Heat Capacity at Constant Volume and Molecular Motion of Crystal, Orientationally Disordered Crystal and Liquid

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The present research aimed at making progress in the thermodynamic understanding on the coupling effects between orientational degrees of freedom and between them and the other degrees of freedom in orientationally disordered crystals.1) Effort was made to construct an automatic high-pressure calorimeter which measures simultaneously both heat capacity and volume at constant pressure to allow one to derive heat capacity at constant volume later from. The calorimeter is workable in the temperature range of 13 K to 500 K and enables high-precision thermometry of 10^<-5> K and heat capacity measurement with inaccuracy around 1 % at high pressures. We have so far succeeded in maintaining high pressure up to 200 MPa with its imprecision of 1 kPa within the high-pressure system. The remaining problem to be solved is in preparation of a thin sample-ampoule of indium which separates the sample and high pressure medium within the calorimeter cell.2) Orientationally disordered crystals, 2-haloge … More nothiophenes and pyridinium salts, were studied through heat capacity measurements under atmosphere. In the former crystals, glass transitions of one step and of two steps were observed in 2-bromothiophene and 2-chlorothiophene, respectively, and their properties changed continuously with a variation of composition between the two substances. These indicate that two reorientational processes couple with each other in both the dynamic and thermodynamic properties. In the latter cystals, pyridium iodide exhibited one phase transition while the chloride two phase transitions leading to the same high-temperature states in their orientational degrees of freedom. In pyridinium hexafluorophosphate, orientational disorderings of the cation and anion proceeded under strong coupling with each other showing only one phase transition as in the iodide. These suggest that the coupling effects stated above play quite important role in the evolution of orientational disordering both in the static and dynamic aspects. Less