Heat Capacity, Transformations, and Thermal Disorder in the Solid Electrolyte RbAg4I5

Abstract

The heat capacity of RbAg₄I₅ has been measured in an adiabatic calorimeter between 5 and 340°K. The compound undergoes transformations at 121.8 and at 209°K and exhibits a very sizable configurational heat capacity. The latent heat and entropy changes of the apparently first‐order transformation at 121.8°K are 190.9 ± 0.8 cal mole^− 1 and 1.568 ± 0.007 cal deg^− 1·mole^− 1, respectively. The second transformation appears to be of the lambda type. Upper limits to the heat and entropy changes are 225 cal deg^− 1 and 1.0 cal deg^− 1·mole^− 1, respectively. The configurational entropy is 10.81 cal deg^− 1·mole^− 1 at 300°K. A residual entropy of 2.25 ± 1.6 cal deg^− 1·mole^− 1 was found. The standard entropy for the compound is 147.07 cal deg^− 1·mole^− 1. The $\mathrm{\Delta S°,\; \Delta H°}$ , and $\mathrm{\Delta G°}$ of formation are 20.55 ± 2.2 cal deg^− 1·mole^− 1, − 136.2 ± 0.5 kcal mole^− 1, and − 142.3 ± 0.1 kcal mole^− 1, respectively. The heat capacity is exceptionally high at all temperatures above ∼50°K. This is attributed to a continuously increasing disorder among the silver ions as temperature is increased. A simple model for the disordering process based on incompletely filled sets of silver sites with different energy levels is suggested to account for the residual entropy and excess heat capacity.