Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 120
MO is either Nb2O5 or Bi2O3) which are reported to form highly stable glasses with the
ratio of WO3:MO = 1 and WO3:MO ≠ 1 [35, 39].
Na2O disrupts the glassy network of TeO2, forming a range of at least five distinct
polyhedra (see section 2), whereas other oxides such WO3 cause less disruption (only
forming two distinct structural units ([WO4] and [WO6]). W 6+ , Nb 5+ and Zn 2+ oxides have
similar electronic polarisability (πion), optical basicity (Λ) and bonding to Te 2+ in the
oxide form [32]; these properties are summarized in table (4.3) for oxides used in this
study.
Table (4.3): Optical basicity (Λ), electronic polarisability of cation (πion) and bonding in
oxides used in this study [32].
Oxide ΛΛΛΛ ππππion / Å 3 Bonding type Bonding details
GeO2 ≈ 0.3 – 0.7 ≈ 0.002 – 0.2
TeO2
ZnO
Nb2O5
WO3
Na2O
PbO
Bi2O3
≈ 0.8 – 1.1 ≈ 0.2 – 0.8
> 1.1 ≈ 0.8 – 3.7
Semi-covalent or
semi-ionic
(acidic)
Ionic
(basic)
Very ionic
(very basic)
Effect of ZnF2 on the series (90-x)TeO2.xZnF2.10Na2O (mol. %), for 5 ≤ x ≤ 30
Large overlap
between O 2p
and metal
valence orbitals
Smaller overlap
between O 2p
and metal
valence orbitals
Small overlap
between O 2p
and metal
valence orbitals
A number of researchers has investigated fluoride-containing tellurite glasses,
particularly PbF2 and ZnF2 containing compositions [10, 20, 40-52]. Ternary systems