Approximant of dodecagonal quasicrystal formed in Mn–Si–V alloy

Abstract

The crystal structure of a hexagonal phase formed in Mn₇₀Si₂₀V₁₀ has been analyzed by the Rietveld method using synchrotron radiation diffraction data. Analysis was carried out by assuming an average metallic element Mn_0.875V_0.125 via high resolution electron microscopy. The space group of the refined model is P6/mmm (No. 191) and the unit cell includes 18.9 Si and 78.1 metallic atoms. The lattice parameters are a = 17.058(6) Å, c = 4.640(1) Å. This hexagonal phase is regarded as an approximant of a dodecagonal quasicrystal. The electron diffraction pattern along the c-axis exhibits pseudo 12-fold diffraction symmetry. In the c-projection, the structure is interpreted as the tiling of two types of local structural units, namely an equilateral triangle and square with the same edge length of 4.57 Å, which correspond to A15-type and Zr₄Al₃-type structures in the three dimensions, respectively. Both bond orientational order and scaling property, characteristic of a dodecagonal quasicrystal, are imitated in this periodic tiling.

Keywords:

• approximant
• dodecagonal phase
• quasicrystal
• Mn-based alloy

Acknowledgements

We thank 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. 2010A1094).