Synthesis and structure of transparent zinc-niobate-tellurite glasses with low hydroxyl content

Low-hydroxyl (OH)-content, transparent zinc-niobate-tellurite glasses (TeO2-Nb2O5-ZnO and TeO2-Nb2O5-ZnCl2) were synthesized by using dry-air atmospheres, stirring of the melt, and preheat treatment before melt preparation. A low OH level (79.8 +/- 6.3 ppm) was achieved by performing a preheat treatment of mixed raw chemicals with zinc chloride and then melting in both open-and dry-air atmospheres with several counts of intermittent stirring. However, this sequence of processes resulted mainly in non-crystallized, opaque products. The basis of this opacity was investigated by performing Raman spectroscopy, X-ray diffraction (XRD), and Xray fluorescence (XRF) measurements on both the opaque and transparent glasses. Raman spectroscopy revealed that the proportion of Te-O-Te bonds, relative to tellurite unit structures (TeO4, TeO3+1, and TeO3), in the opaque glasses decreased with increasing isolated structures of TeO32-. This decrease resulted possibly from atomic-scaled inhomogeneity and immiscible domain formation in the melts and corresponding glasses, which have less amounts of residual OH groups.