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 < 150 K. The curve of heat capacity against temperature is continuous and rises smoothly to T ≈ 140 K, where a small anomaly appears. Above T ≈ 150 K, the curve resumes its smooth path. The anomaly, whose source is unknown, is centred at T ≈ 143 K. The greatest difference of the curve from that defined by the curve of the lattice vibrations is 0.01·Cp. m, which lies outside the error in the measurements. The excess molar enthalpy associated with the anomaly amounts to Δ150 K135 KHom = (1.3±0.3)·R·K (R = 8.31451 J·K-1·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)·R.