Synthesis, thermal and structural properties of pure TeO2 glass and zinc-tellurite glasses

We have synthesized pure TeO2 glass and glasses in the systems xZnO − (1 − x)TeO2 (0 ≤ x ≤ 0.50) and yAl2O3 − (1 − y)TeO2 (0 ≤ y ≤ 0.03) by melting in Pt crucibles, and measured their glass transition temperature (Tg), density (ρ) and Raman spectra to correlate glass properties with structure. For pure TeO2 glass, synthesized using our newly developed intermittent quenching technique, we find onset- and midpoint-Tg at 301.1 and 306.7 °C and ρ = 5.62 g/cm3, in clear disagreement with TeO2 glass melted in alumina crucible for which we find Tg ≈ 380 °C and ρ = 4.86 g/cm3. This latter method, used frequently in the literature, was shown by Raman spectroscopy to introduce Al2O3 in the tellurite matrix which becomes cross-linked by Te-O-Al bridges, resulting in the increase of Tg and decrease of ρ. Raman spectroscopy showed also that doping TeO2 with ZnO or Al2O3 causes the progressive conversion of TeO4 trigonal bipyramids to TeO3 + 1 polyhedra with two terminal oxygens, and then to TeO3 trigonal pyramids with three terminal oxygens. This structural transformation is reflected in the composition dependence of the volume per mole TeO2 evaluated from density data. The ZnO-dependence of this parameter is described by two linear parts with an inflection point at x = 0.25, which indicates an increasing rate of forming terminal TeO bonds at higher ZnO contents. The Tg was found to increase with ZnO and Al2O3 contents and this was attributed to the glass-forming ability of both oxides, while density was found to decrease due mainly to replacement of the heavier TeO2 by the lighter ZnO and Al2O3. The results of this study are discussed with reference to previous works on TeO2 and zinc-tellurite glasses.