Oxygen concentration effect on the behavior of point defects in YBa2Cu3O6.5−7: A first-principles study

Understanding defect behavior in high-temperature superconducting magnet materials, such as yttrium barium copper oxide (YBa2Cu3O7−δ), is vital for predicting changes in their superconducting properties during operation in compact fusion reactors. First-principles density functional theory (DFT) calculations are used to investigate the energetics of point defects in oxygen-deficient YBa2Cu3Ox (6.5 ≤ x < 7) in comparison to YBa2Cu3O7. With decreasing oxygen stoichiometry, the formation energies of oxygen vacancies generally increase, while those of interstitials decrease. The formation energies of single cation defects as a function of oxygen stoichiometry are similar. Oxygen is predicted to diffuse along b-axes via a vacancy mechanism with an activation energy of around 0.5 eV. The diffusion activation energy of oxygen defects in oxygen-deficient YBa2Cu3Ox is larger than that in YBa2Cu3O7 without oxygen vacancies. The mean recombination energy of oxygen Frenkel pairs slightly increases with decreasing oxygen concentration. The formation energy of Frenkel pairs in oxygen-deficient YBa2Cu3Ox, particularly at x ≈ 6.9, is lower relative to YBa2Cu3O7, resulting in a higher number of irradiation defects in YBa2Cu3Ox; additionally, full-chain oxygen vacancy configurations in YBa2Cu3Ox (x = 6.75, 6.50) enhance material stability under irradiation relative to the x ≈ 6.9 structure.