Superionic Conduction in Silver-Iodide and Silver Chromium-Sulfide

During the past few years there has been a renewed interest in the studies of solid electrolytes for two reasons: (i) they stand a high potential for technical applications and (ii) being somewhat intermediate between solids and liquids, they provide interesting systems for theoretical studies. The present investigation is in line with the latter of the above two reasonings. It deals with the influence of mutual interaction and concentration of the mobile Ag ions in the superionic conductor (alpha)-AgI on their distribution and transport. Also an exact calculation for the temperature dependence of the ionic conductivity of a two-dimensional superionic conductor AgCrS(,2) is presented. This study started with a calculation of the configurational entropy of silver ion distribution in the superionic conductor (alpha)-AgI, using the cluster variation method (CVM). Then, starting with a formulation of ionic conductivity based on Kubo's theory, as applied to a lattice gas model with nearest-neighboring hopping, an exact closed form expression for the dynamical current-current correlation functions was obtained. First this formulation was applied to the study of the conductivity of (alpha)-AgI, where the required occupation number correlation functions were evaluated using the CVM. Later, the above formulation was used for AgCrS(,2) which provides a prototype for the two-dimensional conduction on a half-filled honeycomb lattice. Here an exact calculation becomes possible in analogy to the Ising model. The configurational entropy results for (alpha) -AgI compare well with experiment and suggest the existence of strong short-range correlations among the silver ions. The ionic conductivity of (alpha)-AgI is found to increase with the nearest-neighbor repulsion between Ag ions until the nearest-neighbor sites of an occupied site become vacant. The concentration dependence of the conductivity seems to indicate that the naturally occurring composition (one Ag ion per six tetrahedral sites) of mobile ions happens to have the highest conductivity. For AgCrS(,2) the theoretically calculated temperature dependence of the ionic conductivity agrees fairly well with experiment. The inclusion of correlated hops might improve the agreement on the low temperature side.