Structural Studies of Superionic Conduction

Abstract

A solid electrolyte with ionic conductivity in excess of 10⁻² (ohm cm)⁻¹ is frequently referred to as a “fast ion” or “superionic” conductor. These materials have been studied extensively in recent years because of their remarkably high ionic conductivity and because of their technological promise as electrolytes in solid state batteries with higher energy and current densities at lower cost than conventional lead-acid batteries. Extended x-ray absorption fine structure (EXAFS) spectroscopy has been a very useful probe of superionic conductors. Analysis of EXAFS spectra from the normal and superionic phases of several cationic conductors has revealed a great deal about correlations between the highly mobile cations and the fixed anion lattice. The nearest neighbor peak in the cation-anion correlation function p _c−a(r) was found to be broad and asymmetric at high temperatures, necessitating the development of EXAFS data analysis techniques which could accommodate such peak shapes. It is becoming increasingly apparent that broad and asymmetric peaks are to be found in many materials, for which these EXAFS data analysis techniques should prove equally useful. The detailed information about p _c−a(r) revealed by the EXAFS studies has led to the elimination of many models which had been proposed for the structure, and for the conduction process as well. Furthermore, enough insight was gained into the underlying cation-anion interaction potentials to enable a calculation of the DC ionic conductivity as a function of temperature. The results of these EXAFS studies of Agl and the cuprous halides have been published in detail elsewhere [1,2], and are summarized below for the case of Agl.