Positron-Annihilation Studies of Frank-Kasper-Type Quasicrystals of the Mg–Zn–Ga–Al and the Mg–Zn–Rare-Earth System

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We have performed positron-annihilation experiments for a Frank-Kasper (F-K)-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg–Zn–Ga–Al, and the stable F-K-type icosahedral quasicrystals of Mg–Zn–RE (RE$=$rare-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of vacant centers in triacontahedral clusters exists in both icosahedral and approximant phases of Mg–Zn–Ga–Al. Furthermore, in the present study, we report a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg–Zn–RE system also contain a high density of structural vacancies, similar to the stable Al-based quasicrystals. In particular, it is estimated experimentally that the structural vacancy densities for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are $3 \times 10^{20}$ and $4 \times 10^{20}$ cm-3, respectively. These results provide important information on the structural modeling of the stable icosahedral quasicrystal Mg–Zn–RE, which is a recent research topic of the icosahedral quasicrystals.
2001-01-15