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

钛酸钡（BaTiO_3）是铁电体的代表之一，目前已进行了大量的研究。加热时，在T_<R-O>=200 K、T_<O-T>=290 K和T_<T-C>=400 K处发生结构相变，四相的晶系分别为菱形(R)、正交(O)、四边形(T)和立方(C)。自发极化方向为r相上沿[111]，o相上沿[011]，t相上沿[001]。最近，我们观察到用顶种溶液技术生长的高纯度单晶在100 K左右的r相中出现了特殊的介电弛豫（偶极子-玻璃转变）。对于偶极子-玻璃跃迁，本研究得到以下结果：(1)电场冷却诱导的单畴态[111]板在100 K左右出现明显的介电弛豫，而在多畴态[001]板上则不明显。因此，弛豫不是由于通常的畴壁运动，而是由于另一个原因。(2)介电常数的温度依赖性和频率依赖性与PbMg_<1/3>Nb_<2/3>O_3等弛豫剂相似。(3)弛豫时间τ对偶极子-玻璃跃迁的温度依赖性用Vogel-Fulcher关系表示，τ=τ_o exp [E/(T-T_o)]，与一般的玻璃跃迁相似。(4)静水压力对介电性能影响的实验结果表明，T_o随压力增大而增大，而转变温度随压力减小。因此偶极子-玻璃化转变与BaTiO_3的结构相变没有直接关系。(5)用通量法生长出Ba_<1-x/2>K_xTiO_3单晶，x=0.01 ~ x=0.03。铁电居里温度T_<C-T>随K的增加而降低，当x=0时，估计T_<C-T>为400k。

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.