A THERMODYNAMIC INVESTIGATION OF DYNAMICAL DISORDER IN PHASE-II OF CBR4

CBr4 is an archetypal example of a pseudospherical molecule that can form an orientationally disordered phase (phase I, 320 K<T<365 K). At lower temperatures, phase II is the stable phase; although it must be more ordered than phase I, different techniques give conflicting results concerning order in phase II. In order to investigate dynamical disorder in phase II, the heat capacity of CBr4 has been measured in the temperature range 30-305 K. No phase transitions were found in this region. The calculated heat capacity, with contributions due to low-frequency translation and rotational-vibrations of the rigid CBr4 molecules, as well as internal motions, fits the experimentally derived C-v data to within 3%. Virtually full excitation of rigid molecule rotation-vibrations at very low temperatures (similar to 45 K) is consistent with the observation of a low-temperature elevation in the calculated Gruneisen parameter. (C) 1995 American Institute of Physics.