Phase transition in hexagonal type BaTiO3

A review is given on the structural phase transition of hexagonal type BaTiO3 at 222 K and on the ferroelectric · ferroelastic phase transition at ∼ 70 K. X-ray studies and Raman studies have disclosed that an E2u phonon mode condenses below 222 K where crystal changes from hexagonal P63/mmc to orthorhombic C2221-D5 2. As for the ferroelectric transition at ∼70 K, dielectric and elastic behaviors strongly suggest that the transition is primarily of a ferroelastic nature associated with softening of the elastic stiffness c66. A probable space group below 70 K is monoclinic P21.