Structural Model of the Orthorhombic Non-Fibonacci Approximant in the Al₁₂Fe₂Cr Alloy

Al_5.103(Fe, Cr), orthorhombic, Imm2, a = 12.34, b = 12.41, c = 30.71 Å, V = 4701 Å³, atoms/cell = 305 (average), D_x 3.4 g cm^-3. A structural model of the non-Fibonacci approximant O-AIFeCr with composition Al_5.103(Fe, Cr) found in an Al₁₂Fe₂Cr alloy has been derived from its high-resolution electron microscopy images and the structure of the hexagonal -Al₄Mn [Shoemaker, Keszler & Shoemaker (1989). Acta Cryst. B45, 13-20], being isostructural to -Al₄Cr. Among the 14 unique positions of transition metal (TM) atoms, 13 are icosahedrally coordinated: five of these icosahedra interpenetrate into each other, forming icosahedral chains along [010], and the remaining eight icosahedra with their twofold axis parallel to this axis. This structural model describes a (010) layer structure with a sequence P^mFP(P^mFP)_I, where a flat layer F (a mirror plane) is sandwiched between two puckered P and P^m layers, and (P^mFP)I is related to P^mFP through a body-centred translation. The interatomic distances are 2.51-2.74 Å for TM-TM, 2.38-2.99 for TM-Al and 2.41-3.18 Å, for Al-Al. Simulated images and electron diffraction patterns calculated based on this structural model are comparable with the experimentally observed results.

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