TEM spectroscopy study of electronic structures of quasicrystals and approximants

The density of states (DOS) of the conduction and valence bands of a single quasicrystal of Al72NI20Co8 were obtained using electron energy-loss spectroscopy and X-ray emission spectroscopy based on transmission electron microscopy. A complete DOS near the Fermi level, which shows a clear pseudo-gap structure compared to that of aluminium, is presented for the first time. The DOS distribution is non-symmetric about the Fermi level. Onset energies of Al L-shell excitation spectra of amorphous, quasicrystalline and crystalline Al75Cu15V10 are compared with that of aluminium. Only that of the quasicrystalline phase shows a shift to the higher energy side by 0.3 eV. Onset energies of Al L-shell excitation spectra of Al-based quasicrystals (Al73Co27, Al63Cu25Fe12, Al63Cu25Ru12 and Al70Pd20Mn10) and corresponding approximant crystals are compared with that of aluminium. The onset energies of these quasicrystals show chemical shifts to the high energy side, indicating a smaller amount of valence charge at Al atom sites in the quasicrystalline phase than that of aluminium. Onset energies of approximant crystals of 0,2-Al73Co27, Al55Cu25Fe12Si8, Al55Cu25Ru12Si8 do not show a shift, and 1/1-Al67Pd11Mn14Si8 and 2/1-Al70Pd23Mn6Si1 show shifts compared with that of aluminium. An amount of the chemical shift of 2/1-Al70Pd23Mn6Si1 is larger than that of 1/1-Al67Pd11Mn14Si8. The results show that a decrease of valence charge at Al sites is characteristic for Al-based quasicrystalline materials. It may be attributed to an increase of covalency in the bonding of the quasicrystalline materials.