Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 17
conductivity. However, it cannot explain all phenomena, such as property effects thought
to be due to short range ordering, and the glass formation in some novel systems also
contradict the criteria of this theory. The kinetic theory of glass formation provides a
more satisfactory explanation of glass formation of known systems, but the structural
theories are still valid and widely used [2].
2.2.2.4. Dietzel and field strength
By extending Goldschmidt’s original consideration of glass formation to radius and
charge of the constituent atoms / ions, Dietzel classified elements according to their field
strength, Fs. This considers the forces (attraction / repulsion) between cations and anions
in the glass [2].
F
s
τ c
=
( ra
+ rc
2
)
(2.1)
where τc is the valence of the cation, r the ionic radius of the cation (c) or anion (a).
Using Zachariasen’s classification, ions can be catagorised into three field strength
groups, Fs = 0.1 to 0.4 (network modifiers), Fs = 0.5 to 1 (intermediates), and Fs = 1.4 to
2 (network formers) [2].
On cooling a binary melt with cations of approximately the same field strength, phase
separation or crystallisation of the pure oxide phases is normally seen (e.g. SiO2-P2O5,
SiO2-B2O3, B2O3-P2O5). To form a single stable crystalline compound normally requires
∆Fs > 0.3. As ∆Fs increases, so does the number of possible stable compounds, and the