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

Abstract We have synthesized pure TeO 2 glass and glasses in the systems xZnO − (1 − x)TeO 2 (0 ≤ x ≤ 0.50) and yAl 2 O 3  − (1 − y)TeO 2 (0 ≤ y ≤ 0.03) by melting in Pt crucibles, and measured their glass transition temperature (T g ), density (ρ) and Raman spectra to correlate glass properties with structure. For pure TeO 2 glass, synthesized using our newly developed intermittent quenching technique, we find onset- and midpoint-T g at 301.1 and 306.7 °C and ρ = 5.62 g/cm 3 , in clear disagreement with TeO 2 glass melted in alumina crucible for which we find T g  ≈ 380 °C and ρ = 4.86 g/cm 3 . This latter method, used frequently in the literature, was shown by Raman spectroscopy to introduce Al 2 O 3 in the tellurite matrix which becomes cross-linked by Te-O-Al bridges, resulting in the increase of T g and decrease of ρ. Raman spectroscopy showed also that doping TeO 2 with ZnO or Al 2 O 3 causes the progressive conversion of TeO 4 trigonal bipyramids to TeO 3 + 1 polyhedra with two terminal oxygens, and then to TeO 3 trigonal pyramids with three terminal oxygens. This structural transformation is reflected in the composition dependence of the volume per mole TeO 2 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 Te O bonds at higher ZnO contents. The T g was found to increase with ZnO and Al 2 O 3 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 TeO 2 by the lighter ZnO and Al 2 O 3 . The results of this study are discussed with reference to previous works on TeO 2 and zinc-tellurite glasses.