Charge transfer and crystal-field theory for rare-earth ions

The tendency of Eu 3+ in a crystal to attract an electron from the surrounding ligands and become Eu 2+ can be represented by a six-body operator acting within the configuration 4f 6 . The matrix elements of this operator within any configuration 4f N with N>6 are expressible in terms of those of the unit tensors U (k) and the double tensor V (1k) . This mechanism of charge transfer generally acts in a sense opposite to that of the spin correlated crystal field. This provides a possible source for the anomalous crystal-field splittings of Ho 3+ 4f 10 3 K J as well as the abrupt decline in the sixth-rank crystal-field parameters in passing from Eu 3+ to Tb 3+ in the LaCl 3 lattice.