Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 16
phenomenon [6]. As a result of this the supercooled liquid ↔ glass transition occurs over
a range of temperatures.
Zachariasen outlined a set of ‘rules’ for glass formation from compounds such as
SiO2, B2O3, P2O5, GeO2, As2S3, and BeF2 [2]:
• The compound will tend to form a glass if it easily forms polyhedral units as the
smallest structural units.
• Polyhedra should not share more than one corner.
• Anions (O 2- , S 2- , F - ) should not link more than two central cations of the
polyhedra.
• The number of corners of the polyhedra must be less than six.
• At least three corners of the polyhedra must link with other polyhedra.
Cations in the glass were the catagorised by Zachariasen according to their role in the
glass network [2].
• Network-formers: Si 4+ , B 3+ , P 5+ , Ge 4+ , As 3+ , Be 2+ , with CN of 3 or 4.
• Network-modifiers: Na + , K + , Ca 2+ , Ba 2+ , with CN ≥ 6.
• Intermediates, which may reinforce (CN = 4) or loosen the network further (CN 6
to 8).
The Zachariasen random network theory can be used to explain many properties of
conventional glasses, such as viscosity-temperature behaviour, and electrical