where T is temperature and F = U -T S is the Helmholtz free energy. The balance between enthalpy and entropy underpins many crystal processes, including defect formation and the miscibility limits for solid solutions of two or more crystals. As noted by Dunitz, enthalpyentropy compensation is a general feature of chemical reactions, and phase transitions in particular. For most crystals, the dominant entropy term is vibrational and there are several descriptions to consider.