Solid state synthesis of strontium oxoferrates from the mechanically activated system SrCO3-Fe2O3

The solid state reactions occurring in the mechanically activated system SrCO3-Fe2O3 (composition XFe (iron cationic fraction) = 0.5-0.923) have been studied. At XFe = 0.5, SrFeO2.5+x (brownmillerite) forms by annealing of the activated mixture at 850°C under nitrogen while the non-stoichiometric perovskite (SrFeO3-x) is the equilibrium product when annealing is performed under air. At XFe = 0.66 a mixture of SrFe2O4 and SrFe2O5 is the final product no matter what is the annealing atmosphere. The enthalpies of reaction at these composition have been determined. By increasing iron content (XFe = 0.8), the annealing at 850°C of the activated mixture leads to SrFe2O4, SrFe2O5 and SrFe12O19 (indicated in the following with the usual term of strontium hexaferrite). Finally at XFe = 0.923, SrFe12O19 is produced from the mechanically activated mixture at temperatures lower (800°C) than in the case of conventional solid state route. In the case of the last two compositions no reaction enthalpy could be determined as the process is too slow.