Tellurite And Fluorotellurite Glasses For Active And Passive

6. Optical properties; MDO 228
tellurite glasses (see chapter 7). The exact nature of these OH bands will be discussed
later in this section.
Fig (6.11) shows the relative percentage of absorption by the different types of OH
groups in the glass series (80-x)TeO2-10Na2O-10ZnO-xMO, where MO is PbO or GeO2,
for MOD006 (x = 3 mol. % PbO), MOD010 (x = 5 mol. % PbO) and MOD012 (x = 5
mol. % GeO2). The PbO glasses contained relatively more free-OH and strongly
hydrogen bonded OH (strong-OH), and less weakly hydrogen bonded OH (weak-OH)
than the GeO2 glass. Germanium has a lower electronegativity than lead (2.01 and 2.33
respectively), which may have promoted the formation of weak-OH. This latter type of
OH hydrogen-bonding exhibited the largest absorption bands in these glasses, indicating
it is the most energetically favourable state for OH in the glass, assuming that the
extinction coefficients of the different types of OH are relatively similar. Therefore,
adding a cation of relatively high electronegativity to the glass (lead), may have
interfered with the hydrogen bonding of the OH, which tends to form due the permanent
dipole on the OH groups (due to the large difference in electronegativity between oxygen,
3.44, and hydrogen, 2.20).
Infrared spectroscopy of glasses of the series (90-x)TeO2-10Na2O-xZnO
Fig. (6.12) shows infrared spectra of glass of the series (90-x)TeO2-10Na2O-xZnO, for x
= 12 mol. % (MOD007) and x = 10 mol. % (MOD013). An increase in ZnO (from 10 to
12 mol. %) shifts the multiphonon edge to higher wavenumbers. Zinc is lighter than
tellurium (Z = 30 and 52 respectively), therefore, substituting ZnO for TeO2 will result in