Chemical Short-Range Order in Selenide and Telluride Glasses

The structure of Ge20SbxSe80-x (x = 5, 15, 20) glasses was investigated by neutron diffraction, X-ray diffraction, and extended X-ray fine structure measurements at the Ge, Sb, and Se K-edges. For each composition, large-scale structural models were obtained by fitting simultaneously the experimental data sets in the framework of the reverse Monte Carlo simulation technique. It was found that the structures of these glasses can be described mostly by the chemically ordered network model. Ge-Se and Sb-Se bonds are preferred; Se-Se bonds in the Se-poor composition (x = 20) and M-M (M = Ge, Sb) bonds in strongly Se-rich glass (x = 5) are not needed. The quality of the fits was significantly improved by introducing Ge-Ge bonding in the nearly stoichiometric composition (x = 15), showing a violation of chemical ordering. The structure of Ge20SbxSe80-x was compared to that of several glasses from the three analogous systems (Ge-As-Se, Ge-As-Te, Ge-Sb-Te), and it was found that chemical short-range order becomes more pronounced upon substituting As with Sb and Se with Te. Ge-As-Se glasses behave as random covalent networks over a very broad composition range. Chemical short-range order and disorder coexist in both Te-rich and Te-poor Ge-As-Te glasses, whereas amorphous Ge14Sb29Te57 and Ge22Sb22Te56 are governed by strict chemical preferences.