Calorimetric, spectroscopic and dielectric studies of a phase transition in methylammonium hexachloroplatinate

Abstract

The heat capacity of (CH 3NH 3) 2[PtCl 6] was measured from 15 to 300 K. A phase transition was found at T c = 123.82±0.05 K. An anomalous λ-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 Δ C diverged at T c following the equation Δ C = A¦( T- T c/ T c¦ −α where A = 24.3±0.4 J K −1 mol −1, T c = 123.87±0.01 K, α = 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, α = 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 CH 3NH 3 + and (CH 3ND 3 +. 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.