Reinterpretation of the unit cell evolution of BaTiO3

Reexamination of BaTiO3 phase transitions by X-ray precession method has clarified that the crystal systems of the so-called three ferroelectric β-, γ- and δ-phases from high to low temperatures have crystal forms quite different from those obtained in the 1940s. In the γ  -phase between 283 and 183 K, the crystal has a coherent hybrid structure composed of tetragonal and monoclinic forms sharing the tetragonal (1¯01)T lattice plane but it is different from another coherent hybrid structure in the room temperature β-phase which we have reported in a recent volume of Phys. Lett. A. In the δ-phase below 183 K, it reveals a tetragonal structure very close to a cubic system. The most notable aspect is that the orthogonal axes of the cubic unit cell of the α-phase are conserved in the form of tetragonal forms of all the phases. The unit cell evolution due to the phase transitions and the crystallographic relation are discussed.