Atomic structure of the Al–Cu–Fe–Si 1/1-cubic approximant
Atomic structure of the Al–Cu–Fe–Si 1/1-cubic approximant
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Al-Cu-Fe-Si 1/1-立方近似的原子结构
DOI：
10.1016/s0921-5093(00)01198-9
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发表时间：
2000
期刊：
影响因子：
6.4
通讯作者：
U. Mizutani
中科院分区：
文献类型：
--
作者：
T. Takeuchi;H. Yamada;M. Takata;Tamotsu Nakata;N. Tanaka;U. Mizutani
结合X射线Rietveld分析和高角环形暗场扫描电子显微镜(HAADF-STEM)，对Mackay型Al53Si9Cu25.5Fe12.51/1-立方体进行了详细的结构分析。电子衍射测量表明它是一个简单的立方体，空间群为Pm3，与Al-Mn-Si1/1立方体(α相)的空间群相同。通过选择Al-Mn-Siα相作为初始结构，通过Rietveld分析得到了原子在每个原子位置上的位置和占有率。用来衡量Rietveld分析精度的因子Rvpp和Ri分别降至2.32%和3.87%。在CsCl型晶胞的顶端和体心位置形成了由三层原子壳层组成的两个不同的二十面体团簇。其中一个团簇中涉及的过渡金属的数量远远高于另一个团簇中的过渡金属。此外，我们还发现，在一个含有较多过渡金属的亚晶格中，通过在团簇的第一壳层引入“结构无序”，R因子大大降低。由于第一壳层的占有率显著降低，结构无序很可能是由该壳层中的空位引起的。沿[100]、[110]和[111]方向拍摄的HAADF图像与Rietveld精化结构计算的投影图像吻合较好，有力地支持了本原子结构模型的有效性。
By combining the X-ray Rietveld analysis with the high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we have made a detailed structural analysis for the Mackay-type Al53Si9Cu25.5Fe12.51/1-cubic approximant. Electron diffraction measurements identified it as a simple cubic with a space group of Pm3, being the same as that of the Al–Mn–Si 1/1-cubic approximant (α-phase). Both atomic position and occupancy in every atomic site are refined by means of the Rietveld analysis with a choice of the Al–Mn–Si α-phase as the starting structure. The factors Rwpand RI, which are taken as measures for the precision of the Rietveld analysis, are reduced to 2.32 and 3.87%, respectively. Two different icosahedral clusters consisting of triply stacked atomic shells are formed at the vertex and body-centered positions in the CsCl-type unit cell. The number of transition metals involved in one of the clusters is much higher than in the other. In addition, we found that the R-factors are substantially reduced by introducing a “structural disorder” in the first shell of the cluster in one of the sublattices containing more transition metals. Since the occupancy in the first shell is reduced significantly, the structural disorder is most likely introduced by vacancies in that shell. The HAADF images taken along the [100], [110] and [111] directions agree well with the projection images calculated from the Rietveld-refined structure, lending a strong support to the validity of the present atomic structure model.