Weigel Tina, Funke Claudia, Zschornak Matthias, Behm Thomas, Stöcker Hartmut, Leisegang Tilmann, Meyer Dirk C
Institute of Experimental Physics, Technische Universität Bergakademie Freiberg, Leipziger Strasse 23, 09596 Freiberg, Germany.
Samara State Technical University, Molodogvardeyskaya Street 224, 443100 Samara, Russian Federation.
J Appl Crystallogr. 2020 Apr 14;53(Pt 3):614-622. doi: 10.1107/S1600576720003143. eCollection 2020 Jun 1.
High-quality single-crystal X-ray diffraction measurements are a prerequisite for obtaining precise and reliable structure data and electron densities. The single crystal should therefore fulfill several conditions, of which a regular defined shape is of particularly high importance for compounds consisting of heavy elements with high X-ray absorption coefficients. The absorption of X-rays passing through a 50 µm-thick LiNbO crystal can reduce the transmission of Mo α radiation by several tens of percent, which makes an absorption correction of the reflection intensities necessary. In order to reduce ambiguities concerning the shape of a crystal, used for the necessary absorption correction, a method for preparation of regularly shaped single crystals out of large samples is presented and evaluated. This method utilizes a focused ion beam to cut crystals with defined size and shape reproducibly and carefully without splintering. For evaluation, a single-crystal X-ray diffraction study using a laboratory diffractometer is presented, comparing differently prepared LiNbO crystals originating from the same macroscopic crystal plate. Results of the data reduction, structure refinement and electron density reconstruction indicate qualitatively similar values for all prepared crystals. Thus, the different preparation techniques have a smaller impact than expected. However, the atomic coordinates, electron densities and atomic charges are supposed to be more reliable since the focused-ion-beam-prepared crystal exhibits the smallest extinction influences. This preparation technique is especially recommended for susceptible samples, for cases where a minimal invasive preparation procedure is needed, and for the preparation of crystals from specific areas, complex material architectures and materials that cannot be prepared with common methods (breaking or grinding).
高质量的单晶X射线衍射测量是获得精确可靠的结构数据和电子密度的前提条件。因此，单晶应满足几个条件，其中对于由具有高X射线吸收系数的重元素组成的化合物而言，规则定义的形状尤为重要。穿过50 µm厚的铌酸锂晶体的X射线吸收可使钼α辐射的透射率降低几十%，这使得对反射强度进行吸收校正是必要的。为了减少用于必要吸收校正的晶体形状方面的不确定性，本文提出并评估了一种从大样品中制备规则形状单晶的方法。该方法利用聚焦离子束可重复且小心地切割出具有确定尺寸和形状的晶体，而不会产生碎片。为了进行评估，本文给出了使用实验室衍射仪进行的单晶X射线衍射研究，比较了源自同一宏观晶体板的不同制备的铌酸锂晶体。数据还原、结构精修和电子密度重建的结果表明，所有制备的晶体在定性上具有相似的值。因此，不同的制备技术的影响比预期的要小。然而，由于聚焦离子束制备的晶体表现出最小的消光影响，其原子坐标、电子密度和原子电荷应该更可靠。这种制备技术特别推荐用于易受影响的样品、需要最小侵入性制备程序的情况，以及从特定区域、复杂材料结构和无法用常规方法（破碎或研磨）制备的材料中制备晶体的情况。
