Impact of ion-host interactions on the5d-to-4fspectra of lanthanide rare-earth-metal ions. II. The Ce-doped elpasolites

Crystal growth of Ce-doped fluoride elpasolites is reported. Single crystals of ${\mathrm{Rb}}_2$ ${\mathrm{NaYF}}_6$ and ${\mathrm{Rb}}_2$ ${\mathrm{NaScF}}_6$ have been obtained by both vertical Bridgman and Czochralski techniques. Attempts to grow ${\mathrm{K}}_2$ ${\mathrm{NaYF}}_6$ by both methods yielded only polycrystalline samples. Ce-doped ${\mathrm{Rb}}_2$ ${\mathrm{NaYF}}_6$ and ${\mathrm{K}}_2$ ${\mathrm{NaYF}}_6$ display two distinct emitting centers, one in the blue and another in the near uv. Ce:${\mathrm{Rb}}_2$ ${\mathrm{NaScF}}_6$ displays only the blue-emitting center, and Ce:${\mathrm{K}}_2$ ${\mathrm{NaScF}}_6$ displays only the uv-emitting center. Growth and spectroscopic investigation of ${\mathrm{Rb}}_2$ ${\mathrm{NaYF}}_6$ doped with other rare earths established that the blue-emitting center is ${\mathrm{Ce}}^{3+}$ in the normal (Y or Sc) site. The uv-emitting center is ${\mathrm{Ce}}^{3+}$ in a larger, lower-symmetry site. It is hypothesized that this second site is related either to disordering or to the intergrowth of a second phase. Absorption, emission, excitation, and vacuum-uv transmission spectra were measured on Ce-doped elpasolites. Covalent contributions to the crystal field are numerically estimated by comparing the observed 5d energy levels with those predicted by an ionic-model calculation. These estimates as well as the qualitative dependence of the 5d energies on the lattice constant indicate that covalent contributions to the crystal field are significant. Design rules are suggested for choosing a fluoride host crystal for a visible-wavelength ${\mathrm{Ce}}^{3+}$ laser.