Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 157
identified: tetragonal triammonium silicon fluoride (NH4)3SiF6F and hexagonal
ammonium fluoride NH4F (space group P63mc, analogous to ZnO). It is probable that the
by-products of the fluorination reaction (see section 3.4), NH3 and HF, reacted with the
underside of the silica lid of the melting liner, to form the crystalline phase: (NH4)3SiF6F.
A likely chemical reaction proposed by this author is given by equation (5.6).
SiO2 + 3NH3 + 7HF (NH4)3SiF6F + 2H2O ↑ (5.6)
If this reaction occurs during fluorination (and possibly melting), SiO2 containment
therefore produces water, which could be reintroduced into the batch material / melt, due
to the vapour pressure of H2O generated inside the liner as the SiO2 degrades. Therefore,
complete OH removal from batch / melt may not be possible using silica containment.
The NH4F deposited on the silica lid was probably formed by the recombination of the
HF and NH3 volatiles from the batch, as shown by equation (5.7).
NH3 + HF NH4F (5.7)
Further addition of HF and NH3 to the NH4F already deposited in the silica lid, could
cause a subsequent reaction to form (NH4)3SiF6F as shown by equation (5.8).
SiO2 + NH4F + 2NH3 + 6HF (NH4)3SiF6F + 2H2O↑ (5.8)