CALORIMETRIC, SPECTROSCOPIC AND DIELECTRIC STUDIES OF A PHASE-TRANSITION IN METHYLAMMONIUM HEXACHLOROPLATINATE

The heat capacity of (CH3NH3)2[PtCl6] was measured from 15 to 300 K. A phase transition was found at T(c) = 123.82 +/- 0.05 K. An anomalous lambda-type heat capacity was separated from the vibrational baseline using normal mode frequencies of the ions and lattice vibrations. The enthalpy and entropy of the transition are 1.49 kJ mol-1 and 14.5 J K-1 mol-1 respectively. The excess heat capacity DELTA-C diverged at T(c) following the equation DELTA-C = A\(T - T(c)\T(c)\-alpha where A = 24.3 +/- 0.4 J K-1 mol-1, T(c) = 123.87 +/- 0.01 K, alpha = 0.221 +/- 0.004 for T < T(c) and A = 3.0 +/- 0.1 J K-1 mol-1, T(c) = 123.77 +/- 0.02 K, alpha = 0.36 +/- 0.01 for T > T(c). The methylammonium ion rotates rapidly about its molecular axis, as deduced from IR line broadening, the orientational correlation time being about 1 ps at 140 K. The activation energy of the reorientation was 3.5 +/- 0.1 kJ mol-1 as derived from three different modes of vibration of CH3NH3+ and CH3ND3+. The dielectric permittivity followed the excess entropy, but not the excess enthalpy, when plotted as a function of temperature in a normalized form. In contrast with Curie-Weiss behaviour, most of the change in permittivity associated with the transition occurred below T(c). This may involve a hitherto unknown mechanism of dielectric response of a molecular ionic system.