There are many materials used infrared cameras and instruments. An ideal material would be cheap, resistant to abrasion, immune to moisture, have zero dispersion, low transmission losses at all wavelengths, and several other properties which may depend on application. Ge for example has a very high index of refraction which allows compact lenses of low curvature, but dramatically increases reflection losses. The reflection losses can be mitigated but not eliminated by coatings.
No material checks all these boxes so each application chooses a material best suited to it. Ge, ZnS, ZnSe and polyethylene are common choices. Less common choices include NaCl and diamond. There are many more that have been.researched but not commercialized.