  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.
