Crystal structure of the primitive orthorhombic ɛ′-(Al,Si)4Cr phase

The crystal structure of the primitive orthorhombic ɛ′-(Al,Si)4Cr phase has been determined by single-crystal X-ray diffraction analysis: Pbnm (No. 62), a = 3.4477(4) nm, b = 2.0098(6) nm, c = 1.2467(6) nm, V = 8.6386(2) nm3, Pearson symbol oP576-5.6, Z = 8, Mr = 2302.68. Experimental composition Al72.1Si7.6Cr20.3. Formula sum Al50.94Si5.45Cr14.91, i.e., Al71.45Si7.64Cr20.91 or (Al,Si)79.09Cr20.91, very close to (Al,Si)4Cr. Dx = 3.541 g/cm3, F(0 0 0) = 8771, λ(Mo Kα) = 0.071073 nm, μ = 4.798 mm−1, T = 293 ± 2 K. Final R = 0.0632 for 9698 reflections with Fo > 4σ(Fo) and 0.0852 for all 13,087 reflections, goodness of fit S = 1.056. There are 94 (62 Al + 8 Si + 24 Cr) independent atomic positions in the unit-cell, of which 6 Al, 5 Si and 21 Cr atom sites have icosahedral coordination. Interatomic distances cover the ranges of 0.2537-0.2840 nm, 0.2287-0.2978 nm and 0.2487-0.3060 nm for Cr-Cr, Cr-Al(Si) and Al(Si)-Al(Si) atomic pairs, respectively. The structure of ɛ′-(Al,Si)4Cr is characterized by a layer structure consisting of PFP(PFP)′ layers stacked along the c-axis, where F and P denote flat and puckered layers, respectively, and (PFP)′ is related to PFP by a 21 rotation. The icosahedral clusters and their arrangements in the primitive ɛ′ are identical to those in the parent structure of ɛ-Al4Cr (Bbmm) and similar to a number of hexagonal and orthorhombic Al∼4TM (transition metals) including the hexagonal μ-Al4Cr/μ-Al4Mn (P63/mmc).