Host dependence of the rare-earth ion energy separation 4f N–4f N−1 n l

The interaction between two 4f electrons in a rare-earth ion by means of the polarization of a ligand in a solid is discussed. It is shown that this interaction gives rise to a host dependent shift of the energy separations between the 4f N and 4f N−1 nl configurations. The results of the theory of these energy shifts are compared to experimental data for Nd3+, Er3+, and Tm3+ in the hosts YF3, LuF3, LiYF4, and LaF3. Also a shift in the Slater integrals and the spin–orbit parameter is predicted by the same interaction.