HEAT-CAPACITY MEASUREMENTS AND THERMODYNAMIC FUNCTIONS OF LANGBEINITE-TYPE K2CD2(SO4)3 AND K2CA2(SO4)3 AT TEMPERATURES FROM 10-K TO 340-K

Heat-capacity measurements were made with an adiabatic calorimeter on well characterized crystalline samples of K2Cd2(SO4)3 and of K2Ca2(SO4)3 at temperatures T from 12 K to 345 K and from 7 K to 340 K, respectively. Smoothed values of the standard molar thermodynamic properties of these materials are presented at round temperatures between 10 K and 340 K. At T = 298.15 K, the heat capacity and entropy are (343.6 ± 0.7) J · K-1 · mol-1 and (436.9 ± 0.8) J · K-1 · mol-1 for K2Cd2(SO4)3, and (338.9 ± 0.7) J · K-1·mol-1 and (416.2 ± 0.8) J · K-1 mol-1 for K2Ca2(SO4)3. The phase transitions for these two compounds have been examined in the temperature range from 300 K to 510 K by d.s.c. Both materials show a single peak in the d.s.c. curve at the temperatures of the cubic-(P213)-to-orthorhombic-(P212121) phase transition. The transition temperatures and enthalpies of transition are (430.7 ± 0.2) K and (2.19 ± 0.03) kJ · mol-1, and (474.5 ± 0.1) K and (2.12 ± 0.02) kJ·mol-1, respectively, for K2Cd2(SO4)3 and K2Ca2(SO4)3. © 1994.