Phase equilibria and oxygen content of intermediate phases in the Sm2O3-SrO-Fe2O3system

Six types of oxide phases are formed in the Sm2O3?SrO?Fe2O3 system at 1100 ?C in air: perovskite type Sr1-?Sm?FeO3-? (0.05 < ? < 0.50 with the cubic structure, S.G. Pm-3m and 0.85 < x < 1.0 with the orthorhombic structure, S.G. Pbnm), and Ruddlesden-Popper series corresponding to n = 1,2,3, i.e. Sr2-ySmyFeO4-?, Sr3-uSmuFe2O7-? (0 < u < 0.3; u = 1.80) and Sr4-rSmrFe3O10-?. The structural parameters of single-phase samples, refined by the Rietveld method, allowed to reveal the variation of unit cell parameters with composition. A subsolidus isothermal isobaric phase diagram (1100 ?C, Po2 = 0.21 atm) for the 1/2Sm2O3?SrO?1/2Fe2O3 system has been constructed. The compositional triangle was divided into 24 phase fields. The values of oxygen content and mean oxidation state of iron in all single phases were determined by iodometric titration. The changes in oxygen content with temperature in Sr2.7Sm0.3Fe2O7-?, Sr1.2Sm0.8FeO4-?, and Sr3.1Sm0.9Fe3O10-? were studied by TGA within the range of 25-1100 ?C in air. They show that the maximum oxidation state of Fe ions in RP oxides at room temperature increases with n, reaching 3.52 for the Sr3.1Sm0.9Fe3O10-? n = 3 member.