THE CRYSTAL-STRUCTURE OF A GAMMA-(AL-CU-RU-SI) CUBIC PHASE - A NEW APPROXIMANT PHASE FOR THE AL-CU-RU ICOSAHEDRAL QUASI-CRYSTAL

The crystal structure of a gamma-(Al-Cu-Ru-Si) cubic phase (Al55.1Cu14 .6Ru20.2Si10.1), considered to be closely related to that of the Al-Cu -Ru icosahedral quasicrystal, has been determined by single-crystal X- ray diffraction. The atomic model, refined to a final R value of 0.034 6, has about 262 atoms with an atomic ratio of Al66.3Cu13.5Ru19.8 in a cubic unit cell with a lattice constant a = 1.538 0(2) nm and the spa ce group Fm3. The fundamental structure is characterized by a sc arran gement of dodecahedral atom clusters with an interval of 0.77nm, which corresponds to a lattice constant of a 1/0 cubic approximant phase fo r the icosahedral quasicrystals. The chemical ordering of the constitu ents in the dodecahedral cluster produces a superstructure with a latt ice constant twice that of a 1/0 cubic approximant phase.