Abstract: Said medium is a halide glass (2, 8, 12, 20) containing uranium ions at least partly in the form U.sup.3+ and having, at least at one point of a region of the near infrared between 2.2 and 3.4 micrometers, an optical attenuation not exceeding 0.01 cm.sup.-1. For the production thereof, preparation takes place of a halide glass containing uranium ions, at least partly in the form U.sup.4+ and/or U.sup.5+, and having at least at this point the aforementioned attenuation. The glass is exposed to ionizing radiation able to produce trivalent uranium ions from the uranium ions of the prepared glass. Application to the production of optical guides, whereof the guiding parts are formed from such a medium.

Abstract: A process for the preparation of uranium tetrafluoride by reduction of gaseous uranium hexafluoride using: (1) sulphur vapor with a minimum of halogen and/or a halide, chosen from among Cl.sub.2, Br.sub.2, S.sub.2 F.sub.2, SCL.sub.2, S.sub.2 Cl.sub.2, and S.sub.2 Br.sub.2 ; or (2) a minimum of one sulphur halide chosen from among S.sub.2 F.sub.2, SCL.sub.2, S.sub.2 CL.sub.2, and S.sub.2 Br.sub.2. The ratio R of the total number of gram-atoms of sulphur present during the reduction reaction to the number of gram-moles of UF.sub.6 is at least 0.6.