Structure determination of a mixed-layer bismuth titanate, Bi₇Ti₄NbO₂₁, by super-high-resolution electron microscopy

The crystal structure of a bismuth titanate, Bi₇Ti₄NbO₂₁, was determined on the basis of super-high- resolution images, taken by a recently constructed 1000 kV electron microscope, in which each cation site was approximately defined. It was found that the crystal has a mixed-layer structure, i.e. Bi₄Ti₃O₁₂ and Bi₃TiNbO₉-like layers are alternately intergrown. The symmetry is orthorhombic with space group I2cm. The lattice parameters are a = 5.45, b = 5.42 and c = 58.1 Å. That the periodicity along c is twice that expected from a simple intergrowth can be accounted for by the rotation of octahedra. The lattice defects found are discussed in terms of twinning of the crystal.