Crystal structure of orthorhombic epsilon-Al4Cr

The crystal structure of the orthorhombic epsilon-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) Angstrom, b = 34.705(2) Angstrom, c = 20.223(1) Angstrom, V = 8787.7(9) Angstrom(3). Pearson symbol oC584-20.48, Z = 8, M-r = 2256, composition Al56.23Cr14.21, D-x = 3.41 g/cm(3); F(000) = 8576, lambda (MoK alpha) = 0.71069 Angstrom, mu = 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 Angstrom similar to 2.646 Angstrom, 2.421 Angstrom similar to 2.978 Angstrom, and 2.480 Angstrom similar to 2.983 Angstrom, 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 epsilon-Al4Cr phase has been pointed out.