CLUSTER-BASED MODEL FOR THE ICOSAHEDRAL AL-CU-LI QUASI-CRYSTAL

We propose a model for the icosahedral Al-Cu-Li quasicrystal, in which the large five-shell 136-atom icosahedral clusters, found in the R-ph ase by Audier et al., occupy the nodes of the tiling of four canonical cells according to Henley. The residual factors calculated for the si ngle-crystal data set of de Boissieu et al. are 6.1% for X-ray and 6-6 % for neutron diffraction. We interpret this result as an indication t hat the outer shells of the large clusters are essentially unfragmente d also in the icosahedral phase.