Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 25
the two extreme cases of coordination in both forms of crystalline TeO2: α-TeO2 and β-
TeO2 and coordination in tellurite glasses.
Table (2.2): Coordination of both forms of crystalline TeO2: paratellurite (α-TeO2) and
tellurite (β-TeO2) and tellurite glasses [2].
Crystal
Coordination Structure
3
4
Trigonal
pyramid (tp)
Trigonal
bipyramid (tbp)
Glass 3, 3+1 and 4 tp and tbp
Te-O bond
distance / pm
Te-O-Te bond
angle / °
195 95
200
209 (equatorial)
191 (axial)
120 + 20 (equatorial)
180 + 30 (axial)
92.1 + 6.6 (equatorial)
162.6 (axial)
Recent work on the structure of tellurite glasses has concluded that the network more
closely resembles paratellurite (α-TeO2), where [TeO4] units are only linked at their
corners [2]. Combining TeO2 with network modifiers (such as Na2O) and intermediates
(such as ZnO) results in structural modification to chain like structures. Similar structures
are seen in tellurite minerals such as zinc tellurite (Zn2Te3O8), and increase the tendency
of glass formation from melt cooling [2]. However, the corner sharing structure seen in
tellurite glasses would not necessarily lead to the conclusion of easy glass formation.
Pauling’s third rule states that the stability of ionic tetrahedra (i.e. ionic bond strength)
decreases across the series: corner-sharing edge-sharing face-sharing [20]. As the
distance between cations across this series decreases, the covalent interaction increases
[20]. Therefore, it could be concluded that melts with a higher proportion of corner-
sharing tetrahedra will have a higher viscosity (due to increased ionic interaction) than
edge- and face-sharing melts. On rapid quenching, crystallisation can be avoided as
viscosity will rise relatively rapidly, possibly inhibiting atomic rearrangement.