Thermodynamics of divalent-metal fluorides VI. Heat capacity and thermodynamic properties of the fast-ion conductor CaSn2F6 at temperatures from 6 K to 310 K

Abstract

The molar heat capacity Cp. m of calcium hexafluorostannate CaSn2F6 was measured at temperatures T from 6 K to 310 K by adiabatic calorimetry. Reproducible heat capacities were obtained only after cooling the sample twice to T T [approximate] 140 K, where a small anomaly appears. Above T [approximate] 150 K, the curve resumes its smooth path. The anomaly, whose source is unknown, is centred at T [approximate] 143 K. The greatest difference of the curve from that defined by the curve of the lattice vibrations is 0.01[middle dot]Cp. m, which lies outside the error in the measurements. The excess molar enthalpy associated with the anomaly amounts to [Delta]150 K135 KHom = (1.3+/-0.3)[middle dot]R[middle dot]K (R = 8.31451 J[middle dot]K-1[middle dot]mol-3), while the excess entropy is zero within the experimental error of the measurements. Smoothed values of the standard thermodynamic quantities for pure CaSn2F6 are tabulated up to T = 310 K and Som(298.15 K) = (33.16+/-0.03)[middle dot]R.