Some new crystalline approximants of Al-Pd-Mn quasicrystals

Abstract

In this letter we report new crystalline approximants found in Al-Pd-Mn alloys. In the Al₇₀Pd₂₀Mn₁₀ alloy, two cubic approximants with a = 1·24 and 2·03 nm, corresponding to substituting the rational ratios 1/1 and 2/1, respectively, for the irrational τ = 1/2(1 + 5^1/2) in the icosahedral quasicrystal, have been found. In an alloy with a somewhat lower Pd content, namely Al₇₀Pd₁₅Mn₁₅, four orthorhombic approximants with a and c equal respectively to 1·26 and 1·48 nm (using 1/1 and 1/1 respectively to substitute for τ in two orthogonal non-equivalent twofold axes of the decagonal quasicrystal), 1·92 and 6·14 nm (2/1 and 5/3), 5·05 and 3·78 nm (5/3 and 3/2), and 8·4 and 6·2 nm (8/5 and 5/3) occurred. Their b parameter corresponding to the periodicity along the tenfold axis of the decagonal quasicrystal is 1·24 nm.