Crystal structure of 1/0-2/1-1/0 Cu–Al–Sc approximant

Abstract

The new crystal structure of an orthorhombic phase formed at the alloy composition Cu_48.1Al_36.4Sc_15.5 was analyzed by means of the Rietveld method using synchrotron radiation diffraction data. The starting model for this analysis was constructed theoretically using the so-called ‘cell constrained melt-quenching technique’. The space group of the final model is Cmmm, and the unit cell includes 49.0 Cu, 39.0 Al and 16.0 Sc atoms. The lattice parameters are a = 8.337(4) Å, b = 22.02(1) Å and c = 8.305(4) Å, which are related to the six-dimensional lattice parameter, a _6D = 6.959 Å, of the corresponding Cu–Al–Sc icosahedral quasicrystal as 1/0, 2/1 and 1/0 approximations, respectively. The characteristics of the structure can be understood as a framework consisting of Sc atoms, which is regarded as a tiling of five local structural units; the largest is an icosahedron similar to that included in the Tsai-type quasicrystal. The second exhibits structural similarity to a Mg₂Zn₁₁-type crystal and the third is an octahedron.

Keywords:

• approximant
• Tsai-type quasicrystal
• copper-based alloy

Acknowledgements

The authors wish to express their thanks to K. Edagawa for valuable comments. They also thank N. Miyazaki and Y. Tanaka for their help in the use of EPMA, and J. Kim and N. Tsuji for their help at SPring-8. The synchrotron radiation experiments were performed at SPring-8 BL02B2 with approval of the Japan Synchrotron Radiation Research Institute (No. 2008B1255).