Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 21
Table (2.1): Electronegativities of good glass forming oxides and network modifiers [14].
Element Pauling electronegativity
Glass network formers
Te 2.1
B 2.0
Si 1.8
P 2.1
Ge 1.7
As 2.0
Sb 1.8
Glass network modifiers
Li 1.0
Na 0.9
K 0.8
These glass formers bond to oxygen in the network with a substantial covalent nature,
with a more or less random distribution of other atoms of lower electronegativity forming
bonds to oxygen of a more ionic nature. This prediction of glass formation based on
electronegativity proved useful as, according to Zachariasen rules for glass formation,
cations in a good glass former should be surrounded by 3 or 4 anions and most of the
anions are bonded to two cations (see section 2.2.2.3). α-TeO2 has a structure similar to
tin oxide in which six oxygen atoms surround each cation, although two of the oxygen
atoms are nearer to the cation than the other four. So the structure may be regarded a
collection of TeO2 groups (see the next section for a more detailed discussion of TeO2
structure, and its relation to the glass network). Therefore, the Zachariasen rules would
not lead one to expect easy glass formation from TeO2. Also, the radius of the Te 4+ ion
(97 pm [15]) would appear too great for tetrahedral coordination in the pure oxide. Using
equation (2.1), Dietzel’s field strength, gives F = 0.71 for TeO2, i.e. falling into the
category of an intermediate. In spite of these unfavourable circumstances (according to