Melting of orientational degrees of freedom

We use calorimetry and dilatometry under hydrostatic pressure, X-ray powder diffraction and available literature data in a series of composition-related orientationally disordered (plastic) crystals to characterize both the plastic and melting transitions and investigate relationships between associated thermodynamic properties. First, general common trends are identified: (i) The temperature range of stability of the plastic phase T (m) -T (t) (where T (t) and T (m) are the plastic and melting transition temperatures, respectively) increases with increasing pressure and (ii) both the rate of this increase, d(T (m) -T (t) )/dp, and the entropy change across the plastic transition analyzed as function of the ratio T (t) /T (m) are quite independent of the particular compound. However, the dependence of the entropy change at the melting transition on T (t) /T (m) at high pressures deviates from the behavior observed at normal pressure for these and other plastic crystals. Second, we find that the usual errors associated with the estimations of second-order contributions in the Clausius-Clapeyron equation are high and thus these terms can be disregarded in practice. Instead, we successfully test the validity of the Clausius-Clapeyron equation at high pressure from direct measurements.