Observation of Oxygen Vacancies in Perovskite Oxide by a 200kV HREM

通式为XYO_3的钙钛矿由于其有趣的铁电性质和磁性而得到了广泛的研究。LaMnO_<3-X>和BaMnO_<3-X>体系由紧密堆积的LaO_3/BaO_3层组成，Mn金属占据层间的八面体位置。由于其磁性取决于氧空位的含量，因此对其氧缺陷结构的分析很有意义。HREM成像条件的建立对氧缺陷结构的反射具有重要意义。在装有成像板的200/300kV HREM中，对LaMnO_<3-X>中的氧空位进行了结构测定和定量。在HREM直接观测的基础上，借助于计算机模拟和图像处理，找到了氧原子和空位在“黑点图像”中成像为亮点的最佳成像条件。在最佳成像条件下，根据动态衍射理论，讨论了成像对比度与氧位原子占比和样品厚度的关系。通过成像板，从观察到的图像对比度中亮点的强度分布定量估计原子占位率。利用分子动力学模拟得到的弛豫结构，基于多层方法的图像对比有助于HREM对观测图像对比度的理解。此外，作为本方法结构测定的一个应用，对牙齿晶体中的羟基磷灰石进行了研究，以确定羟基^-基团的位置。

The perovskites with the general formula XYO_3 have been extensively studied because of their interesting ferroelectric ad magnetic properties. The systems of LaMnO_<3-X> and BaMnO_<3-X> consist of a staking of close-packed layrs of LaO_3/BaO_3 with Mn metals occupying interlayr octahedral site. Analyzes of their oxygen-defective structures are of interest because of the magnetic properties depending on the content of oxygen vacancy. Establishment of imaging condition of a HREM for reflecting oxygen-defective structure is of importance. The structural determination and quantification of oxygen vacancies in LaMnO_<3-X> were carried out in 200/300kV HREM equipped with an imaging plate. On the basis of direct observation with HREM and the aid of computer simulation and image processing, optimum imaging conditions were found, where oxygen atoms and the vacancies are imaged as bright spots in a "dark-spot image". Under the optimum imaging conditions, the dependence of image contrast on the atomic occupancies of the oxygen sites and the thickness of the samples was discussed on the basis of dynamical diffraction theory. By an imaging plate, the atomic occupancies were quantitatively estimated from the intensity distributions of the bright spots in the observed image contrast. Using the relaxd structure derived from molecular dynamics simulation, image contrast based on the multi-slice method could assist the understanding of the observed image contrast by HREM.Besides, as an application of structural determination by the present method, hydroxyapatite in tooth crystals was studied to ascertain the positions of the OH^- groups.