Crystal structure of the non-stoichiometric argyrodite compound Ag7-xGeSe5I1-x (x=0.31).: A highly disordered silver superionic conducting material

Single crystals of the Ag6.69GeSe5I0.69 phase have been obtained by iodine transport of the iodine-partially substituted stoichiometric argyrbdite compound Ar7GeSe5I. This phase crystallizes in the cubic space group F (4) over bar 3m (argyrodite gamma -phase, A = 10.921(2) Angstrom at - 100 degreesC, A = 10.972(3) Angstrom at 25 degreesC, Z = 4). It is highly disordered both at anion and cation sites. Crystal structure refinements were completed by an anharmonic Gram-Charlier development of the atomic displacement factors of iodine and silver atoms. The structure of Ag6.69GeSe5I0.69 was determined at -100 degreesC and +25 degreesC and was refined to R(F) values of 5.80 and 6.51%, respectively. Both iodine and selenium (Sel) anions have been found disordered and iodine is slightly defective on its crystallographic site. This is correlated to the disorder observed for the two Ag1 and Ag2 cations that provides this material with superionic conducting properties. Analysis of the joint probability density function allowed the visualization of the Ag+ diffusion paths within the anionic framework. (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.