Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 115
denoted by
n
Qm 2 [11, 12, 14-18]. Due to this range of polyhedra present in the TeO2-Na2O
glass, for crystallisation to occur, significant structural rearrangement would need to take
place. This energy barrier, and slow kinetics around Tg result in a stable glass [12, 14,
15].
TeO2-PbO glasses form over a much narrower compositional range: 13.6 to 21.8 mol.
% PbO [11], and are far less stable than TeO2-Na2O glasses, with Tx-Tg < 50°C [19].
Stanworth [20] reported glass formation in the binary system TeO2-PbO over a wide
compositional range (15-85 wt. % PbO), however these glasses were melted in silica,
alumina and zirconia crucibles, which this author suggests would have dissolved
substantially into the glasses which therefore perhaps cannot be truly called binary
compositions. Lead (II) oxide enters the glass as an intermediate, forming -O-Pb-O-
linkages composed of [PbO6] octahedra and [PbO4] tetrahedra [21]. PbO addition also
changes the proportion of [TeO4] tbp and [TeO3] tp present in the glass [19]; hence in this
study addition of PbO to the TeO2-Na2O system has upset the stabilisation caused by
Na2O and reduced the Tx-Tg gap expected from the binary TeO2-Na2O (Tx-Tg of 55°C
compared to > 120°C [12, 13]). As only 1 mol. % Yb2O3 was added to MOD001, the
effect on destabilisation is likely to be insignificant compared to that of PbO, and oxide
tellurite glass stability is relatively tolerant to rare-earth additions of this magnitude [22]
(i.e. 0 to 5 mol. %).
Glass MOD002, 78.21TeO2-4.95ZnO-9.9Na2O-4.95PbO-0.99Yb2O3-0.99Er2O3 mol.
% (fig. 4.10), was slightly more stable (Tx-Tg = 85°C) than MOD001 (Tx-Tg = 85°C
compared to 55°C) due to the presence of ZnO and Na2O in the former glass. The
2 where n = the number of bridging oxygens in the structural unit, and m = co-ordination of Te in the
structural unit