Long-Period Modulated Structures of Bi_2Sr_2(Ca_<1-x>Ln_x)Cu_2O_<8+d> Ceramics Studied by High-Resolution Electron Microscopy and Electron Diffraction

由于可测量的超晶格反射数量不足以确定调制结构的大晶胞中的原子坐标，因此基于衍射数据分析具有正弦波样晶格调制的晶体结构往往非常困难。Bi_2Sr_2(Ca_<1-x>Ln_x)Cu_2O_<8+ δ > （Ln=稀土金属）取代体系是调制结构族中的一员。本研究旨在通过对[001]的高分辨率图像和电子衍射图的分析，解析Ln=Y、Nd、Pr和Tb取代体系的调制结构，从而显示出调制周期对Ln浓度的依赖关系以及元素对浓度依赖关系的取代效应。结果总结如下：1。提出了由b_1=4.5b_0、b_2=5b_0或b_3=4b_0的有序排列域组成的长周期调制结构模型。该模型与电子衍射的强度分布更为吻合。基于一维对比度调制[001]高分辨率图像的光度密度分布曲线和密度分布曲线的计算机模拟进行了分析。分析结果表明，晶体中畴的有序排列（畴组态）可由密度分布曲线直接确定。调制周期随浓度的增加而减小。Pr和Tb元素对周期浓度依赖性的取代作用大于Y和Nd元素。以Ca的2价和Y的3价为标准价，Nd和Pr或Tb元素分别作为晶体中的3价和3.2-3.3价。调制周期随镧浓度呈阶梯递减趋势。对楼梯细节的研究正在进行中。在密度分布曲线中发现的域构型与一维竞争体系的基态是等价的。少

It is often very difficult to analyze structures of crystals with sinusoid-like modulations of lattice based on diffraction data, because the number of measurable superlattice reflection is not sufficient to determine atomic coordinates in the large unit cell of modulated structure. The structure of the Bi_2Sr_2(Ca_<1-x>Ln_x)Cu_2O_<8+delta> (Ln=rare earth metal) substitution system is a member of the family of the modulated structures.The present study aims to resolve the modulated structure of the substitution systems of Ln=Y,Nd,Pr and Tb by means of the analyses of [001] high-resolution image and electron diffraction pattern, and then, to show the Ln-concentration dependencies of the modulation period and the substitution effects of the elements to the concentration dependence. The results are summarized as follows.1.A long-period modulated structure model was proposed, consisting of ordered alignments of domains with extent b_1=4.5b_0 and b_2=5b_0 or b_3=4b_0. The model shows the co … More nsistency with the intensity distribution of electron diffraction.2.An analysis was carried out based on photometric density distribution curves of one-dimensional contrast modulation of [001] high-resolution image and computer simulations of the density distribution curve. The result of analysis has elucidated that the ordered alignment of the domains ( domain configuration ) in the crystal has been determined from the density distribution curve directly.3.The modulation period decreases with the Ln-concentration. The Pr and Tb elements have a larger substitution effect to the concentration dependence of period than the Y and Nd elements. Taking the 2 valence of Ca and 3 of Y as the standard ones, the Nd and Pr or Tb elements serve as the 3 and 3.2-3.3 valences in the crystals, respectively.The modulation period prefers to show a stairswise decrease with the Ln-concentration. The study on the details of the stairs is in progress newly. The domain configurations found in density distribution curves are equivalent those of the ground states of the one-dimensionally competitive system. Less