Crystal structure of a cubic Al₇₀Pd₂₃Mn₆Si; a 2/1 rational approximant of an icosahedral phase

The structure of a cubic Al_69.5Pd_23.0Mn_6.2Si_1.3 phase, which is referred as a 2/1 approximant of an icosahedral phase, was determined by means of single crystal X-ray structural analysis: space group Pm3 (No. 200), a = 20.211(2) Ä, V = 8255.9(14) ij MoK_a (? = 0.71069 Å); refined as Al_69.6Pd_24.3Mn_6.1, atom/cell = 512.83, F(000) = 11154.6, µ = 8.72 mm^-1, D_calc = 4.95 Mgm^-3, R(wR2) = 0.0623(0.1691) for 2597 reflections with I > 2.0s(I) and goodness of fit S = 1.096 for 165 variables. In the cubic 2/1 approximant structure, an atom cluster with about 20 Ä in diameter locates at the origin of the unit cell. This large cluster can be described by nine atomic shells with icosahedral symmetry, where the atomic arrangements similar to the Bergman or double Mackay clusters are clearly identified together with the t-inflated relationships in their sizes. The icosahedral quasicrystals in Al-Pd-Mn system are considered to be realized by a quasi-periodic arrangement of the similar atom clusters.