AL-13-X(CO1-YNIY)(4), A NEW APPROXIMANT OF THE DECAGONAL QUASI-CRYSTAL IN THE AL-CO-NI SYSTEM

The structure of Al-13-x(Co1-yNiy)(4), with x = 0.9 and y = 0.12, a ne w monoclinic approximant of the decagonal phase in the system Al-Co-Ni , was determined by means of single-crystal X-ray diffractometry: spac e group C2/m (No.12), a = 17.071(2) Angstrom, b = 4.0993(6) Angstrom, c = 7.4910(9) Angstrom, beta = 116.17(1)degrees, V = 470.5(1) Angstrom (3), Pearson symbol mC34-1.8, D-x = 3.96 Mg/m(3), MoKalpha, F (000) = 530.6, M(r) = 140.5, mu = 7.97 mm(-1), Z = 2, wR = 0.034 and goodness of fit S = 1.15 for 963 reflections and 58 refined parameters. Typical interatomic distances are: d(Al-Co,Ni) = 2.323 Angstrom-2.733 Angstro m, d(Al-Al) = 2.688 Angstrom-2.956 Angstrom, d(Co,Ni-Co,Ni) = 2.891 An gstrom, respectively. Co atoms are coordinated by nine to eleven atoms , Al atoms are surrounded by ten, twelve or 13 neighbors. The relation ships to the structurally related quasicrystal approximants with compo sitions Al(13)TM(4) (TM = Co, Fe, Os, Rh, Ru) as well as to the Al - C o - Ni(Cu) decagonal phases are discussed.