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

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) degrees 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.