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

Abstract

The diffusivity of oxygen interstitials (Di) and of oxygen vacancies (Dv) 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% ≤ ni/v ≤ 9.1%. In general, we found that at a given temperature T and defect site fraction ni/v the vacancy diffusivity Dv was higher than the interstitial diffusivity Di. Isothermal values of Di and Dv were constant at low defect site fractions (ni/v < 0.91%), but the behaviour diverged at higher ni/v: whereas Dv decreased at higher nv, Di increased at higher ni. The analysis also yielded, as a function of ni/v, activation enthalpies (ΔHmig) and entropies (ΔSmig) of vacancy migration and of interstitial migration. A constant value of 0.6 eV was found for low nv, with increases in observed for nv > 0.91%. For low ni a constant value of 1.4 eV was found, with a surprising decrease in for ni > 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 Dv and an increase in being observed. Donor doping with Nb5+, in contrast, was beneficial, resulting in higher Di and a decrease in . 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.