EXPERIMENTAL-VERIFICATION OF THE QUASI-UNIT-CELL MODEL OF QUASI-CRYSTAL STRUCTURE

The atomic structure of quasicrystals(1)-solids with long-range order, but non-periodic atomic lattice structure-is often described as the t hree-dimensional generalization of the planar two-tile Penrose pattern (2). Recently, an alternative model has been proposed(3-5) that descri bes such structures in terms of a single repeating unit(3-5)-the three -dimensional generalization of a pattern composed of identical decagon s. This model is similar in concept to the unit-cell description of pe riodic crystals, with the decagon playing the role of a 'quasi-unit ce ll'. But, unlike the unit cells in periodic crystals, these quasi-unit cells,overlap their neighbours, in the sense that they share atoms. N evertheless, the basic concept of unit cells in both periodic crystals and quasicrystals is essentially the same: solving the entire atomic structure of the solid reduces to determining the distribution of atom s in the unit cell. Here we report experimental evidence for the quasi -unit-cell model by solving the structure of the decagonal quasicrysta l Al72Ni20Co8. The resulting structure is consistent with images obtai ned by electron and X-ray diffraction, and agrees with the measured st oichiometry, density and symmetry of the compound. The quasi-unit-cell model provides a significantly better fit to these results than all p revious alternative models, including Penrose tiling.