Comparative studies of positron annihilation lifetime and coincident Doppler broadening spectra for a binary Cd-based quasicrystal and 1/1-approximant crystal

Abstract

We performed the positron annihilation lifetime and coincident Doppler broadening measurements for binary icosahedral quasicrystal ${\mathrm{Cd}}_{5.7}\mathrm{Ca}$ and its 1/1-cubic ${\mathrm{Cd}}_{6}\mathrm{Ca}$ and ${\mathrm{Cd}}_{6}\mathrm{Yb}$ approximants. Since the obtained positron lifetimes are quite similar to one another, it is likely that the same type of structural vacancies exists in quasicrystal ${\mathrm{Cd}}_{5.7}\mathrm{Ca}$ and 1/1-cubic ${\mathrm{Cd}}_{6}\mathrm{Ca}$ and ${\mathrm{Cd}}_{6}\mathrm{Yb}$ approximants. The vacancy-type defects are concluded to be surrounded mostly by Cd atoms in both quasicrystal ${\mathrm{Cd}}_{5.7}\mathrm{Ca}$ and its 1/1-cubic approximant ${\mathrm{Cd}}_{6}\mathrm{Ca}$ from the high-momentum Doppler broadening spectra. In addition, we studied the temperature dependence of the positron annihilation lifetime in the low temperature region from 10 to $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for 1/1-cubic approximant ${\mathrm{Cd}}_{6}\mathrm{Ca}$ and ${\mathrm{Cd}}_{6}\mathrm{Yb}$ crystals. As a whole, in both 1/1-cubic ${\mathrm{Cd}}_{6}\mathrm{Ca}$ and ${\mathrm{Cd}}_{6}\mathrm{Yb}$ approximants the positron lifetime ${\ensuremath{\tau}}_{1}$ gradually increases with increasing temperature due to isotropic thermal expansion. However, the positron lifetime ${\ensuremath{\tau}}_{1}$ does not change at the order-disorder transition temperature, namely, 100 and $110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for 1/1-cubic ${\mathrm{Cd}}_{6}\mathrm{Ca}$ and ${\mathrm{Cd}}_{6}\mathrm{Yb}$ approximants, respectively. These results suggest that the size of the structural vacancies and local electron density do not change with the ordering.