Transformation of α-Fe₂O₃ to Fe₃O₄ Realized by Mechanochemical Reaction of α-Fe₂O₃ and SrCO₃

A transformation from α-Fe₂O₃ with rhombohedral structure to Fe₃O₄ with spinel face-centered cubic (fcc) structure was realized by ball milling the mixture of SrCO₃ and α-Fe₂O₃ powders with molar ratio of 1:3 under ambient air in the current work, but it was not observed by ball milling the α-Fe₂O₃ powders. The transformation is due to a mechanochemical reaction between SrCO₃ and α-Fe₂O₃ in the milling process, and the α-Fe₂O₃ catalyzes the decomposition of SrCO₃, while the decomposition promotes the transformation of α-Fe₂O₃ to Fe₃O₄. X-ray diffraction, X-ray photoelectron spectrometry, and magnetic measurements indicate that the transformation finishes completely upon milling of 120 hours, and the mixture milled for 120 hours consists of Fe₃O₄ and a Sr element. However, Sr does not incorporate into the Fe₃O₄ but exists in grain boundary of the Fe₃O₄ in a form of a Sr-O bond. It is found that the product obtained by ball milling the mixture is different from that prepared by sintering the mixture at atmospheric pressure, which product is SrFeO_2.97 with cubic structure. The mechanism of the transformation and origin of difference in products obtained in the ball milling and sintering processes are also discussed in the current work.