Atomistic mechanisms of oxygen diffusion in YBa2Cu3O7 –x and YBa2Cu4O8
Mechanisms of oxygen-ion transport in the YBa2Cu3O7 –x(123) and YBa2Cu4O8(124) superconductors have been examined using atomistic simulation techniques, which offer an interesting comparison of structure–property relationships in the Y–Ba–Cu–O system. We have identified possible mechanisms of oxygen defect migration (vacancy and interstitial)via, conventional hopping processes. The results support speculative models in which diffusion is mediated by oxygen-ion vacancies for both oxides. Oxygen diffusion in 123 is attributed to vacancy migration between O(1) chain sites, although not directly but via, O(5) or O(4) sites; direct O(5)–O(5) interstitial motion is not supported by our calculations. The calculated migration energies of 0.99 eV for the O(1)–O(5) jump path in 123 and 2.0 eV for O(1)–O(1) in 124 are in good agreement with experimental values. For 123, the energy corresponds to the substoichiometric structure (x > 0) containing the missing chains of oxygens. The simulations predict that oxygen diffusion in 123 and 124 is mediated by different vacancy mechanisms.