Extended-x-ray-absorption-fine-structure investigation of mobile-ion density in superionic AgI, CuI, CuBr, and CuCl

Abstract

Extended-x-ray-absorption-fine-structure (EXAFS) data on the normal and superionic phases of AgI and the cuprous halides have been analyzed using four structural models: harmonic oscillator, displaced site, anharmonic oscillator, and excluded volume. The most satisfactory description is obtained with the last model, based upon a softened hard-sphere pair potential. The results indicate that the tetrahedral locations in the halogen lattice are preferred by the mobile cations, but that at elevated temperatures substantial cation density also occurs at bridging trigonal sites. For fcc immobile-ion lattices, significant octahedral occupation is also found at elevated temperatures. These results indicate that the likely conduction path between tetrahedral sites is in the $〈110〉$ directions for bcc materials and in the $〈111〉$ directions for fcc materials. The potential-energy barrier heights for conduction in various directions are obtained from the ion density. In addition, differences in the nature of the structural information probed by EXAFS and by diffraction techniques are discussed.