Vibrational analysis of the elpasolites Cs2NaAlF6 and Cs2NaGaF6 doped withCr3+ ions by fluorescence spectroscopy

Interest in 3d transition metal impurities in ionic crystals has increased due to their important role in the laser activity of these materials. Moreo ver, recent advances in tunable solid-state lasers and high-power semicondu ctor laser diode arrays have generated a strong interest in investigating n ew compounds that emit in the visible and near-infrared spectral regions. I n particular, many optical studies have been devoted to Cr3+-doped fluoride crystals as a consequence of the high quality of some Cr3+-based laser mat erials. In the present investigation, the low-temperature emission spectra of Cr3+ ions in the hexagonal elpasolites Cs2NaAlF6 and Cs3NaGaF6 have been measured. Each compound has two crystallographically inequivalent octahedr al sites for the Al3+ and Ga3+ ions that can be occupied by Cr3+ ions. For- both materials, the luminescence spectrum presents two zero-phonon lines ac companied by a well-defined vibrational structure. The different peaks of t he emission broad band are described in terms of phonons of the lattice and normal modes of the octahedral complex [CrF6](3-). A detailed analysis of the vibrational structure observed leads to the conclusion that the E-2 and T-4(2) excited states of the [CrF6](3-) ion are displaced along the e(g) a nd a(1g) and probably the t(2g) coordinates.