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 eq ual 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 eq ual to 0.6), prepared by twin-roller quenching, is discussed on the ba sis of solid-state Si-29 magic angle spinning (MAS) nuclear magnetic r esonance (NMR) results. As in the well known stoichiometrically analog ous oxide systems, the Si-29 chemical shifts are sensitively affected by differences in the structural environments present. For both the su lfide and selenide systems, the NMR spectra permit easy distinction be tween corner- and edge-shared silicon tetrahedra. In addition, seconda ry chemical shift effects are observed, reflecting the number of bridg ing versus non-bridging chalcogen atoms. The sign and magnitude of the se chemical shift trends can be rationalized on the basis of bond ioni cities using a semi-empirical theory approach. The main conclusion con cerning the structures of these glasses are the following. (1) The int roduction of alkali chalcogenides into the network of silicon chalcoge nide glasses generates non-bridging sulfur and selenium sites, with pr eferential destruction of edge-sharing SiX(4/2) tetrahedra. (2) The di stribution 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 thio silicate glasses are structurally more closely related to binary lithi um thiosilicate glasses than to binary sodium thiosilicate glasses.