Surface structures of Al-Pd-Mn and Al-Cu-Fe icosahedral quasicrystals
Surface structures of Al-Pd-Mn and Al-Cu-Fe surfaces, prepared and characterized in ultrahigh vacuum (UHV), have been systematically studied by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The set of four surfaces we have investigated, Al-Pd-Mn twofold, threefold, fivefold surfaces and Al-Cu-Fe fivefold surface, allows comparisons between different high-symmetry surfaces within a single alloy, and between different alloy surfaces having the same symmetry;Ar+ sputtering and annealing in UHV yield two different types of LEED patterns for all four surfaces. The first is obtained upon annealing at relatively low temperature: 500K-800K (depending on different surfaces). The second is obtained after annealing at higher temperature: 650K-900K (depending on different surfaces);The diffraction spots in low temperature LEED patterns are quite broad. The low temperature LEED patterns consist of multiple rotational domains on twofold and fivefold surfaces: two domains on twofold surface and five domains on fivefold surfaces. When separated into single domains, the LEED patterns are periodic and correspond to cubic [beta]-Al(Pd1-xMnx) or [beta]-Al(Cu1-xFex) with CsCl structure. The orientations of the cubic phases are (111) on threefold surface, (110) on twofold and fivefold surfaces;The high temperature LEED patterns have very sharp LEED spots. The symmetries and spot spacings in the high-temperature LEED patterns correspond well to those expected for unreconstructed quasicrystalline surfaces. Thus, the data are consistent with unreconstructed quasicrystalline surfaces;STM study of the Al-Pd-Mn fivefold surface shows that a terrace-step-kink structure starts to form after annealing above 700K. Large, atomic ally-flat terraces were formed after annealing at 900K. Fine structures with fivefold icosahedral symmetry were found on those terraces. Data analysis of our STM images and structure model of icosahedal Al-Pd-Mn suggest that the fine structures in our STM images may be caused by the pseudo Mackay clusters which are the structure units of the structure model.