Molecular dynamics analysis of lattice site dependent oxygen ion diffusion in YBa2Cu3O7 − δ: Exposing the origin of anisotropic oxygen diffusivity

The oxygen diffusion in YBa2Cu3O7 − δ (YBCO) at different temperatures and oxygen contents is explored by molecular dynamics simulations. At low temperatures, the diffusivity of oxygen ions in YBCO is found to be strongly dependent on the lattice site. It is shown that the oxygen ions in Cu–O chains diffuse much faster than that in Ba–O layers and Cu–O planes. However, when temperature is increased, the diffusivity becomes less sensitive to lattice site. Moreover, distinct anisotropy is also observed for the oxygen diffusion on different lattice sites. By explicitly calculating directional mean square displacement, we show that the oxygen ions in the Cu–O planes and Ba–O layers are similarly more prone to diffuse along the c axis direction while in the Cu–O chains the oxygen ions are more likely to migrate along the ab plane. As temperature increases from 700 to 1100 K, the diffusion anisotropies decrease. The underlying microscopic origins for the above peculiarities of oxygen diffusion are analyzed.