Department of earth and Planetary Science, The University of Tokyo
○Kazumasa Sugiyama Hironori Sato

Since the addition of transition metals and silicon into an Al matrix involves the formation of a variety of intermetallic phases which contribute to special mechanical strength and high thermal and electrical conductivity, the ternary phase diagram of Co-Al-Si system was extensively studied [1,2]. On the other hand, much attention has been paid to the Al-Co alloy system because a large number of quasi-crystalline phases together with their approximants have been found in their Al-rich region. The ternary system of Al-Co-Si is also classified in this category [3].
The crystal structures of approximant structures; f-AlCoSi (Pnma; a=1.3859(1)nm, b=2.3019(1)nm, c=0.7322(1)nm,) and W-AlCoSi (Cm;a=3.9658(4)nm, b=0.8139(1)nm, c=2.3658(2)nm, b=90.02(1)^o) grown in the Al-rich region of the Al-Co-Si system, were analyzed by the single crystal X-ray diffraction. Both compounds exhibit surprisingly large unit cells and complex structures. In particular, the structure of W-AlCoSi phase is isostructural with W-AlCoNi phase by providing a fundamental atomic arrangement for the columnar atom cluster observed commonly in the Al-Co-Ni decagonal quasicrystals[4]. The structure of f-AlCoSi phase serves an unique columnar structural unit similar to those found in the approximants of Al₃Mn[5] and Al₃Pd.[6].
[1]K.W.Richter & D.T.Gutierrez, Intermetallics 12(2005)848. [2]K.W.Richter, Y.Prots & Y.Grin, Inorg. Chem. 44(2005)4576. [3]J.Menon & C.Suryanarayana, Mater. Trans. JIM 30(1989)878. [4]K.Sugiyama, S.Nishimura & K.Hiraga, J. Alloy Comp., 342(2002)65. [5]K.Hiraga, M.Kaneko, Y.Matsuo & S.Hashimoto, Philos. Mag. B67(1993)193. [6]Y.Matsuo & K.Hiraga, Philos. Mag. Lett., 70(1994)155.