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首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Maximum entropy method and charge flipping, apowerful combination to visualize the true nature of structural disorder from in situ X-ray powder diffraction data
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Maximum entropy method and charge flipping, apowerful combination to visualize the true nature of structural disorder from in situ X-ray powder diffraction data

机译：最大熵方法和电荷翻转，强大的组合，可从原位X射线粉末衍射数据中可视化结构异常的真实性质

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摘要

In a systematic approach, the ability of the Maximum Entropy Method (MEM) to reconstruct the most probable electron density of highly disordered crystal structures from X-ray powder diffraction data was evaluated. As a case study, theambient temperature crystal structures of disordered Rb2[C2O4] and-Rb2[CO3] and ordered -K2[C2O4] were investigated in detail with the aim of revealing the ‘true’ nature of the apparent disorder. Different combinations of F(based on phased structure factors) and G constraints (basedon structure-factor amplitudes) from different sources wereapplied inMEMcalculations. In particular, a new combination of the MEM with the recently developed charge-flipping algorithm with histogram matching for powder diffraction data (pCF) was successfully introduced to avoid the inevitable bias of the phases of the structure-factor amplitudes by the Rietveld model. Completely ab initio electron-density distributions have been obtained with the MEM applied to a combination of structure-factor amplitudes from Le Bail fits with phases derived from pCF. All features of the crystal structures, in particular the disorder of the oxalate and carbonate anions, and the displacements of the cations, are clearly obtained. This approach bears the potential of a fast method of electron-density determination, even for highly disordered materials. All the MEM maps obtained in this work were compared with the MEM map derived from the best Rietveld refined model. In general, the phased observed structure factors obtained from Rietveld refinement (applying F and G constraints) were found to give the closest description of the experimental data and thus lead to the most accurate image of the actual disorder.
机译：在系统的方法中，评估了最大熵方法（MEM）从X射线粉末衍射数据重建高度无序晶体结构最可能的电子密度的能力。作为案例研究，对无序的Rb2 [C2O4]和-Rb2 [CO3]和有序的-K2 [C2O4]的环境温度晶体结构进行了详细研究，目的是揭示表观疾病的“真实”性质。 MEM计算中应用了来自不同来源的F（基于相控结构因子）和G约束（基于结构因子振幅）的不同组合。特别是，成功引入了MEM与最近开发的电荷翻转算法以及直方图匹配的粉末衍射数据（pCF）的新组合，从而避免了Rietveld模型不可避免的结构因子振幅相位的偏差。通过将MEM应用于Le Bail拟合的结构因子振幅与衍生自pCF的相结合的MEM，已完全获得了从头算起的电子密度分布。清楚地获得了晶体结构的所有特征，特别是草酸根和碳酸根阴离子的无序，以及阳离子的置换。这种方法具有快速电子密度测定方法的潜力，即使对于高度无序的材料也是如此。将在这项工作中获得的所有MEM图与从最佳Rietveld精炼模型得到的MEM图进行比较。通常，发现通过Rietveld精炼（应用F和G约束）获得的分阶段观察到的结构因子可以最精确地描述实验数据，从而获得最准确的实际疾病图像。

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