Bulk Termination of the Quasicrystalline Five-Fold Surface of Al_{70}Pd_{21}Mn_{9}

Abstract

The structure of the Al_{70}Pd_{21}Mn_{9} surface has been investigated using high resolution scanning tunnelling microscopy (STM). From two large five-fold terraces on the surface in a short decorated Fibonacci sequence, atomically resolved surface images have been obtained. One of these terraces carries a rare local configuration in a form of a ring. The location of the corresponding sequence of terminations in the bulk model M of icosahedral i-AlPdMn based on the three-dimensional tiling T*(2F) of an F-phase has been estimated using this ring configuration and the requirement from the LEED work of Gierer et al. that the average atomic density of the terminations is 0.136 atoms per A^2. A termination contains two atomic plane layers separated by a vertical distance of 0.48 A. The position of the bulk terminations is fixed within the layers of Bergman polytopes in the model M: they are 4.08 A in the direction of the bulk from a surface of the most dense Bergman layers. From the coding windows of the top planes in terminations in M we conclude that a Penrose (P1) tiling is possible on almost all five-fold terraces. The shortest edge of the tiling P1, is either 4.8 A or 7.8 A. The experimentally derived tiling of the surface with the ring configuration has an edge-length of 8.0 +- 0.3 A and hence matches the minimal edge-length expected from the model.