The intermediate valence of formally tetravalent compounds has been detected by L3 x-rayabsorption near-edge structure (XANES) in CeO& and in PrO& but not in UO2, which have the same CaFz structure and large f and ligand mixing. The intermediate valence has been found both in Ce02 and in Ce(SO4)2. 4H20, which have similar local structure but different crystal structure. We show that L3 XANES final states are a direct probe of configuration interaction between 4f" and 4f" +'L configurations in the ground state and that the weight of the 4f" +'L in the ground state can be deduced. The many-body final states arise from the characteristic properties of these materials: (i) the presence of localized 4f level above the oxygen 2p band separated by a gap 6e with relevant correlation energy Uff (Uff )5s) and (ii) mixing of 4f localized states with ligand valence orbitals such that the hybridization energy V is of the same order of magnitude as the energy separation b, E between the many-body configuration 4f" and 4f" +'L (V) bE). These insulating materials, which cannot be classified as standard mixed-valence systems, are called here interatomic intermediate-valencesystems.

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Europium-doped gadolinium carbonates particles have been prepared via urea-assisted precipitation. The reaction has been followed step by step with the investigations of the precipitate by transmission electron microscopy and wide-angle X-ray scattering, in relation with infrared absorption and thermal analyses. It has been observed that spherical particles of (Gd0.95Eu0.05)(OH)CO3, monodispersed in size and amorphous, precipitate first and then transform to agglomerated platelike crystals of (Gd0.95Eu0.05)2(CO3)3·2H2O as the precipitation continues. The Eu3+ 5D0 → 7FJ emission spectrum and the 5D0 lifetime in the two carbonate matrices have been measured. Selected hydroxycarbonate nanoparticles (NPs), with diameters of 164 ± 20 nm, have been then transformed to oxide NPs having the cubic crystalline structure C-(Gd0.95Eu0.05)2O3, with the same shape and size. The photoluminescence (PL) properties of hydroxycarbonate and oxide NPs, and of their colloidal suspensions in water, have been investigated. The hydroxycarbonate and the oxide NPs exhibit same PL intensities when excitation is achieved in one of the Eu3+(4f6) levels. Tests of in vitro fluorescence imaging have been performed. The luminescent NPs have been observed after their internalization by human cervical carcinoma (HeLa) cells. It is concluded that the controlled (urea-assisted) precipitation is appropriate to synthesize Gd(OH)CO3:Eu3+ and Gd2O3:Eu3+ nanoparticles having adequate characteristics for biolabeling.