XCV. Symmetry changes in barium titanate at low temperatures and their relation to its ferroelectric properties

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Abstract
The optical changes in BaTiO3 are described, and it is shown that they can be completely explained if the crystal symmetry changes from tetragonal to orthorhombic at -5°C, and then to rhombohedral at -90°C. These changes have been confirmed by X-ray investigations, and it is concluded that the three transitions are caused by the structure becoming successively spontaneously polarized along the [100] [110] and [111] cube directions. The relation of polarization to the cell structure is discussed and it is shown that the simple Lorentz equation is inapplicable if the titanium-oxygen interaction energy is large. An explanation of the three transitions is outlined on the basis of this interaction and some difficulties of electrical dipole cooperative effects in BaTiO3 are discussed.