Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 159
5.3.1.3. XRD of glasses which crystallised during processing or characterisation
During melting
Fig. (5.10) shows an XRD trace of the deposit which resulted from melting a 10g batch of
glass MOF001 (65TeO2-10Na2O-25ZnF2, mol. %) in the laboratory atmosphere for 5
hours in a platinum crucible at 800°C, and dropping to 700°C to cast, with fluorinating
agent (NH4HF2) added (see fig (3.2) for set-up). Most of the melt had volatilised, leaving
behind around 5g of a white solid deposit. This deposit was identified by means of XRD
(fig. (5.10)) as orthorhombic NaZnF3 (space group Pbnm). An amorphous halo can also
be seen on the trace. Any TeO2 left is likely to be found within the glassy fraction of the
sample, as TeO2 glasses can be formed with the addition of a few mol. % of a second
oxide or halide component [6]. As the fluorinating agent ((NH4)HF2) broke down into
NH3 and HF, it has reacted with the batch, and some of the products of this reaction
volatilised. The fluorinating agent could have promoted the formation of the volatile
tellurium fluoride compounds (TeF4 and TeF6) discussed in section 5.3.1.2.. Another
possibility is that the reactions due to the decomposition of (NH4)HF2, and between the
batch and (NH4)HF2 were sufficiently exothermic, in addition to the melt temperature, to
result in the volatilisation of components from the batch which are normally relatively
stable at 800°C, such as TeO2 which boils at 1245°C [5]. This would have changed the
chemistry of the batch, and destabilised the composition, increasing the likelihood of
crystallisation. Therefore, adding (NH4)HF2 directly to the glass batch was not pursued
further, as treating the ZnF2 directly before melting appeared to result in effective drying.