Gd2O3–SrO–Fe2O3 system: The phase diagram and oxygen content in oxides

Abstract

The phase diagram for the ½ Gd2O3–SrO–½ Fe2O3 system was constructed at 1100 °C in air. Six types of phases: Sr1-хGdхFeO3-δ (cubic, SG Pm3̅m, with 0.05≤х≤0.30 and orthorhombic, SG Pbnm, with 0.80≤х≤1.00), Sr2-yGdyFeO4-δ where y=0.75 and 0.80 with the K2NiF4-type structure, Sr3-uGduFe2O7-δ (two tetragonal oxides u=0 and 0.3 with SG I4/mmm and u=1.9 with SG P42/mnm), and Sr3.2Gd0.8Fe3O10-δ with the tetragonal structure (SG I4/mmm) were obtained. The structural parameters of single-phase samples were refined by the Rietveld analysis. A revised phase diagram for the GdFeO3-SrFeO3-δ system in the “T – composition” coordinates is proposed. Oxygen content in single-phase oxides is determined by TGA and iodometric titration. Oxygen content in the perovskite-type oxides gradually increases with Gd concentration. RP-type oxides with n=1 Sr2-yGdyFeO4-δ and with n=2Sr1.1Gd1.9Fe2O7-δ exhibit negligible oxygen losses with temperature while those with n=2 Sr3-uGduFe2O7-δ (0≤u≤0.3) and n=3 Sr3.2Gd0.8Fe3O10-δ demonstrate significant changes in oxygen content associated to higher Fe oxidation states.