Tellurite And Fluorotellurite Glasses For Active And Passive

5. Crystallisation studies; MDO 160
During characterisation
Fig. (5.11) shows an XRD trace of glass MOF005 (70TeO2-10Na2O-20ZnF2, mol. %)
which crystallised at 400°C in the TMA instrument whilst performing viscosity
measurements. The phases identified by XRD (fig. (5.11)) were monoclinic Zn2Te3O8
(space group C2/c) and orthorhombic NaZnF3 (space group Pbnm) [4]. Fig. (5.14) shows
the Gaussian deconvolution of the crystallisation peak from DTA of this glass
composition. The peak is clearly asymmetric with a high temperature shoulder. Two
crystallisation peaks can be resolved from this trace with maximums at: x1 = 410°C and
x2 = 421°C. At 400°C the first crystallisation event (x1) is near its maximum and a second
phase (x2) has also clearly begun to form. Therefore, it would be reasonable to assign the
crystalline phases Zn2Te3O8 and NaZnF3 to these peaks. EDX analysis from optical fibre
which was pulled at around 300°C of this composition showed crystals present around 1
to 2 µm in diameter (see chapter 8). These crystals were fluorine and sodium rich, and
oxygen and tellurium deficient, suggesting the first crystalline product above Tg to be
NaZnF3. Therefore, the DTA peak at 410°C is likely to be NaZnF3 crystallising, and the
peak at 421°C due to the formation of Zn2Te3O8. As the glass is quenched from the melt,
oxide nuclei may form, providing homogeneous nucleation sites for the fluoride phase to
form on heating. Fig. (5.14) illustrates these temperatures, with a dashed line at 400°C
where the glass crystallised in the TMA [4].