Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 238
an oxide glass for comparison). This strongly H-bonded OH band could have been
masked by the fluorotellurite multiphonon edge, or the presence of fluoride in the glass
could have actually prevented the formation of strongly H-bonded OH bonds. As the
number of OH groups in the glass decreased with increasing ZnF2 and / or melting time,
there would have been less chance of neighbouring OH groups to hydrogen bond to.
Also, hydrogen-bonding will occur between OH and F in these glasses (e.g. -F-----OH-),
possibly explaining the difference in OH band structure seen in the IR for the
fluorotellurite glasses compared to the oxide tellurite glasses. If ZnO is substituted for
ZnF2, the multiphonon edge will tend to shift to a higher wavenumber (see fig. (6.26)).
The Zn-F bond (368 kJ.mol -1 ) is stronger than the ZnO bond (159 kJ.mol -1 ) [6], therefore,
applying the Szigeti equation (2.8), this will result in an increase in k, shifting the
multiphonon edge to higher frequencies.
Fig. (6.35) shows that the number of weakly H-bonded OH groups in the glass tended
to decrease with increasing ZnF2, and the number of free-OH increased. This is possibly
due to the decreasing number of OH groups present with increasing fluoride, which
promotes the formation of free-OH due to the lack of close proximity OH groups to
hydrogen bond to, however -F-----HO- type H-bonds will form in these glasses.
Bulk fluorotellurite glass was prepared with the fluorinated ZnF2 described in section
3.4. However, spectra of bulk samples did not show any reduction in OH band intensity.
Effect of fluorination on the optical loss of fluorotellurite fibre is reported in chapter 8.