Heat Capacity of Silver Iodide. II. Theory

Experiment has shown that the specific heat of stoichiometric α‐AgI is abnormally large, $C_{\upsilon }$ ranging from 18 cal/mole·°C at 150°C to 26 cal/mole·°C at 400°C. This result is explained as an order–disorder transition among silver ions and silver ion vacancies on six sublattices in the structure. Because of the complex nature of the lattice sites, each of which consists of a cluster of seven nonequivalent potential wells, there is also a major anomalous contribution to the specific heat from the vibrational partition function. In addition there may be a small contribution from a coupled disordering of iodide ions. The behavior of nonstoichiometric α‐AgI, which has $C_{\upsilon }\text{\hspace{0.17em}≈\hspace{0.17em}12}\mathrm{cal}\mathrm{\hspace{0.17em}/\hspace{0.17em}}\mathrm{mole}\mathrm{·°}\mathrm{C}$ , is attributed to essentially complete disordering above the phase transition at 150°C together with a modification of the vibrational potentials.