Aluminum-Based Quasicrystals Studied by Slow Positron Beam Technique
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Aluminum-Based Quasicrystals Studied by Slow Positron Beam Technique
K. Satoa, H. Uchiyama, I. Kanazawa, R. Tamura1, b, K. Kimura1, F. Komori2, R. Suzuki3, T. Ohdaira3, and S. Takeuchi4, Department of Physics, Tokyo Gakugei University, Koganei, Tokyo, Japan. 1
Department of Materials Sciences, The University of Tokyo, Hongo, Bunkyo, Tokyo, Japan. Institute for Solid State Physics, The University of Tokyo, Roppomgi, Minato, Tokyo, Japan. 3 Quantum Radiation Division, Electrotechnical Laboratory, Umezono, Tsukuba, Ibaraki, Japan. 4 Department of Materials Science and Technology, Science University of Tokyo, Noda, Chiba, Japan. 2
ABSTRACT Icosahedral quasicrystals Al71.5Pd20.3Mn8.2, Al70.7Pd21.34Re7.96, Al62.5Cu25.5Fe12.5, and αAl68.31Mn21.21Si10.48 1/1- approximant were investigated by using a monoenergetic slow positron beam. The structural vacancy densities in the first three samples were determined to be 5.0×1020, 7.7×1020, and 4.7×1020 cm-3, respectively, by analyzing the measured S-parameter. INTRODUCTION Recently, many proposals have been reported for an atomic cluster model of icosahedral quasicrystals (QC’s) by single-crystal x-ray diffraction analysis, single-crystal neutron diffraction analysis, and high resolution transmission electron microscopy and so on. However, a unified view of the structure has not yet been obtained because of the lack of quantitative experimental data. As one of the reasons for this, a quasi-periodic structure of the icosahedral QC’s is mentioned. Positron-annihilation methods overcome this disadvantage, and carry information such as electronic state, vacancies, diffusion, and potentially the Fermi level of icosahedral QC’s. We have already confirmed by the positron-annihilation method that many kinds of stable icosahedral QC’s contain a dense distribution of structural vacancy-type sites without exception [1-4]. This is consistent with our previous expectation of the relationship between the presence of vacant-centers in clusters and the stability of icosahedral QC’s. Now that we confirm that stable icosahedral QC’s contain a dense distribution of structural vacancytype sites, the next step is to determine the structural vacancy densities in icosahedral QC’s. In this study, the stable Al-based icosahedral QC’s Al71.5Pd20.3Mn8.2, Al70.7Pd21.34Re7.96, Al62.5Cu25.5Fe12.5, and α-Al68.31Mn21.21Si10.48 1/1-approximant were investigated by using a slow
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positron beam. We derived the positron trapping radius at a center site in a Mackay icosahedron cluster for α-Al68.31Mn21.21Si10.48 1/1- approximant, whose atomic arrangement is perfectly clarified [5]. With this trapping radius, the structural vacancy densities in the other three QC’s were successfully determined. EXPERIMENTAL Al70.7Pd21.34Re7.96 ingot was prepared by arc melting the compressed raw material powder after being uniformly mixed in an argon atmosphere. Then the ingot was annealed for 12 hours at 1213K in vacuum after sealing it in a quartz tube. Subsequently, the quartz tube enclosing the ingot was quenched in wa
