Intermolecular interactions and augmented support in compressed crystal structures

Strong structural distortions in crystals exposed to high pressure provide a powerful tool for investigating the nature of cohesion forces and their impact on the macroscopic properties and thermodynamic behaviour of materials. Atomic-scale rearrangements releasing local strains can be regarded as the internal-reaction transformations reducing the external pressure stimuli, as described by Le Chatelier's principle. Monotonic transformations, phase transitions and chemical reactions tend to the augmented support, achieved by evenly distributing the strains among a possibly large number of bonds around. General structure-property relations can be systematically in-depth analysed for one compressed compound, as opposed to statistical analyses of data-mined populations of structures, where numerous random effects are involved. The increasingly frequent high-pressure investigations complement the library of monotonic and discontinuous structural adjustments countering the pressure stimuli. Particularly informative are phase transitions, spectacular property changes, the interplay of molecular size and shape with central and directional forces, counterintuitive transformations and those defying the rule of inverse effects of pressure and temperature. Alternative structures, molecular aggregations, interactions and properties can be in situ determined and applied for designing new functional materials. The interplay of external high-pressure stimuli and microscopic structural transformations, focused on intermolecular interactions, has been briefly discussed for a selection of molecular crystals, including ices, urea, benzene, halogenated benzene and imidazole, ferrocene and iodine. • High-pressure methods for investigating intermolecular forces in crystals. • Structure and macroscopic properties of compressed crystals. • Hierarchy of interactions and their changes under pressure and temperature. • Central and directional forces internally and externally squeezed and stretched. • Le Chatelier principle and its structural consequences.