THERMODYNAMIC AND RESONANCE STUDIES OF STRUCTURAL-CHANGES IN CRYSTALS

When disordered crystalline materials are cooled, they often show phase transitions to more ordered phases which are accompanied by rearrangements in structure and/or changes in the dynamics of the constituent molecules or molecular fragments. It then becomes necessary to study all of these aspects (structure, dynamics and thermodynamics) in order to understand the system more completely. We give examples of (a) the naphthalene-tetracyanobenzene system, where a disorder observed in the room-temperature structure led to extensive NMR and heat capacity investigations in order to obtain an understanding of the disordered and ordered phases; (b) the t-butylcalix[4]arene toluene clathrate, where a disordered room-temperature structure posed a number of problems regarding the symmetry and dynamic state of the system, and where subsequent NMR and heat-capacity measurements led to a reinvestigation of the structure and a better structural model; (c) a set of xenon cyclodextrin compounds with different hydration states where no structural information was available, and NMR and thermodynamic measurements were used to constuct a partial phase diagram.