Structural Analysis on Quasicrystals and their Approximants.

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Since the first finding of a quasicrystal in Al-Mn system by Schetman, a number of structural studies have been carried out in order to understand the physico-chemical properties of this new materials. The conventional high-resolution electron microscope is known to be one of the most powerful methods to analyze the structures of quasicrystals and their approximants. However, detailed structural information such as transition metal distribution cannot be obtained from the results of the electron microscope only. On the other hand, X-ray diffraction allows us to determine the precise atomic coordinates for individual constituents and such information is strongly required for discussing a variety of physical properties of quasicrystals. In these contexts, the main purpose of this research project is to characterize the structures of quasicrystals and their approximants by the combination of high-resolution electron microscope and X-ray diffraction. It may be added that the projection met … More hod including high dimensional space group theory is intensively used for elucidate the obtained structural results. For the intensity measurement of X-ray diffraction, we developed a new type of weissenberg camera equipped with imaging plates and the intensity reader for them. The rotating anode type X-ray generator with a maximum load of 18kW is employed as a X-ray source, the graphite monochromator is set in the incident beam pass for selecting MoKalpha radiation. Two imaging plates are installed and the measuring time is extensively reduced in comparison with a conventional four-circle diffractometer. It should be added that the mu-axis controlling system is also introduced for the intensity measurement with no special blind region. By using this weissenberg camera, we foundan dodecahedral atom clusters with 1.2nm and 2.0nm in diameter in Al-Pd-Mn system. These atom cluster are classified as essential structural units for the corresponding icosahedral phase, by indicating tau-inflation rule in their sizes. The structural model by using these atom clusters coupled with the projection method allows us to demonstrate a feasible structural model of Al-Pd-Mn icosahedral quasicrystals. These results were appreciated by many material scientists and we are invited to talk these fruitful results at the international conference of quasicrystals in 1997. These atom clusters with dodecahedral symmetry together with a projection method are suggested to shed light upon the real understanding of icosahedral quasicrystals. Less