First-principles study on enhancement of the oxygen diffusivity in La2−xSrxCuO4 by cation substitution and strain*

Sr-doped lanthanum cuprate (La2−xSrxCuO4), one of the most studied high-TcTc superconductors, has drawn much attention as a candidate for cathode material in solid oxide fuel cells (SOFCs) due to its fast oxygen evolution capability. Although its large ionic conductivity has been phenomenologically explained by diffusion channels formed by interstitial oxygen and oxygen vacancy, its atomistic and systematic mechanism has not been understood clearly. Using density functional theory calculation, we identify vacancy diffusion paths with low activation energies leading to anisotropic oxygen diffusivity. It is found that not only the activation energy but also the formation energy of oxygen vacancy, especially at an apical site, can be drastically reduced by applying biaxial strain. We further reveal that Sr substitutional doing plays two competitive roles in the oxygen diffusivity: (1) increasing the equilibrium concentration of oxygen vacancy enhancing diffusion channel and (2) trapping oxygen vacancy around a Sr atom raising the activation energy. Our study would be applied to improve the efficiency of SOFCs.