Atomic and electronic structure of decagonal Al-Ni-Co alloys and approximant phases

Detailed investigations of the atomic and electronic structures of decagonal AlNiCo alloys have been performed. Several different models for the decagonal structure have been investigated: A model based on a rhombic-hexagon tiling proposed by Henley and models based on a cluster decoration of the Penrose tiling with large rhombus edge. The topology of the structural models has been refined on the basis of the existing x-ray-diffraction data which, however, do;not allow us to specify the chemical decoration uniquely. 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 a,agreement with the experimental spectra is achieved for a large-rhombus-tiling 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 AlCuCo we find a high density of states at the Fermi level, but the chemical ordering is very different: whereas in d-AlCuCo direct Cu-Cu neighbors are suppressed and there is a slight preference for Co-Co homocoordination, in d-AlNiCo a strong Ni-Ni interaction stabilizes the innermost Ni ring, direct Co-Co neighbors are suppressed and there is a strong Co-Al interaction.