POSITRON-ANNIHILATION STUDIES ON AL-CU-FE - THEIR SIGNIFICANCE FOR THE STRUCTURAL MODELS OF ICOSAHEDRAL QUASI-CRYSTALS

We have investigated the vacancy defects in samples of quasicrystallin e Al-Cu-Fe using positron-lifetime measurements as a function of vario us heat treatments and alloy compositions. In general, a two-component positron-annihilation-lifetime spectrum seems characteristic of quasi crystalline Al-Cu-Fe and other icosahedral quasicrystals. This can be interpreted in terms of vacancy defects, whose concentrations are abou t three orders of magnitude more than what one usually observes in cry stalline metals at room temperature. We discuss the significance of th e results of our studies for the structure of icosahedral quasicrystal s, for which several models have been postulated in the literature. Th e concentration of the vacancy defects observed by us seems incompatib le with the icosahedral-twin model or, at first sight, with the space- filling Penrose tiling based models or random-tiling models.