A thermodynamic investigation of dynamical disorder in phase II of CBr4.

Abstract

CBr4 is an archetypal example of a pseudospherical mol. that can form an orientationally disordered phase (phase I, 320 K < T < 365 K). At lower temps., 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 dynamic disorder in phase II, the heat capacity of CBr4 was measured in the temp. range 30-305 K. No phase transitions were found in this region. The calcd. heat capacity, with contributions due to low-frequency translation and rotational-vibrations of the rigid CBr4 mols., as well as internal motions, fits the exptl. derived C? data to within 3%. Virtually full excitation of rigid mol. rotation-vibrations at very low temps. (?45 K) is consistent with the observation of a low-temp. elevation in the calcd. Grueneisen parameter.