Anomalous effects of Cu-doping on structural and thermoelectric properties of the Al-Ir cubic quasicrystalline approximant

Highlights: • Physical properties of Al-Cu-Ir quasicrystalline approximants are investigated. • Doped Cu atoms behave as interstitial atoms in the Ir icosahedral cluster. • Valence electron rules are confirmed by experimental and computational results. The effects of Cu doping on thermoelectric properties and structural properties, such as lattice constant and density, were investigated for the 1/0-cubic quasicrystalline approximant Al_73.3−xCu_xIr_26.7 (x = 0–7). We found that the lattice constant increased as the Cu concentration increased although the atomic radius of Cu is smaller than that of Al. This finding suggested that doped Cu atoms are not only substituted for Al but also added to interstitial sites in the Ir icosahedral cluster to expand the cluster volume. These suggestions are also supported by the fact that the number of atoms per unit cell determined from the density, lattice constant, and analyzed composition increased as Cu content was increased. The Seebeck coefficient (S) of all the samples exhibited metallic behavior with a positive value. The Cu concentration dependence of S showed a maximum value for the x = 3-doping sample. This result indicates that substitutional Cu atoms contribute to hole doping and interstitial Cu atoms contribute to electron doping, which stabilizes the system electronically. The present work may contribute to an understanding of the mechanism of electronic stabilization in quasicrystals and related materials over a wide composition range.