Unambiguous indexing of trigonal crystals from white-beam Laue diffraction patterns
Synchrotron radiation polychromatic micro-focused X-ray Laue diffraction (mXRD) has been proven an effective technique to study the microstructure of materials and minerals from the micron/submicron scale for the rich crystalline information contained in a single Laue diffraction pattern, such as crystal orientation, lattice strain, dislocation density, and so on. However, the indexing of LPs of primitive trigonal lattices using the conventional Laue pattern (LP) indexing algorithms has been problematic.
A primitive trigonal lattice is metrically identical to a primitive hexagonal unit cell with the same c/a value and same orientation. Corresponding positions of Laue reflections are thus indistinguishable, but there are differences in diffraction intensities. Conventional LP indexing software identifies crystal orientation by fitting only diffraction peak positions but ignoring peak intensities. It is therefore not possible to unambiguously index an LP of primitive trigonal crystals. Instead, resulting orientation matrices contain a 60o (or 180o) orientation ambiguity around the c axis.
Dr. Kai Chen, an associate professor at CAMP-Nano, XJTU and his collaborators, Dr. Catherine Dejoie from ETH Zurich, Switzerland and Prof. H.-R. Wenk from Univ. of California Berkeley, USA, developed a new method to index Laue patterns from primitive trigonal crystals by taking the intensities of the diffraction peaks into account. The intensities were compared with theoretical structure factors after correction for the incident X-ray beam flux, X-ray beam polarization, air absorption, detector response and Lorentz factor. In order to allow for the automatic treatment of thousands of LPs as typically obtained during a mXRD scan, the algorithm was implemented in a computer program to realize maps of the two-dimensional microstructure (orientation, strain, stress etc.) distribution in a sample. To illustrate the new method, they mapped the orientation and the residual stress in a deformed quartz grain from a moderately deformed granite using Laue mXRD. This was the first time that a correct lattice stress tensor was obtained by applying the mXRD method. This piece of work was published in Journal of Applied Crystallography (Volume 45, Part 5, pages 982-989, IF=5.15) and now available on-line. (http://scripts.iucr.org/cgi-bin/paper?S0021889812031287)
白光劳厄释疑难 三方晶系得标定
利用连续波长同步辐射X射线微区衍射技术（mXRD）获得的单幅劳厄衍射图样包含丰富的晶体学信息，如晶向、晶格应变、位错密度等，已经在研究材料和矿物在微米、亚微米尺度的微观结构研究与表征领域获得了广泛应用。然而，利用传统的劳厄图样标定法，仅考虑衍射斑位置（即布拉格角）而忽略衍射斑强度，是很难对三方晶系简单晶格劳厄图样进行标定的。
对于具有相同的c/a值以及晶体取向的简单三方晶格与简单六方晶格单胞来说，劳厄衍射图谱是非常相似的，因为两者的劳厄反射发生在相同的位置，只是在衍射强度上有所差异。传统的劳厄图样标定软件仅通过衍射峰的位置来确定晶向，而忽略了衍射峰的强度。因此，利用这种方法有可能会人为地引入绕c轴的 60o(或者 180o )的不确定性。
西安交通大学微纳尺度材料行为研究中心（CAMP-Nano）副教授陈凯博士和他的合作者，瑞士苏黎世联邦理工学院的Catherine Dejoie博士和美国加州大学伯克利分校的H.-R. Wenk教授，开发了一种应用衍射峰强度来标定简单三方晶系劳厄衍射图样的方法。根据入射X光通量，X光偏振，空气吸收，探测器反应和洛伦兹因子对衍射强度进行修正，并将其与结构因子进行比较。陈凯博士和他的合作者通过将这一新型算法写入计算机程序，实现了对mXRD扫描获得的数以千计的衍射图谱的自动处理、标定，进而达到构建样品中二维微观结构（晶向、应变、应力等）分布图谱的目的。为了形象地解释这一方法，他们以石英为例，通过X射线微区劳厄衍射术构建了一个天然形变石英样品中的晶向、道芬孪晶、以及残余应力分布图。这是第一次应用mXRD方法准确获得石英中的晶格应力张量。这项工作被应用晶体学学报接收并在线发表.(Journal of Applied Crystallography, Volume 45, Part 5, pages 982-989, IF=5.15, http://scripts.iucr.org/cgi-bin/paper?S0021889812031287)
