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首页> 外文期刊>Physical review >X-ray Linear Dichroism In Cubic Compounds: The Case Of Cr~(3+) In Mgal_2o_4
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X-ray Linear Dichroism In Cubic Compounds: The Case Of Cr~(3+) In Mgal_2o_4

机译：立方化合物的X射线线性二向色性：Mgal_2o_4中Cr〜（3+）的情况

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摘要

The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in MgAl_2O_4:Cr~(3+) spinel is measured by means of x-ray absorption near-edge structure spectroscopy and compared to calculations based on density functional theory (DFT) and ligand field multiplet (LFM) theory. We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the 1s→3d transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of Cr K pre-edge are identified as the site distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach is concluded that when the pre-edge features are more intense than 4% of the edge jump, pure quadrupole transitions cannot explain alone the origin of the pre-edge. Finally, the shape of the dichroic signal is more sensitive than the isotropic spectrum to the trigonal distortion of the substitutional site. This suggests the possibility to obtain quantitative information on site distortion from the x-ray linear dichroism by performing angular dependent measurements on single crystals.
机译：通过x射线吸收近边缘结构光谱法测量了MgAl_2O_4：Cr〜（3+）尖晶石中Cr K前缘的角度依赖性（x射线线性二色性），并与基于密度泛函理论的计算结果进行了比较（DFT）和配体场多重态（LFM）理论。我们还提出了一种基于对称性考虑因素的有效方法，该方法可从单个替换位点的二色性开始计算立方晶体的二色性。 DFT显示，电偶极跃变不会对预边缘中可见的特征有所贡献，并且可以清楚地看到1s→3d跃迁的分配。但是，DFT无法定量地再现前边缘的角度依赖性，而另一方面，通过LFM计算可以很好地再现该边缘。决定Cr K前缘二向色性的最相关因素是部位变形和3d-3d电子排斥。从这种组合的DFT中，LFM方法得出的结论是，当前边缘特征的强度大于边缘跳变的4％时，纯四极转变无法单独解释前边缘的起源。最后，二向色性信号的形状比各向同性光谱对置换位点的三角畸变更为敏感。这表明有可能通过对单晶进行角度相关的测量来获得有关X射线线性二色性中位点畸变的定量信息。

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