Bulk termination of the quasicrystalline fivefold surface of Al ₇₀Pd₂₁Mn₉

The structure of the Al70Pd21Mn9 surface has been investigated using high-resolution scanning tunneling microscopy. From two large fivefold 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 the 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 low-energy electron diffraction work of Gierer that the average atomic density of the terminations is 0.136 atoms per Angstrom(2). A termination contains two atomic plane layers separated by a vertical distance of 0.48 Angstrom. The position of the bulk terminations is fixed within the layers of Bergman polytopes in the model M: they are 4.08 Angstrom 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 fivefold terraces. The shortest edge of the tiling P1 is either 4.8 or 7.8 Angstrom. The experimentally derived tiling of the surface with the ring configuration has an edge length of 8.0+/-0.3 Angstrom and hence matches the minimal edge length expected from the model.