Thallium bromide iodide (KRS-5) is a material for use in the far-infrared range. It has a wide transparency spectrum of 0.6-40 µm. KRS-5 is used for the production of ATR prisms (attenuated total reflection), windows and lenses for spectroscopy and astronomical telescopes. In combination with germanium, KRS-5 is also used for athermal IR imaging.

Tallium bromide dioxide tends to undergo cold deformation, which can change its shape over time. In addition, KRS-5 is toxic as a thallium solid solution, so the material must be handled with care.