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

SP Waldow and RA De Souza, JOURNAL OF PHYSICS-ENERGY, 2, 024001 (2020).

DOI: 10.1088/2515-7655/ab5cfd

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 (Delta H-mig) and entropies (Delta S-mig) of vacancy migration and of interstitial migration. A constant value of Delta H-mig,(v) approximate to 0.6 eV was found for low n(v), with increases in Delta H-mig,H-v observed for n(v) > 0.91%. For low n(i) a constant value of Delta H-mig,H-i approximate to 1.4 eV was found, with a surprising decrease in Delta H-mig,H-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 Delta H-mig,H-v being observed. Donor doping with Nb5+, in contrast, was beneficial, resulting in higher D-i and a decrease in Delta H-mig,H-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.

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