Oxygen nonstoichiometry, thermo-chemical stability and lattice expansion of La_0.6Sr_0.4FeO_3-δ

The oxygen nonstoichiometry of La_0.6Sr_0.4FeO _{3 - δ} was measured at intermediate temperatures (773 to 1173 K) between 1 bar and the decomposition oxygen partial pressure by thermogravimetry and coulometric titration. The decomposition of the ABO₃ perovskite phase was found to occur at low oxygen partial pressures (below 10 ^{- 20} bar). Using an atmosphere-controlled high-temperature XRD setup, the rhombohedral lattice parameters were obtained between 10^{- 4} and 1 bar at 773 to 1173 K. A phase transition from rhombohedral to cubic might be expected to occur at high temperatures and for δ near the plateau at δ = [Sr] / 2. The lattice expansion was separated into "pure" thermal and chemically induced expansion by combining the lattice parameters with the oxygen nonstoichiometry data. The linear thermal expansion was formulated with a "pure" thermal expansion coefficient of α_th = 11.052 • 10^{- 6} K^{- 1} and a chemical expansion coefficient of α_chem = 1.994 • 10 ^{- 2}. The results were compared with previous data obtained for La_0.6Sr_0.4Co_{1 - y}Fe_yO _{3 - δ} with y = 0.2-0.8. La_0.6Sr _0.4FeO_{3 - δ} was confirmed to show the highest thermo-chemical stability. While the chemical expansion of La _0.6Sr_0.4Co_{1 - y}Fe_yO _{3 - δ} seems little affected by the iron content, the thermal expansion coefficient was the lowest for La_0.6Sr _0.4FeO_{3 - δ}.