Atomic displacements in BiFeO3 as a function of temperature: neutron diffraction study

The parameters of the crystal structure of BiFeO3, described within the space group R3c, have been determined by high-resolution neutron powder diffraction for temperatures from 293 to 923 K. It was found that there is a local minimum for the rhombohedral angle αrh, near the Neel temperature TN ≃ 640 K, a gradual rotation of the FeO6 octahedra and an increase of the Fe—O—Fe angle. The displacement of the Bi3+ ions from the FeO6 octahedra which influence the electric polarization decreases with temperature. One of the Bi—Fe distances also has a local maximum near TN. The atomic vibrations of Bi3+ and O2− ions show a significant anisotropy.