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Dodecagonal quasicrystal in Mn-based quaternary alloys containing Cr, Ni and Si

A dodecagonal quasicrystal showing 12-fold symmetry forms in Mn-rich quaternary alloys containing 5.5 or 7.5 at.% Cr, 5.0 at.% Ni and 17.5 at.% Si. After annealing at 700 C for 130 h, the quasicrystal precipitated in a matrix of \b{eta}-Mn type crystalline phase. The shape of the quasicrystal is needle-like having a length of several tens of micrometers. Electron diffraction as well as powder X-ray diffraction experiments have revealed the following characteristics of the quasicrystal; diffraction symmetry 12/mmm, presence of systematic extinction for h1h2h2h1h5 type reflections with odd h5 index, and then five-dimensional space group P126/mmc. Indexing of the reflections indicated that the dimension of the common edge in the equilateral triangle-square tiling is 4.560 Å, and the periodicity is 4.626 Å along the 12-fold axis. This is the first example of the dodecagonal quasicrystal synthesized by ordinary metallurgical method in 3-d transition-metal alloys.