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

TL;DR

This paper reports the discovery of an icosahedral quasicrystal and a 1/1 cubic approximant in Au-Al-Yb alloys, highlighting their structures, compositions, and stability, and suggesting implications for Yb valence states.

Contribution

It presents the first identification of an icosahedral quasicrystal and its cubic approximant in Au-Al-Yb alloys, including detailed structural and compositional analysis.

Findings

The quasicrystal has a 6D lattice parameter of 7.448 Å.

The approximant is stable at 700°C with a lattice parameter of 14.500 Å.

Yb exhibits intermediate valence potentially related to its unique environment.

Abstract

A P-type icosahedral quasicrystal is formed in Au-Al-Yb alloy of which 6-dimensional lattice parameter a6D = 7.448 {\AA}. The composition of the quasicrystal was analyzed to be Au51Al34Yb15. This quasicrystal is formed in as-cast alloys, and is regarded as metastable because of decomposition into other crystalline phases by annealing at 700 \degree C. Among Tsai-type quasicrystals, this quasicrystal is situated just between Zn-Sc group with smaller a6D and larger Cd-Yb group. Intermediate valence of Yb recently observed in this quasicrystal may be due to this unique situation, namely smaller major component Au-Al than in Cd-Yb. The predominant phase in the annealed specimen is a 1/1 cubic approximant with lattice parameter a = 14.500 {\AA} belonging to the space group Im-3 . This phase is stable at the composition Au51Al35Yb14 at 700 \degree C. Rietveld structural analysis indicated that the crystal structure is understood as periodic arrangement of Tsai-type clusters each including four Au-Al atoms at their centers. Chemical ordering of Au and Al is characteristics of this approximant.