Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 363
altered by external factors such as impurities, pressure, temperature and rate of cooling
[27].
It is interesting to note, that these snowflake shaped crystals (see fig. (8.8)) initially
grow to form a cross shapes, possibly due to a similar mechanism shown in fig. (8.18).
Then from the primary cross-shaped dendrites, secondary dentrites grow diagonally (in
the {110} direction). This is likely to be due to a similar mechanism described above for
a cubic crystal. In orthorhombic NaZnF3, first the cubic {001} plane grows forming a
cross shaped dendritic structure. Eventually, the {001} plane is enveloped by the slower
moving {110} plane and secondary dendrites grow from the primary structure. The
triangular shaped crystals seen in fig. (8.7) are likely to be due to an orientation effect
from the polishing process. Fig. (8.19) shows a possible model for crystallisation of the
snowflake shaped orthorhombic NaZnF3 crystals in glass MOF005 (70TeO2-10Na2O-
20ZnF2 mol. %) and similar compositions.