Electronic structure of quasicrystalline compounds

Theoretical as well as many experimental means have been used to investigate the electronic structures of quasicrystalline compounds. In icosahedral quasicrystalline compounds a pseudo-gap at the Fermi level exists that is also present even at the surface. Its formation involves a Hume-Rothery stabilization mechanism and also Al-transition metal hybridization effects. The specific atomic arrangement in a hierarchy of interpenetrating clusters may also be invoked to participate in the formation of this pseudo-gap. For Al based decagonal quasicrystals containing transition metals, the d–d interaction is such that there is no pseudo-gap in the total density of states whereas it still exists in the Al sub-bands. The electronic states in the quasicrystals and their close approximants are of more localized-like character on both sides of the pseudo-gap than in regular intermetallics that signs a clear tendency to weak electron localization. Extended states are found more than 2 eV above and deep below the Fermi level. The pseudo-gap at the Fermi level is not a genuine characteristic of quasicrystalline compounds as it exists also in related crystalline alloys, but in conventional alloys of the same phase diagram it is less pronounced than in the quasicrystal. The Al 3p pseudo-gap is connected to the metallic character of the specimen since the larger and deeper the pseudo-gap, the less metallic is the sample. This may allow one to tune the metallic character through a series of compounds.