Combined experimental and ab initio based determination of the thermal expansion of La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3

The thermal expansion of La 0.5 Sr 0.5 Co 0.25 Fe 0.75 O 3 ( LSCF 55) 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 LSCF 55 is calculated as α V ,GGA = 50.34 × 10 −6 K −1 . For comparison, the lattice expansion and the volume expansion of LSCF 55 grain are measured by in situ high‐temperature X‐ray diffractometer ( HT ‐ XRD ). An anisotropic thermal expansion of rhombohedral LSCF 55 with α a ,hex = 10.89 × 10 −6 K −1 and α c ,hex = 21.18 × 10 −6 K −1 is obtained. The volumetric thermal expansion coefficient is measured as α V ,HT‐XRD = 43.17 × 10 −6 K −1 . In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF 55 bar specimen are measured using a vertical high‐performance thermo‐mechanical analyzer and yield α l ,bar specimen = 17.37 × 10 −6 K −1 and α V ,bar specimen = 52.11 × 10 −6 K −1 .