Page 50 - Chalcogenide Glasses for Infrared Optics

28 Cha pte r T w o

into account when casting, molding, slumping, extruding, or drawing
30
a glass into fiber. The rules of Zachariasen first pointed out that to
form a glass it was important to select substances with a low coordi-
nation number (4 or less) for both the cation and the anion. Coordina-
tion number is not the only factor.
There must be rigid directional requirements for the chemical
bonding which is found in covalent solids such as chalcogenide
glasses. Position in the periodic table for element A and element B
will determine the coordination number for their binary compound
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AB. According to Mooser and Pearson, the average principal quan-
tum number N of a crystalline binary AB compound is a measure of
x
the metallic character of the bonds present. The difference in Pauling
electronegativity ∆ is a measure of the ionic character of the bonds. A
plot of these two factors by Mooser and Pearson showed that com-
31
pounds of the same crystalline structures fell in the same general
area of the diagram (Fig. 2.5). In fact, similar crystal structures with
the same coordination number (3, 4, 5, 6, or 8) fall in rather specific
areas of the plot. We have already dealt with the electronegativity
differences for bonds. The principal quantum number of the valence
electrons corresponds to the row of the periodic table on which the
element is located.
The same treatment may be applied to chalcogenide glasses.
However, most compositions will not be binary, only two elements.
Most will contain three or more elements. So the average principal
quantum numbers and average electronegativity numbers must be
calculated based on composition percentages for each element. Con-
sider a ternary system with a composition represented by Ax + By + Cz.
You calculate an average N by multiplying the N of each element
x x
by its composition fraction and adding for a total. The same proce-
dure is followed for the electronegativity differences. The point for
each composition could then be plotted as shown in Fig. 2.5. Multi-
component glasses based on silicon, sulfur, selenium, and tellurium
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were treated in this manner. Note all the chalcogenide glasses lie
in the coordination number 4 area. The silicate glasses start in the
4 region but fall in the coordination number 6 region as more metal
oxides are added. Note that pure SiO glass has a network structure
2
that is open. Pure silica when heated is slightly permeable to helium
and to a lesser extent hydrogen. The glass optical properties are
altered by adding metal oxides to the composition which fill in the
voids in the network structure. The heavy metal oxides are from
higher row numbers of the periodic table. Bonding to silicon which
already has a coordination number of 4 leads to areas of coordina-
tion number 6. Silicate glasses containing metal oxides are not per-
meable to helium or hydrogen. For chalcogenide glasses formed
from a melt, coordination number and bond type as determined by
chemical composition are the two most important factors in glass
formation.