THE THERMODYNAMICS OF THE DIVALENT-METAL FLUORIDES .4. HEAT-CAPACITY OF MSNF4, M = PB, BA, OR SR, AT TEMPERATURES FROM 300-K TO 660-K

The low-temperature adiabatic calorimetric measurements on three fast-ion conductors have been extended to 660 K by differential scanning calorimetry (d.s.c.). For strontium tetrafluorostannate SrSnF4 the heat capacities from the d.s.c. merged smoothly with those from the adiabatic measurements and no phase transition was observed up to 660 K. The anomaly between 280 K and 350 K was confirmed and a second anomaly was found near 475 K coincident with changes in the spin-lattice relaxation time of 19F. The heat capacity of BaSnF4 rose smoothly from the anomaly between 210 K and 320 K noted previously in our adiabatic measurements. No evidence of a phase transition was observed up to 660 K, although the slope of the heat capacity against temperature curve increased above 510 K. In lead tetrafluorostannate PbSnF4 the transition from the monoclinic α-phase to tetragonal β-phase was complete at 365 K with an associated ΔtrsHom = (306.0±0.2) J·mol-1 and ΔtrsSom = (0.97±0.02) J·K-1·mol-1. The heat capacity then rose smoothly to 500 K where it increased sharply leading up to the tetragonal β-phase to cubic γ-phase transition at 600 K with ΔtrsHom = (2.53±0.02) kJ·mol-1 and ΔtrsSom = (4.03±0.05) J·K-1·mol-1. Thermodynamic functions are tabulated to 600 K for SrSnF4 and to 650 K for BaSnF4 and PbSnF4. © 1991 Academic Press Limited All rights reserved.