TRANSITION-METAL ION DIMERS FORMED IN CEO2 - AN EPR STUDY

MeCeO oxides (Me = Cu2+, Mn2+, V4+, Mo5+), With an atomic ratio Me/Ce = 0.1, have been prepared by impregnation of ceria (CeO,) with the cor responding Me salt. Subsequently, the solids were oxidized or reduced at given temperatures. An EPR study evidenced the presence of dimers i n the solids. A fine structure was obtained for CuCeO and MoCeO oxides . The signal at half magnetic field intensity (Delta m(s) = +/-2), cha racteristic of dimers, was present in all the MeCeO oxides with a hype rfine structure for Cu2+, Mn2+ or V4+ ions. From the relative intensit y of the half-field and the normal-field signals characterizing the di mer spectrum, the distance between the Me ions has been calculated. Fo r CuCeO and MoCeO oxides, monomer ions have been evidenced as precurso rs of the dimers. The dimer spectrum of Mo5+ ions has been observed on ly at 77 K and not at room temperature. In all cases, the dimers are l ocated in oxygen Vacancies near Ce3+ ions, present in the solid before the introduction of transition-metal ions.