Short-range order and the electronic structure of decagonal Al-Ni-Co

Detailed investigations of the atomic and electronic structures of decagonal Al-Ni-Co alloys have been performed. The topology of the structural model has been refined on the basis of the existing X-ray diffraction data. The chemical order on the decagonal lattice has been optimized via the comparison of the calculated electronic spectra with photoemission and soft-X-ray data and using total-energy calculations. The electronic structure calculations for large periodic approximants with up to 1276 atoms/cell have been performed self-consistently using a real-space tight-binding linear-muffin-tin orbital technique. The best agreement with the experimental spectra is achieved for a model with the innermost ring of the pentagonal columnar clusters occupied by Ni-atoms only. This configuration also has the lowest total energy. As in decagonal Al-Cu-Co we find a high density of states at the Fermi level, but the chemical ordering is very different: whereas in d-Al-Cu-Co direct Cu-Cu neighbours are suppressed and there is a slight preference for Co-Co homocoordination, in d-Al-Ni-Co a strong Ni-Ni interaction stabilizes the innermost Ni-ring, direct Co-Co neighbours are suppressed and there is a strong Co-Al interaction.