29Si NMR structural studies of ionically conductive silicon chalcogenide glasses and model compounds

Abstract

The structure of ionically conductive glasses in the systems (Li 2 S) x (SiS 2 ) 1− x (0.2 ≤ x ≤ 0.6), (Na 2 S) x (SiS 2 ) 1− x (0.1 ≤ x ≤ 0.6), (Ag 2 S) x (SiS 2 ) 1− x (0.5 ≤ x ≤ 0.6) and (Li 2 S) y (Na 2 S) 1− y ) 0.5 (SiS 2 ) 0.5 (0 ≤ y ≤ 1), (Li 2 Se) x (SiSe 2 ) 1− x (0.23 ≤ x ≤ 0.70), and (Na 2 Se) x (SiSe 2 ) 1− x (0.4 ≤ x ≤ 0.6), prepared by twin-roller quenching, is discussed on the basis of solid-state 29 Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) results. As in the well known stoichiometrically analogous oxide systems, the 29 Si 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.