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The symmetries of the rhombohedral, orthorhombic, and tetragonal phases of barium titanate (BaTiO₃) are investigated using convergent-beam electron diffraction. Nanometer-sized local structures with rhombohedral symmetry are observed in both the orthorhombic and tetragonal phases. This indicates that an order-disorder character exists in phase transformations of BaTiO₃. The nanostructures in these phases are discussed in terms of an order-disorder model with off-centered Ti in the 111 directions.
Tsuda et al. (Mon,) studied this question.