DISTRIBUTION AND IONIC-DIFFUSION PATH OF SILVER IN GAMMA-AG8GETE6 - A TEMPERATURE-DEPENDENT ANHARMONIC SINGLE-CRYSTAL STRUCTURE STUDY

The localization and diffusion of silver species in the high temperature allotropic form γ-Ag8GeTe6 (space group F4̄ 3m, a = 1156.56(4)pm, Z = 4) is revisited from temperature dependent X-ray data with the aid of refinements including anharmonic thermal motions (Gram-Charlier expansion of the structure factor). γ-Ag8GeTe6 is built upon the stacking of GeTe4 tetrahedral units and free tellurium atoms that manages five different tetrahedral sites available for silver ions. A coherent silver distribution over the voids is evidenced by testing three models corresponding to different tensor orders for the atomic displacement parameters (ADP). Three diffusion paths between the preponderant silver sites are revealed by means of joint probability density functions (jpdf) and it is shown through the temperature dependence of those jpdf that the silver disorder is of the positional type. © 1993 Academic Press, Inc.