Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 361
the preform surface, altering the surface chemistry, to provide nucleation sites, and also
making reducing conditions. The EDX spectra which were performed at point A (glass)
and B (crystal) on fig. (8.7) show the oxygen content of the crystal was lower than of the
glass (44 and 30 atomic % respectively). The fluorine content of the crystal was over
double that of the glass (22 and 10 atomic % respectively). The sodium content of the
crystal was around four times that of the glass (20 and 5 atomic % respectively). The
tellurium and zinc contents were slightly lower of the crystal (19 and 9 atomic %
respectively) than those of the glass (28 and 13 atomic % respectively). As the electron
beam will have sampled a volume greater than the crystals (around 10 µm compared to 1
to 2 µm respectively), and glass will be inevitably present in the areas between the
dendrites of the crystals, these values of elemental analysis cannot be used to calculate
stoichiometry of phases. However, they can provide an estimate of the composition of
crystalline phases identified (i.e. relative amounts). The crystal shown in fig. (8.7) was
Na and F rich, and Te and O deficient compared to the glass, with Zn approximately the
same. Therefore, it would be reasonable to assume this crystal was the orthorhombic
phase, NaZnF3, indexed in fig. (5.11).
Fig. (8.8) shows an electron micrograph of a snowflake shaped crystal in the glass
fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %), mounted length-ways
in epoxy resin, and cross-sectioned. Again, this crystal had higher atomic % Na and F,
and lower Zn, Te and O than the bulk glass. This type of dendritic or skeleton crystallite
has been reported in a number of glass systems such as silicates and fluorides [27]. These
crystals often bear little resemblance to their mineralogical counterparts. If a sphere is cut
out of a cubic crystal and introduced into a saturated solution (i.e. a glass melt or