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

Abstract 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 $${Pm}\bar{3}m$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>m</mml:mi> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>m</mml:mi> </mml:math> and $${Pbnm}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>b</mml:mi> <mml:mi>n</mml:mi> <mml:mi>m</mml:mi> </mml:math> phases of LSCF within individual grains, instead of the otherwise commonly reported $$R\bar{3}c$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>R</mml:mi> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>c</mml:mi> </mml:math> phase. The observed phase separation is governed primarily by the Sr distribution: the $${Pm}\bar{3}m$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>m</mml:mi> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>m</mml:mi> </mml:math> is Sr-rich, with more than 50% A-site occupancy, whereas the $${Pbnm}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>b</mml:mi> <mml:mi>n</mml:mi> <mml:mi>m</mml:mi> </mml:math> 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 $${Pm}\bar{3}m$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>m</mml:mi> <mml:mover> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>m</mml:mi> </mml:math> phase compared with the La-rich $${Pbnm}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mi>b</mml:mi> <mml:mi>n</mml:mi> <mml:mi>m</mml:mi> </mml:math> 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.