  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.
