Crystal structure of a 2/1 cubic approximant in an Al–Rh–Si alloy

Abstract The structure of a cubic Al 66.5 Rh 26.1 Si 7.3 phase, which is referred as a 2/1 approximant for an icosahedral phase, was determined by means of single crystal X-ray structural analysis: space group Pm 3, a =19.935(2) A, Mo Kα ( λ =0.71069 A); refined as Al 70.4 Rh 29.6 , R ( wR 2)=0.087(0.194) for 1391 reflections with I >2.0 σ ( I ). A dodecahedral atom cluster is located at the origin of a unit cell, and the corresponding atomic arrangement can be described as nine shells with a local icosahedral symmetry, by suggesting a close similarity to the dodecahedral atomic cluster of the 2/1-AlPdMnSi approximant. The 2/1-AlRhSi approximant is a second example of a simple cubic arrangement of dodecahedral atom clusters that are 20 A in diameter.