A review of structural studies of superionic conducting borate and semiconducting vanadate glasses by Mössbauer spectroscopy and DTA

Abstract

Mössbauer and DTA studies of superionic conducting silver borate glasses, containing AgX (X = Cl, Br, and I) and a small amount of Fe 3+ ions as a Mössbauer probe, suggest that composition dependences of ionic conductivity due to mobile Ag + ions are closely correlated with the structural role of halide ions. In the case of superionic conducting borate glasses containing AgI, ionic conductivity shows an increase/decrease with increasing/decreasing free volume in the glass matrix. Mössbauer studies of semiconducting alkali (Li, Na, and K), alkaline earth (Mg and Ba), and other vanadate glasses are reviewed in connection with the coordination number of Fe 3+ ions incorporated into these vanadate glasses. The isomer shift of Fe 3+ ions, present at substitutional sites of tetrahedral V 5+ or V 4+ ions, is less than 0.40 mm s −1 with respect to metallic iron. This is similar to a well-known case for the Fe 3+ ions incorporated into borate and silicate glasses. Alkali vanadate glasses undergo a gradual change from a two-dimensional layer structure, originally composed of VO 5 tetragonal pyramids, to a chain structure composed of VO 4 tetrahedra having nonbridging oxygen (NBO) atoms. On the other hand, the structure of alkaline earth and phosphorus vanadate glasses gradually changes from the original layer structure to a three-dimensional network structure composed of VO 4 tetrahedra having few NBO atoms.