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 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.