The thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3 (LSCF55) is investigated both by first principles phonon calculations combined with the quasi‐harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as αV,GGA=50.34*10‐6k‐1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in‐situ high temperature X‐ray diffractometer (HT‐XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with αa,hex=10.89*10‐6K‐1 and α c,hex=21.18*10‐6K‐1 is obtained. The volumetric thermal expansion coefficient is measured as αV,HT‐XRD=43.17*10‐6K‐1. Additionally, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield αl,bar specimen=17.37*10‐6K‐1 and αV,bar specimen=52.11*10‐6K‐1.
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