Approximants in the Ag–In–M and Au–Sn–M (M = Ca or Rare Earth Metals) Systems

概要

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Formation of 1/1 Yb–Cd type approximants has been verified in the Ag–In–RE (RE: rare earth metals) system for RE = Tm, Tb, Er, Dy, Gd, Pr, Ce, or Eu. The compositions of the 1/1 approximants determined by wavelength dispersive X-ray analysis (WDX), are in the range of 42–48 at. % Ag, 38–42 at. % In, and 14–15 at. % REs. By employing the empirical rule of $e/a\sim 2.0$, where $e/a$ is the valence electron concentration, a number of approximants are newly found in the Au–Sn–RE systems for RE = Sm, Pr, Ce, Dy, Tb, Gd, or Eu, where the 1/1 approximants are formed in the range of 61–64 at. % Au, 20–23 at. % Sn, and 14–15 at. % REs. The 2/1 approximants only form in Ag43In43Eu14 and Au60Sn25M15 for M = Eu, Yb, or Ca, where the atomic size for Eu, Yb, and Ca is larger than that for the other REs and they are all divalent. Basically, the 2/1 and 1/1 approximants share the same $e/a$ as the quasicrystals found in the In–Ag–M, Au–Sn–M, and Cd–M groups. It is likely that the phase selection among the quasicrystalline and the 1/1 and 2/1 approximant phases is dominated by the effective atomic size factor, $R_{\text{r,e}}$.
2008-10-25