Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 116
structure and properties of zinc-tellurite glasses have been studied for some time [23-27].
Like lead (II) oxide, zinc oxide enters the glass network as an intermediate. [ZnO6]
polyhedra enter the network of Te-O chains breaking them in places, but weakly linking
parallel chains in others [24]. However, unlike alkali metal oxide addition, there is not the
same range of polyhedra present; [TeO4], [TeO3] and [ZnO6] interatomic distances are
preserved (see section 2) in the TeO2-ZnO glasses [24]. This is likely to be due to the
covalent nature of ZnO, which will enter the network with less disruption than the more
ionic Na2O. The ternary system TeO2-Na2O-ZnO has been shown to be highly stable [22,
28, 29]. Aida et al. [28] explained this stability using the theory of optical basicity which
was developed by Duffy [30] and has been used by other groups to help to explain the
properties of oxide glasses [31-33]. Optical basicity is a numerical expression of the
average electron donor power of the oxide species. Combining a high optical basicity
alkali metal oxide (Na2O – 1.15) and a low optical basicity metal oxide (ZnO – 1.03)
causes strong interaction in the glass, and suppresses the movement of Na + and Zn 2+ ions,
resulting in high resistance to crystallisation [28]. TeO2 also has a similar optical basicity
(0.99) to ZnO, which would indicate that addition of ZnO would result in less disruption
to the glassy network compared to Na2O.
TeO2-ZnO-Na2O (TZN) compositions
Fig. (4.11) shows the DTA trace of glass MOD006, 77TeO2-10ZnO-10Na2O-3PbO mol.
%. This composition was far more stable than glass MOD002 (78.21TeO2-4.95ZnO-
9.9Na2O-4.95PbO-0.99Yb2O3-0.99Er2O3 mol. %, with Tx-Tg = 85°C), as it did not exhibit