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

Detailed investigations of the atomic and electronic structures of decagona l AlNiCo alloys have been performed. Several different models for the decag onal structure have been investigated: A model based on a rhombic-hexagon t iling proposed by Henley and models based on a cluster decoration of the Pe nrose 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 compari son of the calculated electronic spectra with photoemission and soft-x-ray data and using total-energy calculations. The electronic structure calculat ions for large periodic approximants with up to 1276 atoms/cell have been p erformed self-consistently using a real-space tight-binding linear muffin-t in orbital technique. The best a,agreement with the experimental spectra is achieved for a large-rhombus-tiling model with the innermost ring of the p entagonal columnar clusters occupied by Ni atoms only. This configuration a lso has the lowest total energy. As in decagonal AlCuCo we find a high dens ity of states at the Fermi level, but the chemical ordering is very differe nt: 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 su ppressed and there is a strong Co-Al interaction.