First-principles study of lanthanum strontium manganite: Insights into electronic structure and oxygen vacancy formation

We characterize the structural, electronic, and defect behavior of La_1-xSrxMnO₃ (LSM) (xSr = 0.0, 0.25, and 0.5) by means of density functional theory + U (DFT+U) and hybrid DFT methods. Aliovalent substitution of Sr²⁺ for La³⁺ induces formation of holes in the LSM electronic structure. These holes affect electron and oxide ion transport, two key processes occurring within LSM when used as a solid oxide fuel cell (SOFC) cathode. To improve fundamental understanding of these processes, we investigated the atomic-scale effects of increasing Sr content and two different Mn magnetic moment alignments. In agreement with low-temperature experiments, we find a metallic, ferromagnetic (FM) electronic ground state with holes delocalized across the Mn and O sublattices. We also employ an antiferromagnetic (AFM) arrangement of Mn ions to model LSM's high-temperature paramagnetic state. In contrast to FM LSM, the holes in AFM LSM localize to form Mn⁴⁺ ions, consistent with the observed high-temperature polaronic transport in LSM. The formation of oxygen vacancies governs oxide ion transport in bulk LSM. We find that the ease with which oxygen vacancies form is strongly influenced by the Sr content and the overall magnetic arrangement of Mn ions. These atomic-scale insights enable us to propose new guidelines for enhanced nanoscale LSM-based SOFC cathodes.