Heat capacities and thermodynamic properties of Cu xZn 1− xSiF 6·6H 2O ( x=1, 0.06, and 0) from 14 to 300 K

Abstract

The heat capacities of CuSiF 6·6H 2O, ZnSiF 6·6H 2O, and their solid solution Cu 0.06Zn 0.94 SiF 6·6H 2O were determined by adiabatic calorimetry in the temperature range 14 to 300 K. The heat capacity at constant pressure C p o(298.15 K), the entropy difference { S o(298.15 K)− S o(0)}, the enthalpy difference { H o(298.15 K)− H o(0)}, and the function −{G o(298.15 K) −H o (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 CuSiF 6·6H 2O. For ZnSiF 6·6H 2O 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 Cu 0.06Zn 0.94SiF 6·6H 2O coincide with those for ZnSiF 6·6H 2O within experimental error. An anomaly in the heat capacity of CuSiF 6·6H 2O was observed at T = (294.9±0.2) K. The origin of the anomaly seems to be a phase transition (cooperative Jahn-Teller effect).