Revealing the intragranular phase separation in La1-xSrxCo1-yFeyO3-δ via advanced electron microscopy and spectroscopy

Lanthanum strontium cobalt ferrite (LSCF) is a prototypical mixed ionic and electronic conductor (MIEC), widely used in solid oxide fuel cells (SOFC) and O 2 separation membrane. Despite its technological relevance, the structure-property relationship of LSCF remain incompletely understood, especially at the atomic-scale. Here, we employ advanced transmission electron microscopy (TEM) to probe the intragranular phase separation of LSCF. Quantitative energy-dispersive X-ray spectroscopy (EDXS) and high-resolution imaging reveal the coexistence of Pm3̄m P m 3 ¯ m and Pbnm P b n m phases of LSCF within individual grains, instead of the otherwise commonly reported R3̄c R 3 ¯ c phase. The observed phase separation is governed primarily by the Sr distribution: the Pm3̄m P m 3 ¯ m is Sr-rich, with more than 50% A-site occupancy, whereas the Pbnm P b n m phase preferentially incorporates lanthanide cations. Electron energy loss spectroscopy (EELS) directly reveals that the charge imbalance caused by Sr substitution is mainly compensated by the formation of oxygen vacancies (V O ). Approximately three times higher concentration of V O s without ordering was determined in the Sr-rich Pm3̄m P m 3 ¯ m phase compared with the La-rich Pbnm P b n m phase. Our results provide atomic scale insights into the structure-property relationship of LSCF, advancing the fundamental interpretation of LSCF-based MIEC materials and offering guidance for the rational design of high-performance energy materials.