Process for producing substances optically transparent to infrared rays

In order to obtain substances that are optically transparent in the infrared range, usable in the manufacture of optical fibers or radiation emitters, a metal or metalloid chalcogenide other than an oxide is produced by a double-substitution reaction between a starting chalcogen compound-particularly a hydride such as H2S, H2Se or H2Te-and a salt of the desired metal or metalloid, e.g. a chloride. The starting compound and the reactant salt preferably are vaporized at a temperature below the melting point of the resulting metal chalcogenide which thereupon precipitates in the reaction chamber.