CRYSTAL-STRUCTURE OF BETA-AL4.5FESI

By a combination of X-ray and electron diffraction, the average struct ure of the intermetallic phase beta-Al4.5FeSi has been determined. The crystals grown from the melt were generally of poor quality, the mono clinic space group A2/a was deduced from electron diffraction patterns obtained from small domains. X-ray diffraction data measured at T = 2 98 K with Mo Kalpha radiation (lambda = 0.71069 angstrom) out to sin t heta/lambda = 0.8 angstrom-1 resulted in 1739 observed reflections, of which 980 were unique. Of these, 244 weak reflections affected by dis order were removed. Cell dimensions are a = 6.161 (3), b = 6.175 (3), c = 20.813 (6) angstrom, beta = 90.42 (3)-degrees. As indicated by the chemical composition there are eight Fe and 44 Al/Si atoms in the uni t cell. The average structure was determined by direct methods and ref ined to a conventional R factor of 0.039. The structure may be describ ed in terms of ten-coordinated iron in the centre of a double-capped s quare antiprism of Al/Si atoms, which again are joined together in dou ble layers normal to the c axis by the sharing of anti-prism edges in the layer and cap atoms between the two single layers.