Semiconducting Aluminum Transition-Metal Quasicrystals
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Semiconducting Aluminum Transition-Metal Quasicrystals Marian Krajčí1,2 and Jürgen. Hafner1 1
Institut für Materialphysik and Center for Computational Materials Science, Universität Wien, Sensengasse 8/12, A-1090 Wien, Austria 2 Institute of Physics, Slovak Academy of Sciences, Dubravska cesta 9, SK-84511 Bratislava, Slovak Republic
ABSTRACT We report on a class of icosahedral aluminum-transition-metal (Al-TM) alloys with a true semiconducting behavior. The existence of a semiconducting gap is found to depend critically on a particular kind of Al-TM ordering defined by a simple rule in a 6-dimensional superspace. Any deviation from this 6D order leads to the formation of strongly localized defect states in the gap. The structure of the semiconducting quasicrystals belongs to the face-center-icosahedral structural class, the same to which also icosahedral AlPdRe belongs. The description of the structure is based on a modified Katz-Gratias-Boudard model. We have calculated the electronic structure for series of quasicrystalline approximants. In the density of states of the 1/1 approximant with 128 atoms/unit cell relaxed with Hellmann-Feynman forces we have found a real gap at or close to the Fermi level. To get a semiconductor the critical band-filling is 648 electrons per cell. It can be achieved for several different stoichiometries. We investigated semiconducting 1/1 approximants with various combinations of 3d, 4d and 5d transition-metals. The gap widens as a 3d or 4d metal is replaced by a 5d metal. Calculations for the 2/1 approximant with 544 atoms/cell show the persistence of the gap. The chemical composition for which semiconducting behavior is achieved varies in the sequence of the approximants, but the existence of the gap in the electronic spectrum persists in higher order approximants, provided the Al-TM ordering is respected. Icosahedral AlPdRe represents the quasiperiodic limit in this hierarchy of approximants. As substitutional defects which obviously exist in real samples of any quasicrystal lead to formation of localized states, a real sample of icosahedral AlPdRe seems to be a semiconductor with a band-gap filled by localized states. INTRODUCTION Among the many exotic physical properties of quasicrystals, their tendency to show high electrical resistivities and negative temperature coefficients has been most extensively studied. The possible occurrence of a metal-insulator transition in highly resistive quasicrystals, and in icosahedral AlPdRe in particular, is a highly challenging and controversial subject [1]. The possibility of insulating behavior of an alloy consisting only of metallic elements is fascinating and has aroused intensive interest. In the present work we demonstrate that insulating behavior in fci structural class of quasicrystals has not origin in the quasiperiodicity itself, as it is widespread assumed, but in a special chemical ordering between Al and transition-metal (TM) atoms that leads to the formation of a semiconducting gap in the electronic spectrum. The exist
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