AlIr: An Al-Ir Binary-Phase Superstructure of the NiAl Type.

A binary phase with AlIr composition has been discovered in the Al-Ir binary system. Single-crystal X-ray diffraction analysis reveals that it crystallizes in the trigonal space group 31 with the unit cell parameters = 12.8802(2) Å and = 9.8130(2) Å. This structure is derived from the NiAl structure type. The supercell is due to the ordering of the aluminum atoms, which replace the nickel atoms in the prototype structure. The crystal structure was directly imaged by atomic-scale scanning transmission electron microscopy, and the misalignment of the Al site responsible for the supercell has been clearly evidenced. Its metastable nature has been confirmed by differential thermal analysis measurements. The atomic and electronic structures of AlIr have also been investigated by density functional theory. The structural optimization leads to lattice parameters and atomic positions in good agreement with the experimental ones. The compound is metallic, with a minimum in the density of states located more than 1 eV above the Fermi energy. This suggests a metastable system, in agreement with the electron count found much above 18 electrons per Ir atom, deviating from the Hume-Rothery rule and with the presence of occupied antibonding states revealed by the crystal orbital Hamiltonian population analysis. The relative stability of the compound is ensured by the hybridization between sp-Al and d-Ir states within Ir-centered clusters, while covalent-like interactions in-between the clusters are indicated by the analysis of the electron localizability function.