Icosahedral quasicrystal and 1/1 cubic approximant in Au–Al–Yb alloys

Abstract

A P-type icosahedral quasicrystal is formed in an Au–Al–Yb alloy with a six-dimensional lattice parameter a _6D = 7.448 Å. The composition of the quasicrystal was analyzed to be Au₅₁Al₃₄Yb₁₅. The quasicrystal is formed in as-cast alloys and is regarded as metastable with decomposition to other crystalline phases during annealing at 700°C. Among Tsai-type quasicrystals, this quasicrystal is situated just between the Zn–Sc group with a smaller a _6D and the larger Cd–Yb group. The intermediate valence of Yb recently observed in this quasicrystal may be due to this unique situation, namely a smaller major component in Au-Al-Yb than in Cd–Yb. The predominant phase in the annealed specimen is a 1/1 cubic approximant with lattice parameter a = 14.500 Å belonging to the space group Im . This phase is stable at the composition Au₅₁Al₃₅Yb₁₄ at 700°C. Rietveld structural analysis indicated that the crystal structure is a periodic arrangement of Tsai-type clusters with four Au–Al atoms at their centers. Chemical ordering of Au and Al is characteristics of this approximant.

Keywords:

• quasicrystal
• approximant
• Rietveld refinement
• Tsai-type
• Au-based alloy

Acknowledgements

The authors thank T. Watanuki for interesting discussions and N. Miyazaki for his help and advice in the use of EPMA.