Tellurite And Fluorotellurite Glasses For Active And Passive

3. Glass batching and melting; MDO 78
3.4. Fluorination of ZnF2
To ensure batch materials were as dry as possible, the ZnF2 was fluorinated before
melting, as the as-received chemicals were not phase pure (see chapter 5). The OH -
group can readily substitute with the F - ion in a number of compounds due to similar
ionic radii (140 and ≈ 131 pm respectively [2]). The fluorination reaction was based
on TGA (thermogravimetric analysis) results obtained by Sanghera et al. [3, 4]. This
type of pre-melt drying has been employed for halide glasses for a number of years
[5]. Equation (3.5) and (3.6) summarise the reactions which are believed to take place
during fluorination. Saito et al. [6] are the only group, to the current authors
knowledge, to describe the reaction between similar compounds to those used in this
study (Zn(OH)F and NH4F) in any detail (a patent by Watanabe et al. [7] does briefly
mention the fluorination reaction). From this work, and the work of Sanghera et al. [3,
4] which describes the drying of compounds used to synthesise ZBLAN glasses with
(NH4)HF2, the following chemical reactions are proposed.
(NH4)HF2 + Zn(OH)F ZnF2 + NH3↑ + HF↑ + H2O↑ (3.5)
(NH4)HF2 + Zn(OH)2 ZnF2 + NH3↑ + H2O↑ (3.6)
From XRD patterns obtained (see chapter 5), it was assumed that the as-received ZnF2
was of average composition Zn(OH)F (i.e. batch composed of ZnF2, Zn(OH)F and
Zn(OH)2). Therefore for each gram of fluoride, 0.56 g of (NH4)HF2 was needed for
complete fluorination to ZnF2, although the amount of (NH4)HF2 was likely to have
been excessive (see section 5). Therefore, for each fluorination, 8.4 g of (NH4)HF2