Raman and electronic Raman spectra of lanthanide ions in elpasolite lattices

Abstract

Raman spectra have been recorded from 300--10 K for lanthanide hexachloroelpasolites, ${\mathrm{Cs}}_{2}$${\mathrm{NaLCl}}_{6}$. Electronic Raman-scattering intensity ratios have been calculated using Judd-Ofelt-Axe theory and are in reasonable agreement with experimental results, except for those cases where electron-phonon coupling has important consequences. Bands resulting from temperature-dependent electron-phonon coupling phenomena have been observed not only in the Raman spectrum of ${\mathrm{Cs}}_{2}$${\mathrm{NaYbCl}}_{6}$ but also in that of ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$, and the interaction mechanism is discussed. The coupling element between the coupled a${\mathrm{\ensuremath{\Gamma}}}_{5}$\ensuremath{\sim}${\mathrm{\ensuremath{\Gamma}}}_{1}$+${\ensuremath{\nu}}_{5}$ phonon-electron states of $^{3}\mathrm{H}_{6}$ in ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$ is estimated to be 20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and the corrected wave functions for these coupled states enable a consistent reinterpretation to be made of the optical spectra of ${\mathrm{TmCl}}_{6}^{3\mathrm{\ensuremath{-}}}$. Excited-state electronic Raman scattering has been observed in the 120-K Raman spectrum of ${\mathrm{Cs}}_{2}$${\mathrm{NaTmCl}}_{6}$. Using fixed wavelength argon-ion laser lines, there is evidence for a small resonance enhancement of electronic Raman-scattering intensity, and the unusual mechanism for this is discussed. The mixed crystal system ${\mathrm{Cs}}_{2}$${\mathrm{NaGd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Yb}}_{\mathrm{x}}$${\mathrm{Cl}}_{6}$ exhibits unimodal behavior for the ${\ensuremath{\nu}}_{1}$ and ${\ensuremath{\nu}}_{5}$ vibrations, and the more complex behavior of features in the neighborhood of ${\ensuremath{\nu}}_{2}$ is described. The vibrational energies of the gerade internal ${\mathrm{LCl}}_{6}^{3\mathrm{\ensuremath{-}}}$ modes (determined from Raman spectra) vary linearly with atomic number of L, except where electron-phonon coupling effects occur.