Tsai-Type Quasicrystal and Its Approximant in Au-Al-Tm Alloys

A P-type icosahedral quasicrystal with a six-dimensional lattice parameter $a_{6D}$ = 7.411 Å is formed as an equilibrium phase in Au-Al-Tm alloy, of which composition was analyzed to be $Au_{46}Al_{38}Tm_{16}$ by electron probe microanalysis. This quasicrystal is observed as a predominant phase in both as-cast and $Au_{49}Al_{34}Tm_{17}$ alloys annealed at 910°C, and as one of main phases in the alloy slowly cooled from 1020°C. A 1/1 approximant, $Au_{48}Al_{38}Tm_{14}$, is also formed near the composition of the quasicrystal. This is a body-centered cubic structure (space group: Im3̅) with a lattice parameter a = 14.458 Å that is an isostructure to the recently reported 1/1 Tsai-type approximant in Au-Al-Yb. This approximant is characterized by disorderly arranged four Au-Al atoms centered at the Tsai-type cluster, presence of atoms at 8c site, and chemical ordering of Au and Al at sites forming a partial triacontahedron.