Page 46 - Chalcogenide Glasses for Infrared Optics

24 Cha pte r T w o

Applying these numbers to the alkali halides, we find their bonds
average more than 50 percent ionic character. For the often used alkali
halide NaCl, ∆ is 2.1 which corresponds to 67 percent ionic character.
The same procedure applied to alkaline earth halides averages to
about 50 percent. For the often used alkaline earth halide CaF , ∆ is
2 ,
3.0 which corresponds to 89 percent ionic character. Both families are
broadband in transmission from the ultraviolet and some into the far
infrared. Their refractive indexes are relatively low while their expan-
sion coefficients are large, leading to negative changes in index with
temperature. Some can be grown in large single-crystal form but are
soft and weak and may cleave. Their melts are not viscous. Most are
readily attacked by water. Alkali halides are used mostly in the labo-
ratory. A major exception among the alkaline earth halides for system
use is fine-grain polycrystalline magnesium fluoride. The point is
that these materials are ionic crystalline solids and as such are not
useful in night vision systems used in the field. The same treatment
applied to the combinations in the IVA, VA, and VIA elements illus-
trates the covalent nature of the chalcogenide infrared glasses.

Percent Percent
IVA Atom Pairs ∆ Ionic VA Atom Pairs ∆ Ionic
Si 1.8–O 3.5 1.7 51 P 2.1–O 3.5 1.4 39
Si 1.8–S 2.5 0.7 12 P 2.1–S 2.5 0.4 4
Si 1.8–Se 2.4 0.6 9 P 2.1–Se 2.4 0.3 2
Si 1.8–Te 2.1 0.3 2 P 2.1–Te 2.1 0 0
Ge 1.7–O 3.5 18 55 As 2.0–O 3.5 1.5 43
Ge 1.7–S 2.5 0.8 15 As 2.0–S 2.5 0.5 6
Ge 1.7–Se 2.4 0.7 12 As 2.0–Se 2.4 0.4 4
Ge 1.7–Te 2.1 0.4 4 As 2.0–Te 2.1 0.1 0.5
Sn 1.7–O 3.5 1.8 55 Sb 1.9–O 3.5 1.6 47
Sn 1.7–S 2.5 0.8 15 Sb 1.9–S 2.5 0.6 9
Sn 1.7–Se 2.4 0.7 12 Sb 1.9–Se 2.4 0.5 6
Sn 1.7–Te 2.1 0.4 4 Sb 1.9–Te 2.1 0.2 1
Pb 1.9–O 3.5 1.6 47 Bi 1.9–O 3.5 1.6 47
Pb 1.9–S 2.5 0.6 9 Bi 1.9–S 2.5 0.6 9
Pb 1.9–Se 2.4 0.5 6 Bi 1.9–Se 2.4 0.5 6
Pb 1.9–Te 2.1 0.2 1 Bi 1.9–Te 2.1 0.2 1

Excluding oxygen bonds, the IVA elements bonded with VIA ele-
ments average 8 percent ionic character, and the VA bonds with VIA
elements average only 4 percent. One may ask why ionic character is
important in infrared chalcogenide glasses. Covalent bonding has