Crystal structure of orthorhombic ε-Al₄Cr

The crystal structure of the orthorhombic ε-Al₄Cr phase in the Al-Cr system was determined by single crystal X-ray structure analysis: space group Cmcm (No. 63), a = 12.521(1)Å, b = 34.705(2) Å, c = 20.223(1) Å, V = 8787.7(9) Å Pearson symbol oC584-20.48, Z = 8, M_r = 2256, composition Al_56.23Cr_14.21,Z)_x = 3.41 g/cm³; F(000) = 8576, λ (MoKa) = 0.71069 Å, µ = 4.510 mm^-1, room temperature, R = 0.068 for 2839 reflections, goodness of fit S = 1.084. There are 55 (13 Cr + 42 Al) independent atoms in the unit cell, in which 12 Cr atoms and 28 Al atoms have icosahedral coordination. Interatomic distances cover the ranges of 2.570 Å ~ 2.646 Å, 2.421 Å ~ 2.978 Å, and 2.480 Å ~ 2.983 Å, for Cr-Cr, Cr-Al and Al-Al atomic pairs, respectively. The crystalline phase appears obviously as a layered structure perpendicular to the a-axis. A typical aggregation of the icosahedral clusters in the structure of the ε-Al₄Cr phase has been pointed out.