DIELECTRIC PROPERTIES IN ORIENTED THIN FILMS OF TUNGUSTEN-BRONZE-TYPE FERROELECTRIC Ba_6Ti_2Nb_8O_<30>

(1)为了研究Ba_6Ti_2Nb_8O_2陶瓷介电常数温度特性中异常宽峰的成因<30>，采用MOD法在（111）Pt/Ti/SiO_2/Si衬底上生长了取向薄膜，研究了取向薄膜的介电常数温度特性。尽管MOD膜没有足够的取向性，其相变温度比晶态材料的相变温度（245℃）高出约13℃，但没有观察到宽峰，这表明陶瓷样品的介电性能受到随机方向内应力的强烈影响。(2)结果表明，在（100）SrTiO_3和（100）MgO衬底上生长的薄膜沿着c轴取向良好，薄膜的a轴相对于衬底的a轴倾斜±18.5°。这种倾斜的性质与四方钨青铜结构的晶胞中氧八面体的特殊排列密切相关。(3)首次在陶瓷样品中测量了极化电场特性随温度的变化。结果表明，复极化随温度的升高而减小，在160 ~ 175℃时趋于消失，该温度明显低于单晶的相变温度（245℃）或陶瓷样品介电常数的峰值温度（180 ~ 200℃）。此外，自发极化的幅度低至1μC/cm ^2，这表明只有一小部分畴可能发生极化反转。(4)对100〜150 μm厚的陶瓷进行偏振光光学显微镜观察，检查沿着厚度方向的畴结构。根据自发极化很小的实验事实，发现其尺寸和具有双折射的部分都很小。

(1) In order to investigate the origin for the extraordinary broad peak in the dielectric constant vs. temperature characteristics in Ba_6Ti_2Nb_8O_<30> ceramics, the temperature dependence of the dielectric constant in oriented thin films grown on (111)Pt/Ti/Si0_2/Si substrates by MOD method. Although the MOD films do not have enough orientation and their phase transition temperature shifts by about 13℃ to a higher temperature in comparison with the case of crystalline materials (245℃), it is observed no broad peak ; this suggests that the dielectric properties of ceramic samples are strongly influenced by internal stresses with random directions.(2) It is confirmed that the films grown on (100)SrTiO_3 and (100)MgO substrates align well along c-axis and that the a-axis of the grown films is tilted by ±18.5° with respect to the a-axis of the substrate. This tilted nature is closely related to the peculiar arrangement of oxygen octahedra in the unit cell of the teragonal tungsten-bronze structure.(3) The polarization-electric field characteristics are measured in the first time in ceramic samples as a function of temperature. It is found that the remanent polarization decreases with increasing temperature and tends to vanishe at 160〜175℃ this temperature is significantly lower than the phase transition temperature of single crystals (245℃) or the peak temperature of the dielectric constant in ceramic samples (180〜200℃). Moreover, the magnitude of the spontaneous polarization is as low as 1μC/cm^2, suggesting that only a small fraction of domains can probably make polarization inversion.(4) A polarized-light optical microscope observation is carried out on 100〜150 μm thick ceramics to check the domain structure along the thickness diraction. It is found that the size and the portions having birefringence are quite small in accordance with the experimental fact of very small spontaneous polarization.