Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 126
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. [40] 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 was readily incorporated into the glassy
network, directly replacing oxygen, without depolymerising the network [40]. 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 [40]. This author would
propose a similar structural effect could occur in glasses MOF009 to 013, as zinc is
divalent, like lead, and typically six-coordinated; however some depolymerisation will
occur, as viscosity behaviour of these glasses becomes more fragile with ZnF2 addition
(see chapter 8). When added to TeO2 glasses ZnO is know to form network linkages in
places, but break the network in others [23, 24], therefore it would be reasonable to
assume ZnF2 would have a similar effect, as zinc will bond to fluorine and oxygen in the
final glass (although ZnF2 introduces two F anions into the glass per zinc cation, as
opposed to ZnO which only introduces one anion).