Phase Separation of ϵ-Fe2O3 and BaFe12O19 in a Synthesis Combining Reverse-Micelle and Sol-Gel Techniques

Abstract

Epsilon iron oxide (ϵ-Fe₂O₃) and magnetoplumbite barium ferrite (BaFe₁₂O₁₉) are well-known hard ferrites. For one synthesis method of ϵ-Fe₂O₃, combining reverse-micelle and sol-gel techniques, Ba ions are used to accelerate the formation of nanorod-shaped ϵ-Fe₂O₃. On the other hand, in synthesis of BaFe₁₂O₁₉ both Ba and Fe ions are used to form the final product. However, the coexistence of these two ferrites has not been reported by any synthesis. Herein, we investigated the effect of Ba ions on the final product for the synthesis combining reverse-micelle and sol-gel techniques. At a low Ba ion ratio of [Ba]/[Fe]=0.2, ϵ-Fe₂O₃ nanorods are formed, while at higher Ba ion ratios ([Ba]/[Fe]=0.4, 1, 2), BaFe₁₂O₁₉ appears. The phase diagram at 1000 °C shows that at a Ba ion ratio of [Ba]/[Fe]=1, the ratio of ϵ-Fe₂O₃ and BaFe₁₂O₁₉ is almost 1 : 1. The diagram shows intermediates between these two phases do not form, indicating a phase separation of ϵ-Fe₂O₃ and BaFe₁₂O₁₉. During the sintering process of this synthesis, initially perovskite-type Ba₂Fe₂O₅ and γ-Fe₂O₃ nanoparticles form. Then, in areas where γ-Fe₂O₃ nanoparticles are gathered around Ba₂Fe₂O₅ and react, BaFe₁₂O₁₉ is formed, whereas in areas lacking Ba₂Fe₂O₅, ϵ-Fe₂O₃ nanorods are formed within Ba-ion containing silica glass.