Eutectic (
\(\pu{42 \% TlBr, 58 \% TlI}\)) mixture of thallium bromide and thallium iodide transparent to
infrared radiation used as window in
infrared spectroscopy.
Note:
KRS-5 transmits well down to \(\pu{200 cm-1}\). It is practically insoluble in water but its refractive index is rather high (approximately 2.35). It is rather plastic and deforms with time, and it is poisonous.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 752 (https://doi.org/10.1515/pac-2019-0203)