Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 158
Ammonium fluoride could therefore be an intermediate product, which attacks the liner
producing triammonium silicon fluoride.
Melting
Fig. (5.9) shows an XRD trace of the deposit left on the silica liner and lid after melting
glass MOF005 (70TeO2-10Na2O-20ZnF2, mol. %) for 12 hours at 800°C in the glovebox.
The material can be clearly identified as tetragonal TeO2 (space group P41212), or α-
TeO2. It was expected that any volatilisation and deposit left on the silica containment
would be a fluoride compound, such as ZnF2 (melting point = 872°C and boiling point =
1500°C [5]), due to the marked difference in volatilisation between oxide tellurite and
fluorotellurite glasses (see tables (3.5) and (3.6)). However, TeO2 has slightly lower
melting and boiling points (733 and 1245°C respectively [5]) than ZnF2, and may react in
the melt (e.g. TeO2 + 3ZnF2 TeF4↑ + 2ZnO) forming more volatile fluoride
compounds such as TeF6 and TeF4 (which sublime at -39 and 194°C respectively, with
the latter forming TeF6 on sublimation [5]). The gaseous fluoride would then most likely
react with oxygen in the liner atmosphere and deposit as TeO2 on the silica (e.g. TeF4 +
O2 TeO2 + 2F2↑).