Crystal structure of the b.c.o. Al 4(Cr, Ni) decagonal approximant

Abstract

The crystal structure of the orthorhombic Al 4(Cr,Ni) coexisting with the decagonal quasicrystal in the Al–Cr–Ni system was determined by single crystal X-ray structure analysis: Immm (No. 71), a=1.2505 nm, b=1.2619 nm, c=3.0660 nm, V=4.8382 nm 3. Pearson symbol οI366-58.16, D x =3.451 g/cm 3, R 1=0.0728 for 2257 reflections, goodness of fit S=0.954. There are 39 (28Al+11Cr/Ni) independent atoms in a unit cell, of which four Al and nine (Cr/Ni) atoms have icosahedral coordination. As in many Al–TM (transition metal) intermetallic phases, a number of Al sites in Al 4(Cr,Ni) have a partial occupancy of only 0.5–0.67. The base-centered orthorhombic Al 4(Cr,Ni) consists of six layers along the a-axis, two flat ones each sandwiched between two puckered layers in mirror reflection. The interconnection of icosahedral clusters in this structure also exists in the hexagonal μ-Al 4Mn, κ-AlCrNi, λ-Al 4Mn and ν-Al 12Cr 2Fe phases.