Exchange and crystal field effects on the 4fn 15d levels of Tb³

The levels of the 4f n −1 5d configuration of Ce 3+ ( n = 1), Pr 3+ ( n = 2), and Tb 3+ ( n = 8) in compounds are compared with each other. A model is presented that, by means of energy shift operations performed on the five 5d levels of Ce 3+ , reproduces the energies of those for Pr 3+ and Tb 3+ . Using Tb 3+ data, two main sources of deviations from the shift model are identified. One is related to the size difference between Tb 3+ and Ce 3+ which affects the lattice relaxation and the crystal field splitting of the 5d configuration. The other is related to the isotropic exchange interaction between the 5d electron spin and the total spin of the 4f 7 electrons in Tb 3+ . The exchange splitting is about 1 eV in fluorides, sulfates, and phosphates. In oxides with less strongly bonded oxygen 2p electrons, the exchange splitting decreases to 0.6 eV. The effects of the two deviations on the predictability of the 4f n −1 5d energy levels of Tb 3+ and other lanthanides are discussed.