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

钛酸钡(BaTiO_3)是具有代表性的铁电材料之一，目前已有大量的研究工作。在T_&lt；R-O&gt；=200K，T_&lt；O-T&gt；=290K和T_&lt；T-C&gt；=400K发生结构相变，四相晶系分别为三方(R)、正交(O)、四方(T)和立方(C)。自发极化方向为R相沿[111]方向，O相沿[011]方向，T相沿[001]方向。最近，我们在顶籽法生长的高纯单晶的R相中观察到了一种特殊的介电弛豫(偶极-玻璃转变)，温度在100K左右。对于偶极子-玻璃转变，本研究得到以下结果：(1)在电场冷却下，在单域态的[111]面上出现明显的介电弛豫，而在多域态的[001]面上，100K附近的介电弛豫不明显。因此，弛豫不是由于通常的磁场壁运动，而是由于另一个原因。(2)介电常数的温度依赖关系和频率依赖关系类似于弛豫体，如PbMg_t；1/3&gt；Nb_&lt；2/3&gt；O_3。(3)与通常的玻璃化转变一样，弛豫时间τ对偶极玻璃化转变的温度依赖关系符合伏格尔-富勒关系，τ=τ_o exp[E/(T-T_o)]。(4)根据静水压力对介电行为影响的实验结果，T_o随压力的增加而增大，而转变温度随压力的增加而降低。因此，偶极玻璃转变与BaTiO_3的结构相变没有直接关系。(5)用助熔剂法生长了x=0.01~x=0.03的BaTiO_3单晶。铁电居里温度T_&lt；C-T&gt；随K减小，当x=0时，T_&lt；C-T&gt；估计为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.