SYNCHROTRON X-RAY STUDY OF THE ELECTRON-DENSITY IN ALPHA-AL2O3

Structure factors for corundum, alpha-Al2O3, have been measured using 0.7 and 0.9 angstrom synchrotron radiation as well as Mo Kalpha (lambd a = 0.71069 angstrom) radiation. The stronger structure factors from t wo sets of synchrotron data and three sets of Mo Kalpha tube data for two small crystals are remarkably consistent. In analysing those data, extinction corrections evaluated by minimizing differences between eq uivalent reflection intensities were compared with those from the more common procedure of least-squares determination of the extinction par ameters as a part of the structure refinement. Difference electron den sities evaluated with extinction corrections from equivalent reflectio n intensities are more consistent than those which optimize \F(o)\ ver sus \F(c)\ agreement. Approximate symmetry in the concordant densities is related more closely to the Al-Al geometry than to nearest neighbo ur Al-O interactions. Space group R3cBAR, hexagonal, M(r) = 101.96, a = 4.7540 (5), c = 12.9820 (6) angstrom, V = 254.09 (6) angstrom3, Z = 6, D(x) = 3.997 Mg m-3, mu(0.9 angstrom) = 2.361, mu(0.7 angstrom) = i .097, mu (Mo Kalpha) = 1.139 mm-1, F(000) = 300, T = 293 K, R = 0.024, wR = 0.030, S = 4.84 for the unique 0.7 angstrom synchrotron reflecti on data.