Melting of orientational degrees of freedom
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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.
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