Zinc Sulfide: CaF2
CVD produced ZnS comes in 2 grades. The regular grade is useful for FLIR applications and has good transmission in 8-12 microns range, with reasonable hardness and good strength.
The other grade water clear ZnS called Cleartran (trademark of CVD, Inc.) transmits not only in the range of infrared but also in the visible in the 0.4 to 12 micron range.
CaF2 isused for optical windows, lenses and prisms in 0.15 - 9 micron range. Because of its low absorption at wavelength shorter than 6 microns, CaF2 is particularly popular for high power laser optics in that wave length range. Due to its low refractive index it can be used without anti-reflection coating.
Silicon Barium Fluoride
Silicon is a very useful IR material in 3-5 micron atmospheric window where many infrared Avionics systems are operational. Optical grade polycrystalline Si is used to make lenses, domes and windows, etc.
Beacause of its high thermal conductivity it is very good mirror substrate for high power lasers.
BaF2 is used for optical windows, lenses and prisms in 0.2-11 micron range. The material is very hard but is very sensitive to thermal shocks.
KRS-5 Quartz,Fused Silica(SiO2)
KRS-5 is used for lenses, windows and prisms for applications from 0.6 microns to 50 microns. It is relatively insoluble in water and may be used in cells in contact with aqueous solutions. Fused silica is used for windows, lenses in the 0.16-2.5 micron range. It is available in IR and UV grade, Depanding on your requirments. It is good for high temperature and strength applications.