Svane Bjarke, Tolborg Kasper, Kato Kenichi, Iversen Bo Brummerstedt
Department of Chemistry, Aarhus University, Aarhus, DK-8000, Denmark.
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
Acta Crystallogr A Found Adv. 2021 Mar 1;77(Pt 2):85-95. doi: 10.1107/S2053273320016605. Epub 2021 Jan 28.
Powder X-ray diffraction has some inherent advantages over traditional single-crystal X-ray diffraction in accurately determining electron densities and structural parameters due to the lower requirements for sample crystallinity, simpler corrections and measurement simultaneity. For some simple inorganic materials, it has been shown that these advantages can compensate for disadvantages such as peak overlap and error-prone background subtraction. Although it is challenging to extend powder X-ray diffraction-based electron-density studies to organic materials with significant peak overlap, previous results using a dedicated vacuum diffractometer with a large image-plate camera (AVID) demonstrated that it can be done. However, the vacuum setup with the off-line detector system was found to prohibit a widespread use. Fast microstrip detectors, which have been employed at a number of powder diffraction beamlines, have the potential to facilitate electron-density studies. Nevertheless, no electron-density studies even for materials with slight peak overlap have been performed with microstrip detectors. One of the most critical problems has been a difference in sensitivity between microstrip channels, which substantially defines the dynamic range of a detector. Recently, a robust approach to this problem has been developed and applied to a total scattering measurement system (OHGI) with 15 MYTHEN microstrip modules. In the present study, synchrotron powder X-ray diffraction data obtained with OHGI are evaulated in terms of multipole electron densities and structural parameters (atomic positions and displacement parameters). These results show that, even without a dedicated setup and perfect samples, electron-density modelling can be carried out on high-quality powder X-ray diffraction data. However, it was also found that the required prior information about the sample prohibits widespread use of the method. With the presently obtainable data quality, electron densities of molecular crystals in general are not reliably obtained from powder data, but it is an excellent, possibly superior, alternative to single-crystal measurements for small-unit-cell inorganic solids. If aspherical atomic scattering factors can be obtained from other means (multipole databases, theoretical calculations), then atomic positions (including for hydrogen) and anisotropic atomic displacement parameters (non-hydrogen atoms) of excellent accuracy can be refined from synchrotron powder X-ray diffraction data on organic crystals.
粉末X射线衍射在准确测定电子密度和结构参数方面比传统单晶X射线衍射具有一些固有优势，这是由于对样品结晶度要求较低、校正更简单以及测量具有同时性。对于一些简单的无机材料，已表明这些优势可以弥补诸如峰重叠和背景扣除容易出错等缺点。尽管将基于粉末X射线衍射的电子密度研究扩展到具有明显峰重叠的有机材料具有挑战性，但先前使用配备大型成像板相机的专用真空衍射仪（AVID）所取得的结果表明这是可以做到的。然而，发现带有离线探测器系统的真空装置阻碍了其广泛应用。已在多个粉末衍射光束线中使用的快速微带探测器有潜力促进电子密度研究。然而，即使对于峰重叠较小的材料，也尚未使用微带探测器进行过电子密度研究。最关键的问题之一是微带通道之间的灵敏度差异，这在很大程度上决定了探测器的动态范围。最近，针对这个问题开发了一种稳健的方法，并应用于具有15个MYTHEN微带模块的全散射测量系统（OHGI）。在本研究中，根据多极电子密度和结构参数（原子位置和位移参数）对用OHGI获得的同步辐射粉末X射线衍射数据进行了评估。这些结果表明，即使没有专用装置和完美样品，也可以对高质量的粉末X射线衍射数据进行电子密度建模。然而，还发现关于样品所需的先验信息阻碍了该方法的广泛使用。就目前可获得的数据质量而言，一般无法从粉末数据可靠地获得分子晶体的电子密度，但对于小单胞无机固体，它是单晶测量的一种出色的、可能更好的替代方法。如果可以从其他途径（多极数据库、理论计算）获得非球形原子散射因子，那么就可以从有机晶体的同步辐射粉末X射线衍射数据中精确地精修原子位置（包括氢原子）和各向异性原子位移参数（非氢原子）。
