Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 26
Champarnaud-Mesjard et al. [19, 21] have recently found evidence of γ- and δ-TeO2
phases in crystalline and glassy samples. γ-TeO2 was formed by heat treating binary TeO2
glasses which contain WO3, Nb2O5 or PbO (5-10 mol. %). The glasses were slowly
heated to 440°C and held at this temperature for 60 hours. The crystal structure of this
phase was determined by XRD to be orthorhombic, space group P212121. A band at 430
cm -1 in the Raman spectra of these glasses corresponded to a similar structural unit to γ-
TeO2 [21].
δ-TeO2 is thought to be an intermediate phase between the crystalline and glassy state
(an ‘antiglass’) [19]. This phase was formed by annealing TeO2-WO3 compositions (5-10
mol. %) for 25 hours at 340°C. δ-TeO2 was determined by XRD to be a metastable
fluorite-related cubic structure, space group Fm 3 m [19].
2.3.2.2. TeO2-ZnO
The ZnO crystal structure is hexagonal, space group P63mc, analogous to wurtzite (ZnS).
Oxygen anions are hexagonally close packed (hcp) with alternate tetrahedral voids filled
with zinc cations [20]. Both zinc and oxygen are four coordinated to one another,
therefore the structure can be thought of as interpenetrating hcp sublattices of Zn and O
[20].
Kozhukharov studied the structure of 80TeO2-20ZnO mol. % glass with neutron
diffraction [22]. This study showed that the structural units in the glass ([TeO4], [TeO3]
and [ZnO6]) are similar to those in α-TeO2, and Zn2Te3O8, but predominantly the latter.
The average coordination number of tellurium was found to be 3.35. Interatomic (Te-O,