THE SURFACE-STRUCTURE OF ICOSAHEDRAL AL68PD23MN9 MEASURED BY STM AND LEED

For the first time the surface structure of an icosahedral quasicrysta l has been successfully investigated in Ultra-High Vacuum (UHV) by Sca nning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction ( LEED). After cleaning an i-Al68Pd23Mn9 sample in UHV by cycles of ion- sputtering and annealing at temperatures close to the melting point at omically flat terraces are observed by STM. Successive step heights sh ow quasiperiodic order according to the Fibonacci chain. The normals o f these terraces are parallel to a five-fold axis as revealed by highl y resolved STM images. On the terraces five-fold stars and pentagonal holes are observed. Their orientation is the same on all terraces inve stigated. Additionally to this long-range orientational order, a high degree of quasiperiodic order is found for the pentagonal holes. This indicates that the quasiperiodic order of this highly ordered and ther modynamically stable quasicrystal extends even up to the surface. Both the step heights and the distances between the pentagonal holes well agree with the values derived from a structural model of this material . Five-fold symmetric LEED patterns can be analysed by means of the Fo urier transform of a Fibonacci pentagrid as suggested by the STM data. The analysis yields the same line separations within the pentagrid as deduced from the STM experiments.