First-principles study of formation, diffusion, and recombination of defects in YBa 2 Cu 3 O 7

Abstract In a fusion tokamak device, high temperature superconducting (HTS) magnet materials, such as yttrium barium copper oxide (YBa 2 Cu 3 O 7− δ ), will be exposed to high energy neutrons, which will create defects that may impact the superconducting properties and ultimately lead to degradation of the magnetic field strength. First-principles density functional theory (DFT) calculations have been used to investigate the energetics of point defects in YBa 2 Cu 3 O 7 . The formation energies of Frenkel pairs (FPs) indicate oxygen defects are dominant, followed by Cu defects. Ba and Y defects are less energetically favorable. DFT combined with the nudged elastic band method has been used to evaluate the diffusion activation energy of point defects. The DFT results predict anisotropic diffusivity of oxygen defects. Oxygen prefers to diffuse between the adjacent BaO and basal planes along the b -axis via a vacancy mechanism with an energy of 0.4 eV. Oxygen can diffuse on the basal plane via either interstitial or vacancy mechanism, with the ability to diffuse along the c -axis at relatively high temperatures. Excess Ba and Y prefer to create antisite defects on the Cu sub-lattice with concomitant formation of Cu interstitials, rather than being at interstitial positions. Cu interstitials can diffuse in bulk YBa 2 Cu 3 O 7 with an activation energy of about 1.1 eV. Cation vacancies are much less mobile than interstitials or oxygen vacancies. The required energies for the diffusion of defects and the recombination of oxygen FPs indicate irradiation temperature dependence of property degradation in YBa 2 Cu 3 O 7 and different recovery rates of superconducting properties of irradiated YBa 2 Cu 3 O 7 following thermal annealing with increasing temperatures. Such computational studies, when coupled with cryogenic neutron or ion irradiation, offer the potential to accelerate the qualification of HTS magnet materials for fusion applications.