Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 29
2.3.2.3. TeO2-Na2O
Addition of Na2O, a network modifier, to TeO2 glasses results in more significant
depolymerisation than ZnO, as sodium is monovalent, and will terminate network
linkages wherever bonded to oxygen. Himei et al. [24] studied the structure of alkali (Li,
Na, K, Rb, and Cs) tellurite glasses with X-ray photoelectron spectroscopy (XPS). The
valence band spectra (0 to 30 eV) showed a change in structure from [TeO4] tbp to
[TeO3] tp with alkali addition. The [TeO4] tbp has a lone pair of electrons at one of the
equatorial sites of the tellurium sp 3 d hybrid orbital and [TeO3] sp 3 [24].
Zwanziger et al. [25] probed the sodium distribution in TeO2-Na2O glasses by spin-
echo NMR. Maximum stability has been shown to occur at 20 mol. % Na2O. Na-Na
distances in the glass were shown to be much closer than the random distribution
expected around the 20 mol. % Na2O composition, and sodium was shown to reside in
different sites to crystalline Na2Te4O9. This difference in structure which occurs at
maximum glass stability could provide a high enough energy barrier to prevent structural
rearrangement, and hence inhibit crytallisation [25].
Sakida et al. [26] studied the structure of alkali-metal tellurite (Li, Na, K, Rb, and Cs)
glasses by NMR. Fig. (2.4) shows (a) the range of structural units seen in the glasses, and
(b) mechanisms for M2O entering the glass network.