SI-29 NMR STRUCTURAL STUDIES OF IONICALLY CONDUCTIVE SILICON CHALCOGENIDE GLASSES AND MODEL COMPOUNDS

The structure of ionically conductive glasses in the systems (Li2S)(x) - (SiS2)(1-x) (0.2 less than or equal to x less than or equal to 0.6), (Na2S)(x) - (SiS2)(1-x) (0.1 less than or equal to x less than or equal to 0.6), (Ag2S)(x) - (SiS2)(1-x) (0.5 less than or equal to x less than or equal to 0.6) and ((Li2S)(y) - (Na2S)(1-y))(0.5)(SiS2)(0.5)(0 less than or equal to y less than or equal to 1), (Li2Se)(x) - (SiSe2)(1-x) (0.23 less than or equal to x less than or equal to 0.70), and (Na2Se)(x) - (SiSe2)(1-x) (0.4 less than or equal to x less than or equal to 0.6), prepared by twin-roller quenching, is discussed on the basis of solid-state Si-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) results. As in the well known stoichiometrically analogous oxide systems, the Si-29 chemical shifts are sensitively affected by differences in the structural environments present. For both the sulfide and selenide systems, the NMR spectra permit easy distinction between corner- and edge-shared silicon tetrahedra. In addition, secondary chemical shift effects are observed, reflecting the number of bridging versus non-bridging chalcogen atoms. The sign and magnitude of these chemical shift trends can be rationalized on the basis of bond ionicities using a semi-empirical theory approach. The main conclusion concerning the structures of these glasses are the following. (1) The introduction of alkali chalcogenides into the network of silicon chalcogenide glasses generates non-bridging sulfur and selenium sites, with preferential destruction of edge-sharing SiX(4/2) tetrahedra. (2) The distribution of the non-bridging selenium sites is closer to random than to ordered. (3) The tendency of forming edge-sharing units decreases in the order S --> Se --> O and Na --> Li --> Ag. (4) Mixed Li-Na thiosilicate glasses are structurally more closely related to binary lithium thiosilicate glasses than to binary sodium thiosilicate glasses.