Oxygen vacancy ordering within anion-deficient Ceria

Abstract

The structural properties of anion deficient ceria, CeO 2− δ , 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, δ, and the isotropic thermal vibration parameters, u Ce and u O, starting at log 10 p(O 2)~−11. The increases are continuous, but show a distinct kink at log 10 p(O 2)~−14.5. Analysis of the total scattering (Bragg plus diffuse components) using reverse Monte Carlo (RMC) modelling indicates that the O 2− vacancies preferentially align as pairs in the 〈 111 〉 cubic directions as the degree of nonstoichiometry increases. This behaviour is discussed with reference to the chemical crystallography of the CeO 2–Ce 2O 3 system at ambient temperature and, in particular, to the nature of the long-range ordering of O 2− vacancies within the crystal structure of Ce 7O 12.