The Interatomic Intermediate Valence State of Insulating Correlated Oxides CeO2, PrO2 and TbO2

The intermediate valence of formally tetravalent compounds has been detected by XANES (X-ray Absorption Near Edge Structure) in CeO2, PrO2 and in TbO2 but not in ThO2 and UO2 which have the same CaF2 structure. These materials, in the framework of many body theoretical description of the ground state by the filled-band Anderson impurity model, can be classified as interatomic intermediate valence (IIV) systems whose specific properties are: i) a large mixing of 4f localized states with ligand valence orbitals, ii) a charge transfer energy δE (which determines the gap) smaller than the correlation energy Uff (Uff>δE) and iii) a hybridization energy V between 4f and oxygen 2p orbitals of the same order of magnitude as the energy separation ΔE between many body configuration 4f and 4fn+1 L, where L denotes a ligand hole (V∼E).