Christensen Sebastian, Bindzus Niels, Christensen Mogens, Brummerstedt Iversen Bo
Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus, Denmark.
Acta Crystallogr A Found Adv. 2015 Jan;71(Pt 1):9-19. doi: 10.1107/S2053273314024103. Epub 2015 Jan 1.
Subtle structural features such as disorder and anharmonic motion may be accurately characterized from nuclear density distributions (NDDs). As a viable alternative to neutron diffraction, this paper introduces a new approach named the nuclear-weighted X-ray maximum entropy method (NXMEM) for reconstructing pseudo NDDs. It calculates an electron-weighted nuclear density distribution (eNDD), exploiting that X-ray diffraction delivers data of superior quality, requires smaller sample volumes and has higher availability. NXMEM is tested on two widely different systems: PbTe and Ba(8)Ga(16)Sn(30). The first compound, PbTe, possesses a deceptively simple crystal structure on the macroscopic level that is unable to account for its excellent thermoelectric properties. The key mechanism involves local distortions, and the capability of NXMEM to probe this intriguing feature is established with simulated powder diffraction data. In the second compound, Ba(8)Ga(16)Sn(30), disorder among the Ba guest atoms is analysed with both experimental and simulated single-crystal diffraction data. In all cases, NXMEM outperforms the maximum entropy method by substantially enhancing the nuclear resolution. The induced improvements correlate with the amount of available data, rendering NXMEM especially powerful for powder and low-resolution single-crystal diffraction. The NXMEM procedure can be implemented in existing software and facilitates widespread characterization of disorder in functional materials.
诸如无序和非谐运动等细微结构特征可以从核密度分布（NDDs）中准确地表征出来。作为中子衍射的一种可行替代方法，本文介绍了一种名为核加权X射线最大熵方法（NXMEM）的新方法，用于重建伪NDDs。它利用X射线衍射提供高质量数据、所需样品体积较小且可用性较高的特点，计算电子加权核密度分布（eNDD）。NXMEM在两个差异很大的系统上进行了测试：PbTe和Ba(8)Ga(16)Sn(30)。第一种化合物PbTe在宏观层面上具有看似简单的晶体结构，但无法解释其优异的热电性能。关键机制涉及局部畸变，通过模拟粉末衍射数据证实了NXMEM探测这一有趣特征的能力。在第二种化合物Ba(8)Ga(16)Sn(30)中，利用实验和模拟单晶衍射数据对Ba客体原子之间的无序进行了分析。在所有情况下，NXMEM通过大幅提高核分辨率，优于最大熵方法。所带来的改进与可用数据量相关，这使得NXMEM对于粉末和低分辨率单晶衍射特别有效。NXMEM程序可以在现有软件中实现，并有助于对功能材料中的无序进行广泛表征。
