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An icosahedral-twin is a twenty-face cluster made of ten interlinked dual-tetrahedron (bowtie) crystals, typically joined along triangular (e.g. cubic-111) faces having three-fold symmetry. One can think of their formation as a kind of nano-scale self-assembly. A variety of nanostructures (e.g. condensing argon, metal atoms, and virus capsids) assume icosahedral form on size scales where surface forces eclipse those from the bulk. A twinned-form of these nanostructures is sometimes found to occur e.g. in face-centered-cubic (FCC) metal-atom clusters larger than 10 nm in diameter.〔H. Hofmeister (2004) "Fivefold twinned nanoparticles" in ''Encyclopedia of Nanoscience and Nanotechnology'' (ed. H. S. Nalwa, Amer. Sci. Publ., Stevenson Ranch CA) vol. 3, pp. 431-452 ISBN 1-58883-059-4 (pdf ).〕 This may occur when the building-blocks beneath each of the 20 facets of an initially icosahedral cluster make the case for conversion to a translationally symmetric crystalline form. == Causes == When interatom-bonding doesn't have strong directional preferences, it's not unusual for atoms to gravitate toward a kissing-number of 12 nearest-neighbors. The three most symmetric ways to do this are by icosahedral clustering, or by crystalline face-centered-cubic (cuboctahedral) and/or hexagonal (tri-orthobicupolar) close-packing. Icosahedral arrangements, perhaps because of their slightly smaller surface area, may be preferred for small clusters e.g. noble gas and metal atoms in condensed phases (both liquid〔K. F. Kelton, G. W. Lee, A. K. Gangopadhyay, R. W. Hyers, T. J. Rathz, J. R. Rogers, M. B. Robinson, and D. S. Robinson, "First X-ray scattering studies on electrostatically levitated metallic fluids: Demonstrated influence of local icosahedral order on the nucleation barrier" (2003) ''Phys. Rev. Lett.'' 90, 195504 ((link ))〕 and solid). However, the Achilles-heel for icosahedral clustering about a single point is that it cannot fill space over large distances in a way that is translationally ordered. Hence bulk atoms (i.e. sufficiently large clusters) generally revert to one of the crystalline close-packing configurations instead. In other words, when icosahedral clusters get sufficiently large, the bulk-atom vote wins out over the surface-atom vote, and the atoms beneath each of the 20 facets adopt a face-centered-cubic pyramidal arrangement with tetrahedral (111) facets. Thus icosahedral twins are born, with a certain amount of strain along the interfacial (111) planes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Icosahedral twins」の詳細全文を読む スポンサード リンク
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