Spectra, Energy Levels and Energy Transfer in High Symmetry Lanthanide Compounds

The Ln3+ ion is situated at a site of octahedral symmetry in crystals of the lanthanide elpasolites, M2ALnX6. The syntheses and crystal structures of M2ALnX6 are described, with particular reference to phase transitions. The centrosymmetric environment of Ln3+ confers different spectral properties than are observed for most solid-state lanthanide ion systems, and the selection rules are deduced. The vibrational, electronic Raman and electronic spectra of the lanthanide elpasolite systems are presented and discussed. The electronic spectra are described in terms of the 4fN-4fN zero phonon line and the vibronic sideband structure and the success of intensity calculations is evaluated. The synergy of one- and two-photon studies is emphasized. The differences with the 4fN-4fN−15d spectra of M2ALnX6 are illustrated. The parametrization of the energy level datasets is assessed and the success of recent refinements is analysed. Following a brief review of the theory of energy transfer in crystals, experimental studies of quenching, cross-relaxation, upconversion and photon avalanche in neat and doped elpasolite systems are reviewed. Physical mechanisms are given for spectral features and for energy transfer pathways.