Defect characterization in Ce0sub(2-x) at elevated temperatures

Polycrystalline Ce0sub(2-x) (0 <= x <= 0.21) has been examined at 9000C by means of neutron diffraction to characterize the predominant atomic point defects responsible for nonstoichiometry. The stoichiometry was controlled by adjusting the oxygen partial pressure between 1 and 10-21 atm. For x <= 0.11, good agreement (residual <= 1.0%) is obtained by assuming a random distribution of defects on the oxygen sublattice. Vacancy concentrations obtained from the neutron-data analysis are in agreement with thermogravimetric determinations. Over this region of x, the temperature factors of both the cation and anion increase with an increase in vacancy concentration, which implies greater root-mean-square (r.m.s.) displacement of the atoms. However, for x > 0.11, the analysis of the neutron data indicates a smaller anion vibrational amplitude than in the near-stoichiometric compositions and is accompanied by a shift in the equilibrium position of the oxygen atoms. The effect of anharmonic thermal motion has also been considered, but alone, cannot explain the neutron data. The results of the present study correlate well with the previous X-ray experiment. (author)