In Nd3+ -doped CsCdBr 3 crystals, most of the Nd3+ ions form pairs. A detailed analysis of the optical spectroscopy of these pairs has been carried out using both absorption and luminescence spectra. Due to the low value of the Debye cutoff frequency for the phonons, many excited.1evels are luminescent, thus giving a lot of spectroscopic information and allowing the assignment of 65 levels to their associated irreducible representations. These results have been used to deduce, through a fitting procedure, the free-ion and crystal-field parameters. The results are discussed and compared with those found in other compounds.

a b s t r a c tThis work is devoted to the study of optical properties of the elpasolite Cs 2 NaAlF 6 with 0.1, 1.0, 3.0, 10.0, 30.0 and 50.0% of Cr 3 þ ions. The interest in this system lies on the fact that it presents a high quantum yield in the visible and infrared regions and therefore can be considered for laser applications. The photoluminescence and excitation spectra were obtained at 5 and 300 K, while the absorption spectra were measured at 300 K. The spectra at 300 K show broad bands attributed to the Cr 3 þ ions in two non-equivalent sites, both of them with octahedral coordination, while at 5 K we can observe the vibrational modes of the [CrF 6 ] 3 À complex. The ensemble of our results lead us to conclude that the material has potential applications as a lasing system.

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Résumé. -

-CsCdBr3 doped with trivalent rare earths is an attractive system to study energy transfer phenomena. Trivalent rare earths substituted to Cd' + can be associated in pairs close to a Cd^+ vacancy. In these systems, cooperative relaxation (up conversion and cross relaxation) are very efficient. A comparaison between Nd 3+ absorption and excitation spectra shows that the great majority of Nd 3+ are associated in one kind of pairs even in low doping crystals.