THE STRUCTURE OF DECAGONAL AL70NI15CO15

The quasiperiodic structure of decagonal Al70Ni15Co15 was determined on the basis of X-ray single-crystal intensity data using the n-dimensional embedding mediod. Centrosymmetric five-dimensional space group P10(5)/mmc, three-dimensional reciprocal and direct quasilattice parameters: a(i)* = 0.2636 (1) angstrom-1, i = 1,..., 4, a5* = 0.24506 (3) angstrom-1, a(i) = 3.794 (1) angstrom, i = 1,..., 4, a5 = 4.0807 (3) angstrom. Five-dimensional unit-cell parameters: d(i) = 3.393 (1), d5 = 4.0807 (3) angstrom, alpha(ij) = 60-degrees, alpha(i5) = 90-degrees, i, j = 1,..., 4, V = 302.4 angstrom5, M(r) = 36.54, D(x) = 4.5 Mg m-3, mu = 10.1 mm-1, Mo Kalpha, wR = 0.078, R = 0.091 for 253 unique reflections and 21 variables. A five-dimensional structure model was obtained by Patterson syntheses and refined by the least-squares technique. High-resolution electron-density maps were calculated by die maximum-entropy method. The structure is fortnally built up by two quasiperiodic atomic layers with stacking sequence Aa (a denotes the layer A rotated around 2pi/10). It is essentially isotypic to that of decagonal Al65Cu20Co15 and shows locally a close resemblance to monoclinic Al13Co4. Columnar clusters parallel to the local ten-fold screw axes, formed by ten pentagonal antiprismatic columns surrounding a central column, are found to be the basic structural building elements. Quasiperiodicity is forced by the formation of interconnected networks of closed icosagonal rings of pentagonal and rectangular structure motifs.