Valence stability of lanthanide ions in inorganic compounds

New methods are available to position the energy levels of divalent lanthanide impurities in wide band gap inorganic compounds (halides, chalcogenides) relative to the conduction and valence bands. These methods are used to determine the energy difference (E-Ff) between the 4f(n) ground state of the lanthanide and the Fermi energy assumed to be located midway between the top of the filled valence band and the bottom of the empty conduction band. It is demonstrated that EFf provides a measure for the stability of the divalent lanthanide against oxidation to the trivalent state and the stability of the trivalent one against reduction. The relation between EFf and the lanthanide valence state and how EFf depends on type of lanthanide ion and type of inorganic compound is addressed.