Search and Synthesis of New Family of Quasicrystals
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Search and Synthesis of New Family of Quasicrystals Tsutomu Ishimasa, Shiro Kashimoto and Ryo Maezawa Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan ABSTRACT Starting from the Zn17Sc3 cubic approximant, new icosahedral quasicrystal was searched by substituting Zn by other metals, M, at the alloy composition of Zn75M10Sc15. In the cases of M = Mn, Fe, Co, Ni, Pd, Pt, Ag and Au, new P-type quasicrystals were discovered in as-cast alloys. In the cases of M = Fe, Co, Ni, Pd and Ag, the quasicrystals are thermodynamically stable at approximately 700 ºC. This result indicates that use of an approximant crystal as a starting material is very efficient way to search new quasicrystal alloy, and many kinds of metals stabilize the quasicrystal structures; i.e. noble metals and transition elements including Mn, Fe, Co and Ni in addition to Mg. Taking the variety in base metals of Tsai-type approximants into account, this variety in additional components suggests many possibilities of undiscovered quasicrystals. The equality of the electron concentration, e/a ≈ 2.1, in Hume-Rothery rule may be a guide to these quasicrystals.
INTRODUCTION Experimental studies in the recent three years have revealed the existence of new family of icosahedral quasicrystal. The increasing knowledge has forced us to change the common understanding of a quasicrystal with respect to formation conditions, structural properties, and also physical properties. Before the discovery of the Cd-Yb quasicrystal [1, 2], base metals of quasicrystal alloys were limited: we knew only Al-, Zn-, Mg-, and Ti-based quasicrystals. Among them, stable quasicrystals with high structural perfection belong to either the Al-based or Zn-based alloys. Therefore our understanding was mainly based on the properties of these stable quasicrystals. For example, we believed that an F-type icosahedral quasicrystal may have structural advantages comparing with P-type, because F-type is due to the atomic ordering, and because all high quality icosahedral quasicrystals belong to F-type at that time. Now we know that this is not true: for example, the Zn-Mg-Sc quasicrystal belongs to P-type, and is stable with high degree of structural perfection [3]. Furthermore we know that Cd, Cu [4], and Ag [5] also can be base metals, and then there are many possibilities to find new quasicrystals. The purpose of the present study is to demonstrate this possibility. New family of quasicrystals is characterized by a unique atomic cluster called Tsai-type, which is different from well-known Bergman- and Mackay-types. These quasicrystals have structural relation to the approximants which also include Tsai-type cluster.
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TSAI-TYPE APPROXIMANT Structure model of the Zn17Sc3 crystal is presented in Fig. 1(a), which was analyzed by Andrusyak et al. [6] by means of single-crystal X-ray diffraction method. This structure is interpreted as an arrangement of Tsai-type cluster in the style of a body centered cubic structure.
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