ELECTRON-TRANSFER REACTIONS IN NONSTOICHIOMETRIC CERIA AND URANIA

The fluorite oxides ceria and urania can be made non-stoichiometric by loss and gain, respectively, of oxygen atoms, thereby forming CeO2-x and UO2+x. The former now contains Ce3+ and Ce4+ ions and both single crystal and powder forms are blue and exhibit electron transfer in the form of intervalence spectra. The absorption spectra of pure and redu ced ceria crystals are interpreted and used to help resolve electrical conductivity studies. Solid solutions of ceria and urania are blue-bl ack and here electron transfer is of the form Ce(IV)+U(IV)-->Ce(III)+U (V). The intervalence contribution to the absorption spectrum of ultra -thin single crystal wafers of UO2+x is only available as a difference spectrum, using the UO(2)available as a difference spectrum, using th e UO2 spectrum as reference. The U5+ ions formed and contributing to t he intervalence spectra in freshly oxidised samples, quenched from UO2 +x at high temperature, were found to be metastable at ambient tempera ture and slowly oxidised to U6+. The intervalence spectral profile cha nged significantly and slowly with time. Thus in aged crystals the ele ctron transfer in the intervalence spectrum is principally due to U(IV )+U(VI)-->U(V)+U(V). Monitoring these changes should yield information on the precipitation of interstitial clusters containing oxygen out o f the urania matrix into aggregates and hence the growth as U3O7 on su rfaces and at grain boundaries.