Locating lanthanide impurity levels in the forbidden band of host crystals

The systematics of f→d transition energies of divalent lanthanides and charge transfer energies of trivalent lanthanides with type of lanthanide ion and type of host crystal are combined into a three parameter model which predicts the absolute location of the energy levels of divalent lanthanides relative to the valence and conduction band states. Including information on the f→d transition in Ce3+ together with some assumptions or further experimental data, the levels of trivalent lanthanides can also be deduced. CaF2 and YPO4 are used as examples to demonstrate the model.