Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 156
amount of Zn(OH)F, an orthorhombic phase, present (fig. (5.6)) [4]. Only cationic
impurities were quoted on the assay sheet, therefore anionic impurities such as OH - were
not included by Alfa Aesar for calculating purity. OH - can readily substitute for F - in
many inorganic compounds and minerals, due to their similar ionic radius and
electronegitivity. There was some tetragonal ZnF2 present (space group P42/mnm) in the
powder which was identified on the trace.
Fluorinated ZnF2
The XRD trace of the purified ZnF2 powder (fig. (5.7)), shows that the fluorination
process was highly effective, removing a high proportion of the hydroxide phase from the
batch [4] (see X-ray photoelectron spectroscopy (XPS) spectra in chapter 7 for
quantification). The tetragonal phase ZnF2 (space group P42/mnm) has an analogous
structure to rutile (TiO2). The zinc cations sit in octahedral holes, between the fluorine
anions. Each zinc ion is surrounded by six fluorine ions, with each fluorine surrounded by
three zinc ions [1].
5.3.1.2. XRD of deposits from melting and fluorination
Fluorination
Fig. (5.8) shows an XRD trace of the deposit left from fluorination of ZnF2 with
(NH4)HF2, on the silica liner lid in glovebox. Two crystalline phases can clearly be