Heat capacity, thermodynamic properties, and transitions of silver iodide

Abstract

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 ([beta] to [alpha])- and ([beta]/[gamma] to [alpha])-transitions and the heat capacities of single crystals of [beta]-, of finely divided [beta]/[gamma]-, and of the [alpha]-phases were measured in adiabatic calorimetric cryostats and thermostats from 70 to 700 K and from 310 to 523 K. The values of Cp, m, Smo, and [Phi]mo at 298.15 K are 6.707R, 13.764R, and 8.511R for the [beta]-phase and since the molar volumes of [beta]- and [gamma]-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 [alpha]-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 [gamma]-AgI in one of them (below the transition temperature). The (partial) enthalpy of the ([beta]/[gamma] to [alpha])-phase transition was found to be (758.7 +/- 0.8)R [middle dot] K and that of the ([beta] to [alpha])-phase transition was found to be (758.0 +/- 0.3)R [middle dot] K over the range 405 to 425 K {mean: (758.4 +/- 0.4)R [middle dot] K}. The (total) enthalpy of transition is about 1150R [middle dot] 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 [beta]/[gamma]-phase, and in and beyond the transition region with those of numerous prior investigators.