Interralation Between Hume-Rothery Mechanism, Hybridization and Covalent Bonds in Aluminum- and Boron-based Icrosahedral
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Metallic-covalent bonding conversion in icosahedral clusters of Al and B by small changes of the structure was demonstrated by molecular orbital calculations. According to the electron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al12Re, α-AlMnSi, α-rhombohedral boron). The multiple-shell atomic structure, the electrical and optical conductivity are compared for metal doped β-rhombohedral boron and AlLiCu or AlPdRe icosahedral quasicrystal. Photoemission and electron energy loss spectroscopy investigations for V doped β-rhombohedral have been discussed. Conclusions are the following two. It is difficult to distinguish a Hume-Rothery, i.e. structure-induced, pseudogap, and a covalent bonding, i.e. hybridization, pseudogap for materials with a highly symmetric Brillouin zone and a strong potential for valence electrons. Because hybridization is necessary not only for covalent bonds but also for metallic ones, it is better to use the covalent bonding pseudogap than the hybridization pseudogap.
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Some Al-based icosahedral quasicrystals and approximants exhibit semiconductorlike electron transport, although they consist of almost metallic elements and have a finite electron density of states (DOS) at the Fermi energy, EF. They are, therefore, considered to be intermediate substance between semiconductors and metals. Unique electric properties, particularly anomalous high resistivity, of some Al-based icosahedral quasicrystals have been reported by many researchers. The origin of the high resistivity has been explained by the combination of a Hume-Rothery pseudogap in the electron density of states at EF and the localization tendency of electrons near EF [1]. The Hume-Rothery pseudogap reduces the number of relevant carriers and the localization tendency causes minimal carrier mobility. The Hume-Rothery mechanism has been considered to be the origin of the stability of quasicrystalline structure. On the other hand, the hybridization between the Al p band and the transition metal d band has been pointed out to be additionally responsible for the opening in the pseudogap near EF for quasicrystals including transition metals [2]. Both structure and physical properties of a higher-order approximant crystal are similar
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to those of quasicrystal. However, Kimura et al. pointed out that in Al-based icosahedral quasicrystals and approximants, the electrical conduc
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