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arXiv 0704.0878cond-mat.mtrl-sci
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MgAl2O4中取代基Cr3 +周围的结构弛豫

Structural relaxation around substitutional Cr3+ in MgAl2O4

Amélie Juhin, Georges Calas, Delphine Cabaret, Laurence Galoisy, Jean-Louis Hazemann

摘要:

利用Cr K边扩展X射线吸收精细结构(EXAFS)和X射线吸收近边结构(XANES)谱研究了Cr 3+离子在MgAl 2 O 4尖晶石中的结构环境。结构弛豫和XANES谱的第一性原理计算已经进行,与实验吻合良好。Cr-O距离与MgCr_2O_4相近,表明第一邻层原子完全弛豫,第二层原子部分弛豫。这些观察结果表明MgAl_2 O_4-MgCr_2 O_4固溶体不遵守Vegard定律。尽管有一些角位畸变,在Cr取代Al的过程中,尖晶石结构的B-位的局部D3 d对称性被保留。在这里,我们表明,松弛是由应变诱导的键屈曲,与角倾斜的Mg为中心的四面体周围的Cr为中心的八面体。相比之下,没有显着的改变之间的角度的边共享八面体,建立链沿三个四重轴的立方结构。

英文摘要:

The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement to the experiment. The Cr-O distance is close to that in MgCr2O4, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4 solid solution. Despite some angular site distortion, the local D3d symmetry of the B-site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accomodated by strain-induced bond buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three four-fold axes of the cubic structure.

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