Abstract
Abstract A Mössbauer spectrum of xNa2O·(95−x)TeO2·5Fe2O3 glasses (0≤x≤35) consists of a paramagnetic quadrupole doublet peak with an isomer shift of 0.39±0.01 mm s−1 with respect to metallic iron. Quadrupole splitting of the doublet peak changes gradually from 0.76 to 0.60 mm s−1 when Na2O content is changed from 0 to 35 mol% at 5 mol% intervals. These results suggest that Fe3+ ions are present at substitutional sites of Te4+ ions constituting distorted TeO4 trigonal bipyramids. Each of the distorted TeO4 trigonal bipyramids has an oxygen vacancy and a lone electron pair at one of the equatorial sites. Decreased quadrupole splitting is ascribed to an increased symmetry of FeO4 (and also TeO4) trigonal bipyramids, due to a formation of nonbridging oxygen atoms followed by a gradual change of the glass matrix from a two-(β-TeO2) or three-(α-TeO2) dimensional network structure to a lower dimensional network structure. These structural changes are also deduced from a composition dependency of glass transition temperatures (Tg), which decrease continuously from 318 to 232 °C with increasing Na2O content.
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