Structure of a ferroelectric and ferroelastic monodomain crystal of the perovskite BiFeO 3

X-ray diffraction data on a ferroelectric and ferroelastic monodomain single crystal of BiFeO3 at 294 K are reported. Bismuth iron oxide, M(r) = 312.83, R3c, lambda-(Mo K-alpha) = 0.7107-angstrom, a(hex) = 5.57874 (16), c(hex) = 13.8688 (3)-angstrom, V(hex) = 373.802 (17) angstrom-3, a(rh) = 5.6343-angstrom, alpha-rh = 59.348-degrees, V(rh) = 124.601-angstrom-3, Z = 6, D(x) = 8.337 Mg m-3, mu = 75.97 mm-1, F(000) = 798, R = 2.4, wR = 1.9% for 422 independent reflections. The structure can be described as a rhombohedrally distorted simple cubic perovskite cell. The oxygen octahedron is distorted with minimum and maximum O-O distances of 2.710 (7) and 3.015 (9)-angstrom, respectively, and rotated by about +/-alpha = 13.8 (3)-degrees around the threefold axis. The iron atom is shifted away from the centre of the deformed oxygen octahedron by about 0.134 (7)-angstrom along the threefold axis. The bismuth atom is shifted with respect to two neighbouring octahedron centres by about 0.540 (7)-angstrom along the threefold axis. Bi-Fe distances are 3.0617 (11) and 3.8726 (11)-angstrom. The oxygen atom is displaced by 0.2877 (6)-angstrom away from the Bi4 plane. The iron-oxygen chain has angles of 165.04 (21) (O-Fe-O) and 154.1 (4)-degrees (Fe-O-Fe), and dihedral angles of 127.8 (9) (O-Fe-O-Fe) and 121.6 (9)-degrees (Fe-O-Fe-O). The ferroelectric poling mechanism was analyzed: atom displacements were calculated for 60, 120 and 180-degrees switching of P(s) and found to be 0.44, 0.62 and 0.82-angstrom for iron. Average oxygen displacements for +alpha (-alpha) rotated octahedra were 0.87 (0.89), 1.13 (1.24) and 1.34 (1.53)-angstrom. Average bismuth shifts were less than 0.06-angstrom. As the displacements increase in the order 60, 120 and 180-degrees switching of P(s), the 180-degrees reversal must have a very high activation energy and will be avoided.