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