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

Abstract

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) nm 3, Pearson symbol oP576-5.6, Z = 8, Mr = 2302.68. Experimental composition Al 72.1Si 7.6Cr 20.3. Formula sum Al 50.94Si 5.45Cr 14.91, i.e., Al 71.45Si 7.64Cr 20.91 or (Al,Si) 79.09Cr 20.91, very close to (Al,Si) 4Cr. Dx = 3.541 g/cm 3, F(0 0 0) = 8771, λ(Mo K α) = 0.071073 nm, μ = 4.798 mm −1, T = 293 ± 2 K. Final R = 0.0632 for 9698 reflections with F o > 4 σ( F o ) 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 2 1 rotation. The icosahedral clusters and their arrangements in the primitive ɛ′ are identical to those in the parent structure of ɛ-Al 4Cr ( Bbmm) and similar to a number of hexagonal and orthorhombic Al ∼4TM (transition metals) including the hexagonal μ-Al 4Cr/μ-Al 4Mn ( P6 3/ mmc).