As an overview, this is a consideration of a group of inorganic materials which transmit IR light in that region of the electro-magnetic spectrum from about 1 micron to beyond 25 microns. In addition to the transmission, some properties in visible light will be mentioned as well as refractive index, hardness, and thermal conductivity.
W. L. Truett, W. L. Truett,
"Infrared (IR) Transmitting Materials", Proc. SPIE 0520, Thermosense VII: Thermal Infrared Sensing for Diagnostics and Control, (20 March 1985); doi: 10.1117/12.946125; https://doi.org/10.1117/12.946125