The ferroelectric-paraelectric phase transition in BiFeO3: crystal structure of the orthorhombic beta-phase

A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO3 undergoes a phase transition from the ferroelectric alpha phase (rhombohedral, R3c) to a paraelectric beta phase (orthorhombic, Pbnm) between 820 oC and 830 oC. Co-existence of both phases over a finite temperature interval, together with abrupt changes in key structural parameters, confirm that the transition is firstorder. The beta phase corresponds to the GdFeO3-type perovskite. The metastability of BiFeO3 relative to Bi2Fe4O9 above 810 oC precludes observation of a reported cubic gamma phase above 925 oC under the present experimental conditions.