Vibrational and electronic spectra of

Single crystals of doped with molar concentrations of between 0.1 and 20% have been synthesized by the hydrothermal method, and studied by absorption, excitation and luminescence spectroscopy at temperatures down to 4 K. The observed spectral features were consistently assigned with reference to those of the corresponding europium hexachloroelpasolite. No distortion from octahedral symmetry was evident for the site in the 0.1 mol%-doped crystal, but small splittings of magnetic dipole zero-phonon lines in the spectra of the 20%-doped crystals are observed below 20 K. The vibrational behaviour of has been interpreted, and the energy-level scheme of the ion was deduced from the observed and inferred zero-phonon-line positions in the optical spectra. A parametrized Hamiltonian was fitted to 21 crystal-field levels of in . The fourth- and sixth-degree crystal-field parameters were about 1.6 times larger than for . The temperature shifts of the crystal-field levels were found to be negative, in contrast to the mostly positive shifts previously reported for actinide systems in octahedral symmetry, and were largely determined by the changes in the Slater parameters rather than the crystal-field parameters. A preliminary study has been made of the decay kinetics of the levels. is quenched by a cross-relaxation mechanism, and also by a thermally activated mechanism. The lifetime of does not exhibit a noticeable temperature or concentration dependence.