Application of Mössbauer Spectroscopy to a Structural Study of Superionic Conducting AgI–Ag2O–B2O3 Glasses

Abstract

Abstract A structural study of silver borate glasses containing 20 mol% AgI and 1 mol% 57Fe2O3 has been performed by Mössbauer spectroscopy. A Mössbauer spectrum of these glasses consists of an asymmetric quadrupole doublet with an isomer shift smaller than 0.4 mm s−1 with respect to metallic iron. This indicates that the individual Fe3+ ions are present at the substitutional sites of the tetrahedral boron atoms constituting BO4 units as a network former. An increase in the Ag2O content results in continuous decreases in the isomer shift and the quadrupole splitting when the Ag2O content exceeds about 16 mol%, which corresponds to the Ag2O/B2O3 ratio of 0.25 and the alkali oxide content of 20 mol% in the binary borate glasses. These results are ascribed to the formation of nonbridging oxygen atoms in the tetrahedral BO4 and FeO4 units. A glass transition temperature (Tg) of the glasses shows a distinct composition dependency, having a maximum around 16 mol% Ag2O. These lead to the conclusions that planar BO3 units are changed into the tetrahedral BO4 units in the lower Ag2O content region and that the nonbridging oxygen atoms are formed in the BO4 and FeO4 units when the Ag2O content is higher than 16 mol%. These conclusions are well consistent with the composition dependency of the electric conductivity which shows a minimum when the Ag2O content is 10–15 mol%.