CE3--]CE4+ CONVERSION IN CERIA-DOPED ZIRCONIA SINGLE-CRYSTALS INDUCEDBY OXIDE-REDUCTION TREATMENTS([)

Optical absorption and resonant Raman techniques have been used to stu dy the Ce3+<->Ce4+ conversion in cerium-doped yttria-stabilized zircon ia single crystals (YSZ). The reduced material presents an intense ora nge coloration associated with a broad absorption band at 460 nm that we assign to a 4f-->5d transition. Its experimental oscillator strengt h is approximate to 1x10(-2). The Raman spectra consist of the lattice phonon bands at 600 cm(-1) and below together with an intense electro nic Raman peak at 2086 cm(-1). The anomalous intensity of this peak is due to a resonance effect by the excitation in the 4f-->5d transition . The peak is associated with the F-2(7/2)-->F-2(5/2) electronic trans ition within the 4f(1) configuration of Ce3+. Heating in air at 1000 d egrees C produces a complete discoloration of the sample. The effect o f the oxidation can be fully reversed by reducing the sample at the sa me temperature. Optical spectroscopy has been used to determine the ki netics of the oxide-reduction process. Isothermal annealing in the tem perature range 750-950 degrees C were performed in both oxidizing and reducing atmospheres. The results indicate that the oxidation process is diffusion-limited with an activation energy of approximate to 1.7 e V. For the reduction two alternative limiting processes are proposed: electron diffusion and ionization reaction.