Crystal Structure and Thermal Expansion of La _{1− <i>x</i>} Sr _<i>x</i> FeO _3−δ Materials

Abstract

Crystal structure and thermal properties of La 1− x Sr x FeO 3−δ ( x = 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high‐temperature X‐ray diffraction and thermal analysis in air and nitrogen ( p (O 2 ) ∼ 10 −3 atm) atmosphere. The first‐order phase transition from orthorhombic‐to‐rhombohedral La 1− x Sr x FeO 3−δ ( x = 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase‐transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO 3 and the observed volume contraction of the phase transition. Rhombohedral La 1− x Sr x FeO 3−δ ( x = 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second‐order phase‐transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3 –SrFeO 3−δ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr‐rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.