Oxygen Deficiency, Ferroelectricity and Dipole-Grass Transition on Barium Titanate

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Barium titanate (BaTiO_3) is one of the representative ferroelectrics and a lot of research has been accomplished. It undergoes structural phase transitions at T_<R-O>=200 K, T_<O-T>=290 K and T_<T-C>=400 K on heating, and the crystal systems of the four phases are rhombohedral (R), orthorhombic (O), tetragonal (T) and cubic (C). The direction of the spontaneous polarization is along the [111] on the R-phase, along the [011] on the O-phase and along the [001] on the T-phase. Recently, we observed a peculiar dielectric relaxation (dipole-glass transition) at around 100 K in the R-phase on the high purity single crystal grown by a top seeded solution technique. As for the dipole-glass transition, following results were obtained in this study : (1) The dielectric relaxation at around 100 K appears obviously on the [111] plate of a single domain state induced by an electric field cooling, while it is obscure on the [001] plate of a multi domain state. Thus the relaxation is not due to a usual domain wall motion but is due to another reason. (2) Temperature dependence and frequency dependence of the dielectric constant resemble those of relaxor such as PbMg_<1/3>Nb_<2/3>O_3. (3) The temperature dependence of the relaxation time τ on the dipole-glass transition is represented with the Vogel-Fulcher relation, τ=τ_o exp [E/(T-T_o)], like usual glass transition. (4) According to an experimental result of a hydrostatic pressure effect on the dielectric behavior, T_o increases with pressures, contrasting to the result that the transition temperatures decrease with pressures. Thus the dipole-glass transition is not directly related to the structural phase transition of BaTiO_3. (5) Single crystals of Ba_<1-x/2>K_xTiO_3 with x=0.01 to x=0.03 were grown by a flux method. The ferroelectric Curie temperature T_<C-T> decreases with K and T_<C-T> is estimated as 400 K if x=0.