Atomic structures of ternary Yb-Cd-Mg icosahedral quasicrystals and a 1/1 approximant
Atomic structures of ternary Yb-Cd-Mg icosahedral quasicrystals and a 1/1 approximant
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三元Yb-Cd-Mg二十面体准晶和1/1近似原子结构
DOI：
10.1107/s2052520617013270
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
An-Pang Tsai
中科院分区：
文献类型：
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作者：
Tsunetomo Yamada;Hiroyuki Takakura;Marc de Boissieu;An-Pang Tsai
三元二十面体（i）Yb-Cd-Mg准晶的原子结构 和相应的1/1近似物（AP），其组成为Yb_（13.3）Cd_（70.3）Mg_（16.5）。发现iQC的结构与母体i-YbCd 5.7是同构的，其由所谓的Tsai型菱形三十面体（RTH）簇和双Friauf多面体组成，并且发现1/1AP的结构与YbCd 6是同构的，其由相同类型的RTH簇的体心堆积描述。在iQC中，发现Cd/Mg的占位有Cd优先位、Mg优先位和Cd/Mg混合位3种类型，Mg原子在Mg优先位的占位概率在Mg含量为20 at.%附近呈现饱和行为。 这种选择性的镁占据被认为是二十面体晶格常数随着镁含量的增加而非线性增加的原因。1/1 AP具有类似的选择性Mg占领的蔡型RTH簇的壳结构方面的iQC。在iQC和1/1 AP中，Mg优先位点在它们的配位数中具有较少数量的Yb原子。此外，短程有序（s.r.o.）在iQC的衍射图案上，在对应于面心型（F型）二十面体超晶格的位置处观察到漫散射。F型s.r.o.发现是由于镁的替代。
Atomic structures of ternary icosahedral (i) Yb–Cd–Mg quasicrystals (QCs) with five different Mg contents up to 46.4 at.% and a corresponding 1/1 approximant (AP), which has a composition of Yb13.3Cd70.3Mg16.5, have been analysed by single-crystal X-ray diffraction. The structures of the iQCs were found to be isostructural to the parent i-YbCd5.7, which consists of a so-called Tsai-type rhombic triacontahedron (RTH) cluster and double Friauf polyhedron, and that of the 1/1 AP was found to be isostructural to YbCd6, which is described by a body-centred packing of the same type of RTH cluster. In the iQCs, it was found that there are three types of Cd/Mg occupation, namely, Cd preferential site, Mg preferential site and Cd/Mg mixed site, and the occupation probabilities of Mg atoms at the Mg preferential site show a saturation behaviour around the Mg content of 20 at.%. This selective Mg occupation is identified as a cause of the non-linear increase in the icosahedral lattice constant with increasing Mg content. The 1/1 AP has a similar selective Mg occupation to that of the iQCs in terms of the shell structures of the Tsai-type RTH cluster. In both iQCs and the 1/1 AP, the Mg preferential sites have a smaller number of Yb atoms among their coordination numbers. Moreover, short-range order (s.r.o.) diffuse scattering was observed on the diffraction patterns of the iQCs at the positions corresponding to a face-centred-type (F-type) icosahedral superlattice. The F-type s.r.o. was found to result from the Mg substitution.