Crystal structure of Bi4Ti3O12

The crystal structure of Bi4Ti3O12 has been determined from x-ray and neutron diffraction patterns. Physical properties indicate monoclinic symmetry, but the diffraction data are consistent with a polar orthorhombic structure, space group B2cb, with a = 5.448(2), b = 5.411(2), c = 32,83(1) Å, and Z = 4. Integrated intensities of 2500 x-ray reflections were measured using an automated diffractometer, and the intensities of 28 neutron peaks were obtained from powder patterns. Final R-factors between the refined structure and the x-ray and neutron data were 0.07 and 0.09, respectively. Titanium is the principal contributor to polarization along a, shifting an average of 0.32 Å from the octahedron centers. The spontaneous polarization, calculated on the basis of point charges on the ions, is 28 μC/cm2 along a, somewhat smaller than the observed value of 50 ± 5 μC/cm2. The bismuth bonding is very asymmetric, causing rotations of the octahedra about a and c. A model involving the correlated reorientation of lone-pair electrons is offered as a possible explanation of the monoclinic behavior, accounting satisfactorily for the small (4 μC/cm2) component of P, along c.