CALORIMETRIC STUDY OF PHASE-TRANSITIONS IN SEF6 AND WF6 CRYSTALS

The heat capacities of SeF6 and WF6 were measured with an adiabatic ca lorimeter in the temperature range between 6 and 290 K. SeF6 and WF6 t ransformed from the crystalline form to the plastically crystalline an d finally to the liquid form. Their temperatures and entropies are as follows: SeF6, T-trs = 134.50 +/- 0.02 K, Delta(trs)S = 28.12 +/- 0.01 J K-1 mol(-1), T-fus = 239.24 +/- 0.01 K, Delta(fus)S = 21.87 +/- 0.0 1 J K-1 mol(-1) WF6, T-trs = 264.95 +/- 0.02 K, Delta(trs)S = 36.50 +/ - 0.01 J K-1 mol(-1), T-fus = 275.00 +/- 0.02 K, Delta(fus)S = 14.88 /- 0.01 1 K-1 mol(-1). The sublimation enthalpy of SeF6 was also measu red at 205.10 K. The molar enthalpy and entropy of sublimation at 205. 10 K were 24.96 +/- 0.04 kJ mol(-1) and 121.7 +/- 0.2 J K-1 mol(-1), r espectively. The third-law entropy of SeF6 at 298.15 K and 10(5) Pa wa s calculated from the present results to be 312.9 +/- 0.5 J K-1 mol(-1 ). This value agrees well with the statistical value (313.6 +/- 0.4 J K-1 mol(-1)) calculated from the spectroscopic data, indicating that S eF6 becomes completely ordered at 0 K. The transition entropies of SF6 , SeF6, and WF6 were reproduced by a model considering the entropy ass ociated with the excited levels of the rotational vibration. It was fo und that the large transition entropy of the MF(6) family is due to th e broad potential surface caused by the orientational frustration inhe rent to the bcc structure of the high-temperature phase.