Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 234
The band at around 1870 cm -1 (5.35 µm) attributed to [WO4] tetrahedra appeared to grow
with increasing WO3. Shalout et al. [28] found that the number of [WO4] tetrahedra
increased in binary TeO2-WO3 glasses with increasing WO3, up to around 30 mol. %
WO3. All the WO3 glasses prepared in this study contained between 5 and 25 mol %
WO3, therefore it would be reasonable to assume the bands growing out of the
multiphonon edge were due to equilibrium between [WO6]oct ↔ [WO4]tet in the glass
with varying WO3 content.
Fig. (6.21) shows the OH band envelope of the WO3 containing glasses, and fig. (6.22)
to (6.24) show the associated Gaussian deconvolution. It is interesting to note that these
glasses exhibit very different shaped OH bands from the TeO2-Na2O-ZnO containing
glasses discussed earlier in this section (fig. (6.17) for example). Much like silicate
glasses, the OH bands for tungsten-tellurite glasses show a decrease in frequency (and
wavenumber), and an increase in intensity and width, with the increasing degree of
hydrogen bonding [11]; i.e. free-OH weakly H-bonded OH strongly H-bonded OH.
Fig. (6.25) illustrates this. This similar trend to silicates could be due to similarities in the
bonding in the tungsten-tellurite glasses. Like the Si-O bond (799.6 kJ.mol -1 ), the W-O
(672 kJ.mol -1 ) and Nb-O (771 kJ.mol -1 ) bonds are very strong compared to Zn-O (159
kJ.mol -1 ), Pb-O (382.0 kJ.mol -1 ) and Te-O (376.1 kJ.mol -1 ) [6]. This change in bonding
environment could affect the population of states of the OH groups in the glass, favouring
strongly hydrogen bonded OH, as opposed to weakly bonded OH in the TeO2-Na2O-ZnO
containing glasses. The cations in the tungsten tellurite glasses were also high in valence
(Te +4 , W +6 , Nb +5 and Bi +3 ) compared to the earlier compositions (Te +4 , Zn +2 , Na + , Pb +2
and Ge +4 ). This could also alter the bonding environment and population of OH types.