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

摘要

The heat capacity of RbAg4I5has been measured in an adiabatic calorimeter between 5 and 340deg;K. The compound undergoes transformations at 121.8 and at 209deg;K and exhibits a very sizable configurational heat capacity. The latent heat and entropy changes of the apparently firsthyphen;order transformation at 121.8deg;K are 190.9thinsp;plusmn;thinsp;0.8 cal moleminus;thinsp;1and 1.568thinsp;plusmn;thinsp;0.007 cal degminus;thinsp;1middot;moleminus;thinsp;1, respectively. The second transformation appears to be of the lambda type. Upper limits to the heat and entropy changes are 225 cal degminus;thinsp;1and 1.0 cal degminus;thinsp;1middot;moleminus;thinsp;1, respectively. The configurational entropy is 10.81 cal degminus;thinsp;1middot;moleminus;thinsp;1at 300deg;K. A residual entropy of 2.25thinsp;plusmn;thinsp;1.6 cal degminus;thinsp;1middot;moleminus;thinsp;1was found. The standard entropy for the compound is 147.07 cal degminus;thinsp;1middot;moleminus;thinsp;1. TheDgr;Sdeg;, Dgr;Hdeg;, andDgr;Gdeg;of formation are 20.55thinsp;plusmn;thinsp;2.2 cal degminus;thinsp;1middot;moleminus;thinsp;1, minus;thinsp;136.2thinsp;plusmn;thinsp;0.5 kcal moleminus;thinsp;1, and minus;thinsp;142.3thinsp;plusmn;thinsp;0.1 kcal moleminus;thinsp;1, respectively. The heat capacity is exceptionally high at all temperatures above sim;50deg;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.