Decoding the surface instability of perovskite oxides at the atomic level: Sr segregation in La_1-xSr_xMnO_3±δ in SOFC electrodes

One of the most important and best-studied perovskite materials for energy conversion applications is La_1-xSr_xMnO₃ (LSM). As the oxygen exchange at the surface is typically rate-limiting, tuning of the electrode surface is a key aspect in the development and optimization of materials for energy conversion applications. Sr-doped perovskite materials form passivating Sr-rich layers which inhibit the oxygen exchange at the surface, thus reducing the efficiency of the SOFC.
We assess the stability of LSM surface and bulk using a combination of DFT calculations on the GGA+U level and DFT-based thermodynamics. We find the clean LSM(001) surface to be unstable and prone to reconstructions and defect segregation.
Considering a wide variety of near-surface defects, as well as the growth of SrO as clusters, particles or homoepitaxial layers, we come to the conclusion that Sr segregation should be self-limiting because the surface dipole moment is removed by SrO overlayers that do not cover the entire surface. These surface terminations are stable and not prone to further segregation of Sr.
These results allow us to develop a knowledge-based doping strategy that prevents Sr segregation in LSM. Our modeling strategy and principle findings are transferable to other perovskite oxides.