Heat Capacities of Cubic and Hexagonal Ammonium Hexafluosilicate from 25° to 300°K

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Abstract
Heat capacities of cubic and hexagonal ammonium hexafluosilicate were measured between 25° and 300°K. The hexagonal heat capacity showed an anomaly at 38.6°K with an entropy of transition of R ln2. This was attributed to an order—disorder reorientation of the anion. These heat‐capacity measurements, when combined with heat of solution measurements, gave a third‐law verification with ΔS0°=0.03±0.2 cal deg—1 mole—1 for the change in state (NH4)2SiF6(hex)=(NH4)2SiF6(cub). The heat capacities were analyzed to determine the motions of the ammonium ions in the two polymorphs. These analyses showed that the ammonium ions in the cubic modification are undergoing torsional oscillation with a frequency of 235 cm—1, and that the ammonium ions in the hexagonal modification are undergoing pseudofree three‐dimensional rotation at room temperature. The thermodynamic functions CP, S°, (H° — E0°)/T, and — (F° — E0°)/T were calculated. The values, in cal deg—1 mole—1 at 298.15°K are: 59.25, 67.99, 35.21, 32.78 for the cubic modification, and 54.52, 66.98, 33.93, 33.05 for the hexagonal. The entropy of the aqueous hexafluosilicate ion was determined to be 30.3±3 cal deg—1 mole—1.