  We report X-ray structural studies of the metal-insulator phase transition in
bismuth ferrite, BiFeO3, both as a function of temperature and of pressure (931
oC at atmospheric pressure and ca. 45 GPa at ambient temperature). Based on the
experimental results, we argue that the metallic gamma-phase is not
rhombohedral but is instead the same cubic Pm3m structure whether obtained via
high temperature or high pressure, that the MI transition is second order or
very nearly so, that this is a band-type transition due to semi-metal band
overlap in the cubic phase and not a Mott transition, and that it is primarily
structural and not an S=5/2 to S=1/2 high-spin/low-spin electronic transition.
Our data are compatible with the orthorhombic Pbnm structure for the beta-phase
determined definitively by the neutron scattering study of Arnold et al .[Phys.
Rev. Lett. 2009]; the details of this beta-phase had also been controversial,
with a remarkable collection of five crystal classes (cubic, tetragonal,
orthorhombic, monoclinic, and rhombohedral!) all claimed in recent
publications.
