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The 1/1 and 2/1 Approximants in the Sc−Mg−Zn Quasicrystal System:  Triacontahedral Clusters as Fundamental Building Blocks

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posted on 2006-10-11, 00:00 authored by Qisheng Lin, John D. Corbett
Single-crystal structures are reported for Sc3Mg0.18(1)Zn17.73(3), the 1/1 approximant crystal (AC), and Sc11.18(9)Mg2.5(1)Zn73.6(2), the 2/1 AC, in the corresponding icosahedral quasicrystal (i-QC) system. The 1/1 AC crystallizes in space group Im3̄, a = 13.863(2) Å, Z = 8, and the 2/1 AC, in Pa3̄, a = 22.412 (2) Å, Z = 8. The latter, which is valuable in pointing the way to the QC structure, is the best ordered and refined 2/1 example to date. The fundamental building blocks in both ACs are triacontahedral clusters centered by smaller multiply endohedral Tsai-type arrays; the former are condensed through body-centered-cubic packing in the 1/1 and primitive cubic packing in the 2/1 AC. Novel prolate rhombohedra centered by Sc−Sc dimers are also generated between triacontahedra in the 2/1 AC.

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