Oxygen vacancy ordering within anion-deficient Ceria

The structural properties of anion deficient ceria, CeO2-delta, have been studied as a function of oxygen partial pressure, p(O-2), over the range 0 >= log(10)p(O-2) >= -18.9 at 1273(2) K using the neutron powder diffraction technique. Rietveld refinement of the diffraction data collected on decreasing p(O-2) showed increases in the cubic lattice parameter, a, the oxygen nonstoichiometry, delta, and the isotropic thermal vibration parameters, u(Ce) and u(O), starting at log(10)p(O-2)similar to-11. The increases are continuous, but show a distinct kink at log(10)p(O-2)similar to-14.5. Analysis of the total scattering (Bragg plus diffuse components) using reverse Monte Carlo (RMC) modelling indicates that the O2- vacancies preferentially align as pairs in the cubic directions as the degree of nonstoichiometry increases. This behaviour is discussed with reference to the chemical crystallography of the CeO2-Ce2O3 system at ambient temperature and, in particular, to the nature of the long-range ordering of O2- vacancies within the crystal structure of Ce7O12. (C) 2009 Elsevier Inc. All rights reserved.