Application of Small-Crystal Diffraction Technique Using Synchrotron Radiation to Structural Studies of Ceramics

岩溶密度分布研究的实验和理论方法在材料科学领域变得越来越重要。然而，有几种效应限制了从不完美晶体中观察到的运动结构因子的准确性。它们有消光、多次散射、吸收、计数统计、热扩散散射等。使用较小的晶体和/或较短的波长可以显著降低消光和吸收。由于两种解决方案都倾向于降低衍射强度，强烈建议使用同步辐射来进行准确的结构分析。在本研究中，我们采用了一种改进的单晶衍射技术来克服上述问题。该技术的特点是：(1)使用尺寸约为10 mm的小晶体；(2)在14A束线上使用垂直偏振同步辐射，并配备适合于电荷密度研究的水平式四圆衍射仪。例如，尺寸约10 mm的K_2NiF_4型CaYAlO_4单晶的三维衍射数据几乎不受二次消光的影响，对于200强反射，二次消光透射率Y_s的最小值为0.988。对于安装在银阳极四圆衍射仪上的直径为112 mm的球形样品，该值应与计算的0.5855相比较。对CaYAlO_4无消光数据的分析提供了常规方法无法获得的成分、原子位置和电子密度分布的详细信息。对BaTiO_3、含铅弛豫材料、CuInSe_2、MnS_2、LaTA_3O_9、LaNb_3O_9稀土倍半氧化物等陶瓷材料的结构研究取得了成功。

Both experimental and theoretical approaches to the cahrge density distribution study become more important in the field of materials science. However, several kinds of effects limit the accuracy of the observed kinematical structure factors from imperfect crystals. They are extinction, multiple scattering, absorption, counting statistics, thermal diffuse scattering, etc. The extinction and absorption can be reduced significantrly by using a smaller crystal and/or a shorter wavelength. Since both solutions tend to decrease the diffracted intensities, the use of synchrotron radiations is strongly recommended for accurate structure analyzes.In this study, we employed an improved single crystal diffraction technique to overcome the problems mentioned above. The technique has features of (1) using a small crystal of about 10mm in size, (2) using a vertically polarized synchrotron radiations at the beam line 14A equipped with a horizontal-type four-circle diffractometer suitable for the charge density study.Promising results have come out. For example, the three-dimensional diffraction data from a K_2NiF_4-type CaYAlO_4 single crystal of about 10mm in size have been found to be scarcely affected by the secondary extinction with a minimum value for the secondary extinction transmission factor Ys of 0.988 for the 200 strongest reflection. The value should be compared to 0.5855 calculated for a spherical specimen with diameter of 112mm mounted on the Ag rotating-anode four-circle diffractometer. The analysis of the extinction-free data on CaYAlO_4 provided detailed information about the composition, atomic positions and the electron density distribution, which have never been obtained by the conventional methods.Various structural studies were performed successfully on ceramic materials, such as BaTiO_3, lead-containing relaxr materials, CuInSe_2, MnS_2, LaTa_3O_9, LaNb_3O9 rare-earth sesquioxides, etc.