Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 162
5.3.1.4. Heat treated Er +3 -doped compositions
Fig. (5.12) and (5.13) show XRD traces of glass MOF017 (69.86TeO2-9.98Na2O-
19.96ZnF2-0.2ErF3 mol. %) before and after a heat treatment for 1 hr. at 275°C
respectively. Although the heat treated sample was opaque, no distinct crystalline phase
was identified by XRD, indicating either liquid / liquid phase separation, which could
result in opacity and lack of crystallites shown by fig. (5.13), or that the crystals are too
small to be resolved by the diffractometer, but still contribute to scattering. As crystal
size decreases, Debye diffraction rings, and hence peaks broaden [8]. If crystal size is
sufficiently small (of the order of nm), peaks can become indistinguishable from the
amorphous halo. The untreated glass exhibits the characteristic halo at around 30° 2θ,
with a lower intensity broader halo centered at around 55° 2θ. After the heat treatment
the main halos shape has developed a lower angle shoulder at around 23° 2θ and there is
possibly a peak developing at around 40° 2θ. Kukkonen [9] and Beggiora [10] et al.
studied the crystallisation of SiO2-Al2O3-CdF2-PbF2-ZnF2-YF3-ErF3 glass-ceramics.
Erbium (III) was found to preferentially partition to PbFx nano-crystals. Crystalline peaks
appeared from the amorphous halo of the untreated glass with increasing time and
temperature. In the TeO2-Na2O-ZnF2-ErF3 system, Er +3 could preferentially partition to
one of the phases, possibly the lower temperature fluoride phase (NaZnF3).