The impact of ion-host interactions on the 5d-to-4f spectra of lanthanide rare earth ions

Dielectric crystals doped with Ce3+ , Eu2 +, Sm2 +, and Yb2 + display broadband 5d-to-4f emission, and are candidate solid-state tunable laser materials. The absorption and emission wavelengths, however, are very host-sensitive, and a better understanding of 5d crystal field interactions is needed in order to design high performance materials. The adequacy of a purely ionic model of the crystal field is experimentally tested. Crystal growth of Ce-doped fluoride elpasolites is reported. Single crystals of Rb2NaYF6 and Rb2NaScF 6 have been obtained by both vertical Bridgman and Czochralski techniques. Attempts to grow K2NaYF 6 by both methods yielded only polycrystalline samples. Ce-doped Rb2NaYF 6 and K2NaYF6 display two distinct emitting centers, one in the blue and another in the near uv. Ce:Rb 2NaScF6 displays only the blue-emitting center, and Ce:K 2NaScF 6 displays only the uv-emitting center. Growth and spectroscopic investigation of Rb2NaYF6 doped with other rare earths established that the blue-emitting center is Ce3 + in the normal (Y or Sc) site. The uv-emitting center is Ce3+ 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 qualitive dependence of the 5d energies on the lattice constant indicate that covalent contributions to the crystal field are significant. The long-wavelength emission of Ce-doped elpasolites as compared to Ce:LiYF 4 is due to a lower 5d center of gravity, a larger Stokes shift, and larger 4f splittings. The anomalously broad emission bandwidth suggests that the Jahn-Teller effect on the emitting 5d level softens the