Phase transitions in the SrSnO₃-SrFeO₃ solid solutions: X-ray diffraction and Mössbauer studies

SrSn_1-xFe_xO_y (0≤x≤1) oxides were prepared by conventional solid-state reaction in air using high-purity (≥99%) SrCO₃, SnO₂ and Fe(C₂O) ₂·2H₂O at elevated temperatures of 1300°C for 24h and furnace cooled. Samples obtained from 1300°C were annealed at 620°C for 2 days and quenched in liquid nitrogen (LN). Powder XRD analysis by Rietveld refinement and Fe Mössbauer spectroscopy measurements were employed to characterize synthesized perovskites. Samples obtained from furnace cooled and LN quenched undergo two compositionally driven phase transitions, which are supposed to be of second order. The x=0-0.3 members crystallize in orthorhombic parent SrSnO₃ structure (Space group Pbnm), whereas samples x=0.4-0.9 have a simple cubic perovskite cell and end-member SrFeO _2.74 composition crystallize orthorhombic structure (Space group Cmmm). The composition of the first phase transition (x≈0.3) is slightly shifted to higher x with decreasing annealing temperature. Mössbauer data show that the Fe⁴⁺/Fe^tot ratio is depending on composition under constant synthesis conditions. The phase compositions have been discussed in terms of ternary solid solution of compounds SrSnO ₃-SrFeO_2.74-SrFeO_2.5 superior to a simple binary solid solution (SrSnO₃-SrFeO₃).