Is excess faster than deficient? A molecular-dynamics study of oxygen-interstitial and oxygen-vacancy diffusion in CeO2

Abstract

Abstract The diffusivity of oxygen interstitials (D i) and of oxygen vacancies (D v) in fluorite-structured CeO2 was studied by means of classical molecular dynamic simulation techniques. Simulations were performed on cells that were either oxygen abundant or oxygen deficient at temperatures 1500 ≤ T / K ≤ 2000 for defect site fractions 0.18% ≤ n i/v ≤ 9.1%. In general, we found that at a given temperature T and defect site fraction n i/v the vacancy diffusivity D v was higher than the interstitial diffusivity D i. Isothermal values of D i and D v were constant at low defect site fractions (n i/v < 0.91%), but the behaviour diverged at higher n i/v: whereas D v decreased at higher n v, D i increased at higher n i. The analysis also yielded, as a function of n i/v, activation enthalpies (ΔH mig) and entropies (ΔS mig) of vacancy migration and of interstitial migration. A constant value of Δ H mig , v ≈ 0.6 eV was found for low n v, with increases in Δ H mig , v observed for n v > 0.91%. For low n i a constant value of Δ H mig , i ≈ 1.4 eV was found, with a surprising decrease in Δ H mig , i for n i > 0.91%. The effect of dopants on the behaviour of the defect diffusivities was also studied. Doping with Gd3+ had a detrimental effect on vacancy diffusion, with a slight decrease in D v and an increase in Δ H mig , v being observed. Donor doping with Nb5+, in contrast, was beneficial, resulting in higher D i and a decrease in Δ H mig , i . We suggest that the migration mechanism of oxygen interstitials in CeO2, non-collinear interstitialcy, is responsible for the lower defect diffusivity and higher migration barrier.