Abstract:

The existence of impuritiy phases such as Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ has led to high leakage current in BiFeO₃ multiferric materials, which consequently restricts further understanding of its coupling between magnetic and polarization orders. Prior to the attempts to synthesize pure-phase BiFeO₃ ceramics, the phase transition involved in the reaction sintering should be clarified. In the present, the phase transformations during the reaction sintering process of BiFeO₃ ceramics with different molar ratio of Bi₂O₃/Fe₂O₃ in air were studied via High Temperature X-ray Diffraction technique (HT-XRD), Rietveld refinement quantification, and High Temperature Raman Spectroscopy (HT-Raman). The thermal stabilities of BiFeO₃, Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ ceramics were also studied by such methods. The qualitative phase distributions after heating were analyzed by Electron Backscattered Diffraction (EBSD). Results show that the phase transition from monoclinic to cubic for Bi₂O₃ was well done, which usually taken place at 700 ℃. The Fe₂O₃ did not react with Bi₂O₃ to form BiFeO₃ until that transition finished. In addition, BiFeO₃, Bi₂₅FeO₃₉ and Bi₂Fe₄O₉ phases are not in thermodynamic stable state during the cooling process for Bi excess samples. Bi₂O₃ excess can effectively inhibit the formation of impurities and promote the sintering of BiFeO₃ phase. The phase content of BiFeO₃ mainly depends on the molar ratio of Bi₂O₃/Fe₂O₃, and 1.03 : 1 is optimum. Combining with our previous research results, it is found that the effective parameters for the synthesis of BiFeO₃ strongly depend on the excessive Bi and rapid heating and cooling rate. This work may provide useful experimental guidance for the preparation of pure-phase BiFeO₃ ceramics.

Key words: BiFeO₃, molar ratio of Bi₂O₃/Fe₂O₃, reaction sintering, phase transition, High Temperature X-ray Diffraction