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Free keywords:
quasicrystal; approximant; gallium; zinc; magnesium
Abstract:
A tiling approach towards primitive hypercubic icosahedral quasicrystals of the Bergman cluster type is presented. A set of decoration rules for Bergman cluster approximants was derived from the crystal structures of the 1/1-(Al,Zn)(3)Mg-2 and the 2/1-(Al,Zn)(3+delta)Mg2-delta Fibonacci approximants. Both structures are well described as a Bergman cluster packing with two types of cluster connections. Clusters connected along threefold axes share Common hexagons and clusters along twofold axes share common edges. The cluster networks are compatible with canonical cell filings. An approximant of 3/2-2/1-2/1 type in the Ga-Mg-Zn system ((Ga,Zn)(43)Mg-26, Z=16, Cmc2(1), oC1104 a=36.8 Angstrom, b=c=22.8 Angstrom) was modelled using the next higher canonical cell and the decoration rules. Suitable single crystals of the 3/2-2/1-2/1 approximant were obtained by melting and annealing ternary alloys with target compositions with x(Mg)=0.38-0.44 and a free electron per atom ratio of e/a=2.09-2.19. The structure was successfully refined to (Ga,Zn)(175-delta)Mg97+delta (delta approximate to 3, (Ga,Zn)(67)Mg-37, x(Ga) approximate to 0.09-0.19, Z=4, Cmc2(1), a=36.840(7) Angstrom, b=22.782(5) Angstrom, c=22.931(5) Angstrom, N-ind = 23398 (9105 >2sigma(F-2); N-var = 576, R- 1/wR(2) = 0.170/0.154). (C) 2002 Elsevier Science B.V. All rights reserved.
