Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 32
showed clustering of sodium cations around the eutectic (20 mol. % Na2O) composition
in the glass [27].
2.3.2.4. TeO2-ZnF2
The tetragonal phase ZnF2 (space group P42/mnm) has an analogous structure to rutile
(TiO2). The zinc cations sit in octahedral holes, between the fluorine anions. Each zinc
ion is surrounded by six fluorine ions, with each fluorine surrounded by three zinc ions
[20]. This suggests that in tellurite glasses, [ZnF6] units will enter the glassy network, and
a number of structural units could exist in the series [ZnF6-xOx], for 1 ≤ x ≤ 6. This
substitution is possible due to the similar ionic radius of F - and O -2 , and the six-
coordination of zinc to both anions. Therefore, ZnF2 can be substituted for ZnO in
tellurite glasses, resulting in compositions with stability equal to, and exceeding those of
pure oxide compositions.
Sidebottom et al. [29] studied the Raman spectra of the ternary glass system TeO2-
ZnO-ZnF2. The Raman bands due to symmetric stretching modes of TeO2 structural units
([TeO4], [TeO3+1] and [TeO3]) in the 650-750 cm -1 region were unaffected by fluoride
addition. However, the bending mode at 420 cm -1 increased in intensity with increasing
ZnF2 content. This indicates that fluorine is readily incorporated into the glassy network,
directly replacing oxygen, without depolymerising the network further [29]. In this glass
system, the increase in bending mode intensity at 420 cm -1 is due to the conversion of
[TeO3] units to [Te(O,F)3+1], with increasing fluoride content [29]. However,