THE STRUCTURE OF DECAGONAL AL70NI15CO15

The quasiperiodic structure of decagonal Al70Ni15Co15 was determined o n the basis of X-ray single-crystal intensity data using the n-dimensi onal embedding mediod. Centrosymmetric five-dimensional space group P1 0(5)/mmc, three-dimensional reciprocal and direct quasilattice paramet ers: a(i) = 0.2636 (1) angstrom-1, i = 1,..., 4, a5* = 0.24506 (3) an gstrom-1, a(i) = 3.794 (1) angstrom, i = 1,..., 4, a5 = 4.0807 (3) ang strom. 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, m u = 10.1 mm-1, Mo Kalpha, wR = 0.078, R = 0.091 for 253 unique reflect ions and 21 variables. A five-dimensional structure model was obtained by Patterson syntheses and refined by the least-squares technique. Hi gh-resolution electron-density maps were calculated by die maximum-ent ropy 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 Al65C u20Co15 and shows locally a close resemblance to monoclinic Al13Co4. C olumnar 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 icosag onal rings of pentagonal and rectangular structure motifs.