CRYSTAL-STRUCTURE OF ORTHORHOMBIC EPSILON-AL4CR

The crystal structure of the orthorhombic epsilon-Al4Cr phase in the A l-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) Angs trom, c = 20.223(1) Angstrom, V = 8787.7(9) Angstrom(3). Pearson symbo l 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, goodn ess of fit S = 1.084. There are 55 (13 Cr + 42 Al) independent atoms i n the unit cell, in which 12 Cr atoms and 28 Al atoms have icosahedral coordination. Interatomic distances cover the ranges of 2.570 Angstro m 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 obviousl y as a layered structure perpendicular to the a-axis. A typical aggreg ation of the icosahedral clusters in the structure of the epsilon-Al4C r phase has been pointed out.