Melting of orientational degrees of freedom

We use calorimetry and dilatometry under hydrostatic pres7 sure, X-ray powder diffraction and available literature data in a series 8 of composition-related orientationally disordered (plastic) crystals to 9 characterize both the plastic and melting transitions and investigate 10 relationships between associated thermodynamic properties. First, gen11 eral common trends are identified: (i) The temperature range of sta12 bility of the plastic phase Tm-Tt (where Tt and Tm are the plastic and 13 melting transition temperatures, respectively) increases with increas14 ing pressure and (ii) both the rate of this increase, d(Tm-Tt)/dp, and 15 the entropy change across the plastic transition analyzed as function 16 of the ratio Tt/Tm are quite independent of the particular compound. 17 However, the dependence of the entropy change at the melting transi18 tion on Tt/Tm at high pressures deviate from the behavior observed at 19 normal pressure for these and other plastic crystals. Second, we find 20 that the usual errors associated with the estimations of second-order 21 contributions in the Clausius-Clapeyron equation are high and thus 22 these terms can be disregarded in practice. Instead, we successfully 23 test the validity of the Clausius-Clapeyron equation at high pressure 24 from direct measurements. 25