Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 114
4.3. Discussion
Effect of rare-earth and lead oxide addition
Glass MOD001, 79TeO2-10Na2O-10PbO-1Yb2O3 mol. %, (fig. (4.10)) was rather
unstable; thus Tx-Tg = 55°C, compared with other glasses investigated in this study, with
Tx-Tg > 150°C. The binary system TeO2-Na2O has been reported to form stable glasses
over a wide compositional range: 5.5 to 37.8 mol. % Na2O [11] with maximum stability
occurring at 20 mol. % Na2O, for which Tx-Tg > 120°C [12, 13]. This binary glass
corresponds to the eutectic composition on the equilibrium phase diagram, and the phase
Na2Te4O9 [12]. Sodium oxide is a network modifier, i.e. it breaks up the covalent TeO2
network by forming ionic bonds with non-bridging oxygens (NBOs), as illustrated by
equation (4.6):
≡Te-O-Te≡ + Na2O 2[≡Te-O - Na + ] (4.6)
Thus addition of Na2O to TeO2 results in a decrease in Tg and opens up the Tx-Tg gap.
TeO2 does not form a glass without the addition of at least one component (oxide or
halide), however a glass can be formed with small additions of these second components
[11]. The structure of pure TeO2 at ambient temperature and pressure is based on [TeO4]
trigonal bipyramids (tbp), which transform to [TeO3] trigonal pyramids (tp) with
increasing alkali metal oxide addition. This range of structural units (see section 2) is