Experimentally determining the intrinsic center point of Bi2O3–Fe2O3 phase diagram for growing pure BiFeO3 crystals

Abstract

It has been a major challenge to obtain pure BiFeO₃ (BFO) crystals completely without secondary Bi₂Fe₄O₉ or Bi₂₅FeO₄₀ phases, especially for Bi₂Fe₄O₉. Studies on the Bi₂O₃–Fe₂O₃ phase diagram using flux method have been performed to experimentally determine the intrinsic center point, which is located around the center of its temperature–composition phase diagram at atmospheric pressure and separates BFO, Bi₂₅FeO₄₀, Bi₂Fe₄O₉ and liquid phases. The center point is rather critical for growing pure BFO, the most important compound in the phase diagram, but unfortunately this has never been clearly clarified due to various intrinsic or extrinsic difficulties. Gold crucibles have eventually been found to have much less influence during investigation of Bi₂O₃–Fe₂O₃ thermodynamics than platinum, silver and alumina ones, and were therefore used it to determine the intrinsic center point at 25.4(6) mol% Fe₂O₃ and 852(5) °C. According to the results of this communication, to obtain BiFeO₃ crystals using Bi₂O₃–Fe₂O₃ flux method completely without a secondary Bi₂Fe₄O₉ phase and various parasitic magnetism, we advise to use high purity starting materials with Fe₂O₃ composition below 25 mol% using a gold crucible.