparable quality to the experimental PDFs collected within the disordered regime; nevertheless, our data show a subtle but consistent preference for the anisotropic Potts model proposed in Phys Rev. B 79, 174106 (2009). The key implications of this model are electronic and magnetic isotropy together with the loss of local inversion symmetry at the Mn site. We conclude with a critical assessment of the interpretation of PDF measurements when characterising local symmetry breaking in functional materials.

The molecular crystals adamantane, C H , and adamantanecarboxylic acid, C H COOH, undergo order-disorder phase transitions at 208 and 250 K, respectively. Reverse Monte Carlo refinement of total neutron scattering data collected from deuterated samples immediately above these phase transitions shows that the high-temperature phases are well described by models in which the adamantyl groups are disordered over two sites. No correlation between the orientations of neighbouring molecules is observed. These results demonstrate that the intermolecular potential energy of these materials depends strongly on the orientation of the reference molecule but only very weakly on the orientations of its neighbours.

Two crystallographically similar phases of a dielectric framework material differ markedly in the rigidity of the cyanide framework.