Covalent bonding and semiconducting bandgap formation in Al–transition-metal quasicrystalline approximants

We demonstrate that in some quasiperiodic transition-metal aluminides a special bonding between the Al(s,p) and transition-metal (TM) d-orbitals is responsible for the formation of a semiconducting gap in the electronic spectrum. We analyzed the electronic charge distribution and have observed an enhanced charge density along the Al-TM bonds that is characteristic for covalent bonding. The semiconducting gap is found in three lowest approximants of the hierarchy of approximants, which converges to the icosahedral quasicrystals of the fci class, to which also i-AlPdRe belongs. We predict the existence of novel truly semiconducting quasicrystalline 1/1-approximants with composition e.g. Al₈₈Pd₂₄Ir₁₆, Al₈₈Pd₃₆W₄, or Al₈₈Au₁₂Ir₂₈ with the bandgap at the Fermi level up to 0.7 eV. These results allow us to predict the existence of semiconducting quasicrystals of a similar Al-TM composition. The possibility of the existence of a narrow semiconducting bandgap suggests an explanation of the anomalously high resistivity of the icosahedral AlPdRe quasicrystals. However, the observed bandgap formation and covalent bonding is not specific to quasicrystals, and has been observed also in crystalline Al-TM compounds. We conclude that it is not the quasicrystallinity but the unusual Al-TM bonding that is responsible for a substantial part of anomalies observed in the transport properties of Al-TM quasicrystals. © 2004 Elsevier B.V. All rights reserved.