Occurrence of Strontium–Iron Oxide SrFe12O19 in the Fe2O3–Na2O–SrSO4 System

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Abstract
The formation of SrFe12O19 in the Fe2O3–Na2CO3–SrSO4 system was found to proceed through three stages, first an endothermic reaction between Fe2O3 and Na2CO3 to form Na2Fe2O4 and CO2, second an exothermic reaction of Na2Fe2O4 with SrSO4 to produce Na2SO4, Sr7Fe10O22, and SrFe12O19, third an exothermic reaction between Sr7Fe10O22 and Fe2O3 towards more SrFe12O19. The yield of SrFe12O19 depended on the molar ratio of the reactants and on temperature. Below 700 °C the formation did not proceed beyond the second stage. Samples prefired at 900–1200 °C contained SrFe12O19 in all cases when the original relative molar concentrations xFe2O3,yNa2CO3, and zSrSO4 satisfied the condition (x − y + z)/z > 5/7 or x/y > 5/7 ≥ x/z. Substitution of Na2CO3 in the reaction mixtures with equivalent amounts of NaOH or other precursors of Na2O did not affect the second and the third stage of the reaction and the final yield of SrFe12O19. On the ternary diagram of the Fe2O3–Na2O–SrSO4 system the SrFe12O19 region was represented by the area of a quadrilateral with the corner points located at the composition points 98-1-1, 1-1-98, 26-36-38, and 50-49-1 mole percent. The highest yields were obtained from the mixtures represented by the molar ratio 6Fe2O3:1 Na2O:1SrSO4. The hexagonal lattice parameters of SrFe12O19 were determined as a0 = 5.885 Å and c0 = 23.047 Å. X-Ray diffraction reflections of SrFe12O19 were indexed and reassignment of several indices in ASTM 7-276 for BaFe12O19 was suggested.

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