Design and Development of Bismuth Layr-structured Compounds with Large Electrical Anisotropies

铋层状结构化合物具有其中铋层和准钙钛矿块沿c轴方向沿着堆叠的晶体结构。本研究的目的是设计和开发具有大的电各向异性的无机材料，即根据晶体学方向的不同而具有不同的电性能。用<12><15>助熔剂法和熔体凝固法制备了具有铋层状结构的钛酸铋（Bi_4Ti_3O_）和钛酸铋钡（BaBi_4Ti_4O_）单晶，测量了它们的介电常数和直流电导率。与没有晶体取向的多晶样品的各向异性进行了比较。在单晶中，在钛酸铋和钛酸钡铋中，在a（B）轴方向（平行于Bi层）上的介电常数（ε）和DC电导率（σ）大于在c轴方向（垂直于Bi层）上的介电常数（ε）和DC电导率（σ）。在676 ℃（居里温度）下，各向异性为10。在钛酸铋中，在150 ° C时，σ为10^3，在420 ° C（居里温度）时，σ为58。在钛酸钡铋中，在150 ℃下，σ为10。多晶样品的性质在两个方向上表现出单晶样品的中间行为。复阻抗测量也表明，即使在顺电相居里温度以上的介电和导电性能的各向异性。铋层被认为在铋层结构化合物中起着绝缘和顺电性质的层的作用。这些结果对于控制层状化合物的电学性能和利用材料的晶体各向异性设计功能器件具有重要的参考价值。

The bismuth layr-structured compounds have the crystal structure in which bismuth layrs and quasiperovskite blocks are stacked along the c-axis direction. The objective in this study is to design and develop the inorganic materials with large electrical anisotropies, that is, different electrical properties depending on the crystallographic directions. Dielectric permittivity and DC conductivity in bismuth titanate (Bi_4Ti_3O_<12>) and barium bismuth titanate (BaBi_4Ti_4O_<15>), which have the bismuth layr-structures, were measured on single crystals prepared by a flux method or a melt-solidification method. The anisotropies were compared with those of polycrystalline samples without crystallographic orientation. In the single crystals, dielectric permittivity (epsilon) and DC conductivity (sigma) were larger in the a(b)-axis direction (parallel to the Bi layr) than those in the c-axis direction (perpendicular to the Bi layr) both in bismuth titanate and barium bismuth titanate. Anisotropies were 10 for epsilon at 676ﾟC(Curie Temp.) and 10^3 for sigma at 150ﾟC in bismuth titanate, and 58 for epsilon at 420ﾟC (Curie Temp.) and 10 for sigma at 150ﾟC in barium bismuth titanate. The properties of polycrystalline samples showed an intermediate behavior of those of single crystals in the two directions. Complex impedance measurements also showed the anisotropy in the dielectric and conducting properties even in the paraelectric phase above the Curie temperature. The bismuth layr was thought to act as a layr with insulating and paraelectric properties in the bismuth layr-structured compounds. Those results world be very valuable to control electrical properties in layr-structured compounds and to design the functional devises utilizing crystallographic anisotropy of materials.