Eur. Phys. J. Special Topics 226, 1017-1029 (2017)
https://doi.org/10.1140/epjst/e2016-60315-4

Regular Article

Melting of orientational degrees of freedom

Departament de Física, ETSEIB, Universitat Politècnica de Catalunya, Diagonal 647, Barcelona, 08028 Catalonia, Spain

^a e-mail: pol.lloveras@upc.edu

Received: 30 September 2016
Revised: 17 October 2016
Published online: 18 April 2017

Abstract

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.