Heat capacity, thermodynamic properties, and transitions of silver iodide

The heat capacity of silver iodide was measured using adiabatic calorimetric cryostats over the ranges 7 to 350 K and from 2 to 75 K. The (β to α)- and ( β γ to α)-transitions and the heat capacities of single crystals of β-, of finely divided β γ -, and of the α-phases were measured in adiabatic calorimetric cryostats and thermostats from 70 to 700 K and from 310 to 523 K. The values of C p, m, S m o, and Φ m o at 298.15 K are 6.707R, 13.764R, and 8.511R for the β-phase and since the molar volumes of β- and γ-phases are identical and the structures differ only in their stacking sequence, their heat capacities are essentially identical. Moreover, the molar transition-enthalpy increments to the α-phase of the two samples are also equal within the accuracy of the present measurements in spite of the structural differences occasioned by the presence of some γ-AgI in one of them (below the transition temperature). The (partial) enthalpy of the ( β γ to α)-phase transition was found to be (758.7 ± 0.8)R · K and that of the (β to α)-phase transition was found to be (758.0 ± 0.3)R · K over the range 405 to 425 K {mean: (758.4 ± 0.4)R · K}. The (total) enthalpy of transition is about 1150R · K. An abnormal trend in the heat capacity in the low-temperature region was noted. The heat-capacity values are compared with those of prior overlapping measurements by Nernst and Schwers, by Pitzer, and by Madison et al. for the β γ -phase , and in and beyond the transition region with those of numerous prior investigators.