Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 123
Table (4.4): Van der Waals (rvdw), ionic (rion), covalent (rcov) and Bragg-Slater (rb-s) radii
of elements used in this study [53, 54].
Element
Radius / pm
rvdw rion rcov rb-s (3)
O -2 150 140 73 60
F - 150-160 133 71 50
Na + 230 102 154 180
Zn +2 140 74 120 135
Ge +4 - 53 122 125
Nb +5 - 64 - 145
Te +4 210 97 135 140
La +3 - 103 - 195
Er +3 - 89 - -
Yb +3
- 87 - -
W +6 - 60 - -
Pb +2 200 119 - -
Bi +3 - 103 - 160
Zn +2 has a slightly lower ionic radii compared to Te +4 (97 and 74 pm respectively [54]),
therefore Zn +2 can be introduced into the glass network with less disruption. Pb +2 has a
higher radius (119 pm) [54] and will disrupt the network more easily. Na + has a similar
radius to tellurium (102 pm) [54], however it does not enter the glass as an intermediate
like Zn +2 and Pb +2 . As discussed earlier, sodium oxide enters the glass as a network
modifier and forms a range of polyhedra which stabilise the glass by means of providing
a higher energy barrier for atomic and molecular rearrangement.
It is interesting to note that F - has a similar ionic radius to O -2 (133 and 140 pm
respectively) [54], so oxide batch materials should be readily substituted by fluoride
batch materials and stable compositions could result, e.g. stable glasses were produced
here from the oxide ternary TeO2-Na2O-ZnO and also the analogous oxyfluoride ternary
TeO2-Na2O-ZnF2.
(3) Bragg-Slater radii roughly represents the size of the inner core of electrons