Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 155
(Puratronic, 99.9995%). The crystal structure is hexagonal, space group P63mc,
analogous to wurtzite (ZnS). Oxygen anions are hexagonally close packed (hcp) with
alternate tetrahedral voids filled with zinc cations [1]. Both zinc and oxygen are four
coordinated to one another, therefore the structure can be thought of as interpenetrating
hcp sublattices of Zn and O [1].
Na2CO3
Fig. (5.5) shows an XRD trace of the Na2CO3 powder used in this study sourced from
Alfa Aesar (Puratronic, 99.997%). There was relatively high background noise on the
XRD trace, most likely due to the low atomic weight of the atoms in the powder (Na, C
and O, Z = 11, 6 and 8 respectively). This will result in less scattering of the incident X-
rays compared to heavier compounds (e.g. TeO2 and ZnF2), and hence less X-ray counts
at the detector for the hkl reflections of the NaCO3 crystals. However, the pattern
matched that of pure Na2CO3, crystal structure monoclinic, space group C2/m. Na2CO3
decomposes in the glass melt to Na2O, therefore, the crystal structure of the carbonate is
unrelated to the final state of the oxide in the glass.
As-received ZnF2
Fig. (5.6) shows an XRD trace of the ZnF2 powder used in this study, sourced from Alfa
Aesar. Although the quoted purity was high (Puratronic, 99.995%) there was a significant