TEM spectroscopy study of electronic structures of quasicrystals and approximants

Abstract

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 τ0,τ2-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.

Acknowledgements

The authors thank Mr. F. Sato for his skilful technical assistance. The construction of XES spectrometers was performed with the help of the workshop of IMRAM, Tohoku University. The development and improvement of the XES spectrometer was supported by a Grant-in-Aid for Scientific Research (B) (No. 12440079, No. 13554010) from the Ministry of Education, Science, Sports and Culture of Japan.