A theoretical investigation of 4f→5d transition of trivalent rare earth ions in fluorides and complex oxides

Abstract

We have investigated 4f n→4f n−15d excitation energies for trivalent rare earth ions in LaF 3, LiYF 4, YF 3, LiGdF 4, K 2YF 5, YAlO 3, LaBO 3 and YPO 4 using a spin density functional procedure based on the atomic sphere approximation (computer code ASW). The local relaxation of the activator ions in the host lattices has been calculated by the projector augmented wave method (computer code VASP). The experimentally observed 4f n→4f n−15d excitation energies are shown to depend linearly on the calculated energy difference between the one-electron f- and d-like states of the rare earth ions Δ ε f−d. The linear relationship, Δ E= mΔ ε+ ε shift, involves two empirical parameters m and ε shift. m is found to be the same for all ions and hosts, and ε shift is different for the different ions but independent of the host lattice. In the present case, m and ε shift have been determined empirically by a least-square fitting procedure from the experimental and calculated Δ E for the hosts LaF 3, YF 3 and LiYF 4. The error of the predicted excitation energies is lower than 0.3 eV for the systems. This error increases up to 0.6 eV for some heavy rare earth ions for which the calculated occupied one-electron f-like states overlap energetically with the host valence band states (Er and Tm in the hosts LaF 3, YPO 4 and LiYF 4).