Atomic structures of ternary Yb–Cd–Mg icosahedral quasicrystals and a 1/1 approximant. Issue 6 (11th December 2017)

• Record Type: Journal Article
• Authors: Yamada, Tsunetomo; Takakura, Hiroyuki; de Boissieu, Marc; Tsai, An-Pang
• Is Part Of: Acta crystallographica. Volume 73:Issue 6(2017:Dec.)
• Page Start: 1125
• Page End: 1141
• Publication Date: 2017-12-11
• Subjects: quasicrystals -- approximants -- superspace crystallography -- structure analysis -- X‐ray scattering
• DOI: 10.1107/S2052520617013270
• Languages: English
• ISSNs: 2052-5206

Abstract

Abstract : A systematic study of the structures of ternary icosahedral Yb–Cd–Mg quasicrystals with different Mg contents has been carried out by single‐crystal X‐ray diffraction; occupational and positional disorder in the resulting structures was compared with that in a Yb–Cd–Mg 1/1 approximant. Abstract : 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 Yb13.3 Cd70.3 Mg16.5, 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.