HEAT-CAPACITIES AND THERMODYNAMIC PROPERTIES OF CUXZN1-XSIF6.6H2O (X=1, 0.06, AND 0) FROM 14 TO 300 K

The heat capacities of CuSiF6·6H2O, ZnSiF6·6H2O, and their solid solution Cu0.06Zn0.94 SiF6·6H2O were determined by adiabatic calorimetry in the temperature range 14 to 300 K. The heat capacity at constant pressure Cpo(298.15 K), the entropy difference {So(298.15 K)-So(0)}, the enthalpy difference {Ho(298.15 K)-Ho(0)}, and the function -{Go(298.15 K)-Ho(0)} 298.15 K were found to be 373.5 J K-1 mol-1, 367.3 J K-1 mol-1, 59240 J mol-1, and 168.6 J K-1 mol-1, respectively, for CuSiF6·6H2O. For ZnSiF6·6H2O these functions are 428.6 J K-1 mol-1, 437.0 J K-1 mol-1, 69330 J mol-1, and 204.5 J K-1 mol-1, respectively. Thermodynamic properties of the solution Cu0.06Zn0.94SiF6·6H2O coincide with those for ZnSiF6·6H2O within experimental error. An anomaly in the heat capacity of CuSiF6·6H2O was observed at T = (294.9±0.2) K. The origin of the anomaly seems to be a phase transition (cooperative Jahn-Teller effect). © 1979.