Anisotropic and Nonlinear Thermal and Chemical Expansion of La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ (<i>x</i> = 0.3, 0.4, 0.5) Perovskite Materials

La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ (<i>x</i> = 0.3, 0.4, 0.5) materials were investigated by in situ high-temperature X-ray diffraction in pure O₂ and N₂. A rhombohedral symmetry was found at ambient temperature for the perovskite materials in agreement with previous reports. The rhombohedral distortion of the perovskite structure (space group <i>R</i>3̅<i>c</i>) from the cubic prototype decreased with increasing temperature, and the materials became cubic perovskites (<i>Pm</i>3̅<i>m</i>) at elevated temperatures. The cubic to rhombohedral phase transition temperature decreased with increasing Sr-substitution level and with decreasing partial pressure of oxygen. The thermal expansion of the materials below the phase transition was strongly anisotropic. The two order parameters of the phase transition, the antiferrodistortive tilting and compression of the FeO_6/2 octahedra, were determined by Rietveld refinement of the diffraction patterns. A large chemical expansion of the materials because of thermal reduction of the oxidation state of Fe was observed. The chemical expansion was strongly anisotropic below the rhombohedral to cubic phase transition temperature and strongly nonlinear with respect to the oxygen non-stoichiometry. The microscopic origin of the anisotropic nature of the chemical expansion is discussed as well as the nonlinear dependence of chemical expansion with respect to the concentration of oxygen vacancies