  The structural and polar properties of BiFeO3 at finite temperature are
investigated using an atomistic shell model fitted to first-principles
calculations. Molecular Dynamics simulations show a direct transition from the
low-temperature R3c ferroelectric phase to the Pbnm orthorhombic phase without
evidence of any intermediate bridging phase between them. The high-temperature
phase is characterized by the presence of two sublattices with opposite
polarizations, and it displays the characteristic double-hysteresis loop under
the action of an external electric field. The microscopic analysis reveals that
the change in the polar direction and the large lattice strains observed during
the antiferroelectric-ferroelectric phase transition originate from the
interplay between polarization, oxygen octahedron rotations and strain. As a
result, the induced ferroelectric phase recovers the symmetry of the low
temperature R3c phase.
