CRYSTAL-STRUCTURE OF A HEXAGONAL PHASE AND ITS RELATION TO A QUASI-CRYSTALLINE PHASE IN ZN-MG-Y ALLOY

The crystal structure of a Zn-Mg-Y hexagonal phase, considered to be r elated to that of the quasicrystalline phase, has been determined by s ingle-crystal X-ray diffraction. The atomic model, refined to a final R value of 0.027, has the composition Zn65.22Mg27.92Y6.86 and 92 atoms in a unit cell with lattice constants a = 14.579(2) Angstrom and c = 8.687(1) Angstrom and the space group P6(3)/mmc. The Y atoms are situa ted at two different sites; one is fully occupied by Y atoms, while th e other is shared with Mg atoms. The structure can be characterized by a layer structure stacked along the c axis, and also viewed as a colu mnar structure composed of fused Friauf polyhedras sharing hexagonal r ings. The similarity of this hexagonal phase to the MgZn2 phase is sho wn. The structural relationship between this hexagonal phase and the i cosahedral phase is also discussed.