Phase Relations between Temperature-Induced Phase Transition, Hysteresis and Bending Strain in an Elastically Flexible Molecular Crystal
Relations between structural properties and their variations due to phase transitions under variable thermodynamic conditions are 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 the mechanical properties of molecular crystals. Here, we report on a structural transition at T > 320 K that is accompanied by negative area expansion of the plane normal to the bending axis, and a nonlinear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles, the transition at Tc is found to be reversible; albeit, from 360 K ≤ T ≤ 380 K to T = 100 K, structurally irreversible states are observed that are characterized by hysteresis in the area of the planes and intermolecular distances. The phase transformation, as well as irreversibilities, are associated with significant crystal degradation, as suggested by increasing mosaicity as a function of T, which qualitatively indicates the evolution of defects in the crystals. 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 similarity in its order in the high-temperature phase, as well as those in the irreversible structural states, to that calculated from reported deformations during mechanical bending at T = 100 K.