Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 163
5.4. Summary
Fluoride batch materials (ZnF2) were shown by XRD to be significantly ‘wet’ in the form
of Zn(OH)F. This as-received powder was treated with a fluorinating agent, (NH4)HF2,
resulting in increased purity (confirmed by XRD and XPS) [4]. Addition of this
fluorinating agent directly to the batch of a fluorotellurite melt caused ≈ 50 wt. %
volatilisation, and therefore was not pursued further. The fluorination of the precursors
resulted in volatiles (NH3 and HF) which attacked the silica containment vessel. These
chemical reactions formed solid deposits ((NH4)3SiF6F and NH4F) which reintroduced
molecular water into the atmosphere, and possibly to the ZnF2.
Two crystalline phases were identified (by XRD, energy dispersive X-ray (EDX)
analysis and DTA) to crystallise from the 70TeO2-10Na2O-20ZnF2 mol. % glass at
400°C: NaZnF3, and Zn2Te3O8 [4]. It is possible that the competition between these two
phases stabilises the glass as it approaches the eutectic composition with ZnF2 addition.
Er +3 -doped 70TeO2-10Na2O-20ZnF2 mol. % glasses which were heat treated, did not
show sharply defined XRD peaks, although the amorphous halo was modified. This lack
of observable crystallinity, even in opaque samples could be due to the small size of
crystallites.
5.5. References
[1] J. A. K. Tareen and T. R. N. Kutty, A basic course in crystallography. Bangalore:
Universities Press, 2001.