Structural vacancies and their local atomic environment in the Zn-Mg-Sc alloy system studied by positron annihilation spectroscopy

Recently discovered P-type icosahedral quasicrystal Zn80Mg5Sc15 of high structural perfection and its crystalline 1/1-approximant Zn85Sc15 are investigated by positron annihilation spectroscopy. Positrons are trapped by high concentration of the vacancy-type sites surrounded by Zn atoms in both the quasicrystal and approximant. Based on the atomic structure of the approximant, the vacancy-type sites are identified to be inside the dodecahedral 20 Zn first shell, suggesting that the quasicrystal is composed of the same cluster units as the approximant. In contrast to the proposed structure model of Zn85Sc15 where inside of the dodecahedral first shell is empty, the present results indicate the existence of several atoms inside the dodecahedral first shell. Positron diffusion experiments using a slow positron beam reveal that the structural vacancy density in the quasicrystal is 35% lower than that in the approximant