ostphotonics
https://www.ostphotonics.com/products/kbr-substrates.html
Crystals such as sodium chloride (NaCl), potassium bromide (KBr) and potassium chloride (KCl) are uniform, bright and high transmittance within 0.250-20 μm when thickness less than 10mm, and no impurity absorption. These single crystals are ideal materials for making optical elements for prisms, lenses, filters, and various windows in infrared devices and optical instruments.