Off-center phonon scattering sites inEu8Ga16Ge30andSr8Ga16Ge30

Abstract

We present a detailed extended x-ray absorption fine structure (EXAFS) analysis of the thermoelectric clathrates ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, both of which have an unusually low thermal conductivity attributed to a ``rattler'' motion of the filler atoms Eu and Sr. The EXAFS results show that the $\mathrm{Ga}∕\mathrm{Ge}$ lattice is quite stiff with a high correlated Debye temperature $\ensuremath{\sim}400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Eu is on-center in the site 1 cage but off-center $(0.445\ifmmode\pm\else\textpm\fi{}0.020\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ in the large cage called the Eu2 site. The results for Sr are similar, but $\ensuremath{\sim}75%$ are off-center $0.40\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $\ensuremath{\sim}25%$ are on-center in the Sr2 site. Both results are in reasonable agreement with diffraction results. The temperature dependence of the nearest neighbor pair distribution widths yield low Einstein temperatures ($80\ifmmode\pm\else\textpm\fi{}10$ and $100\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for Eu1 and Sr1, respectively, and $95\ifmmode\pm\else\textpm\fi{}10$ and $125\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the shortest $\mathrm{Eu}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ and $\mathrm{Sr}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ pairs). In contrast, the more distant $\mathrm{Eu}2∕\mathrm{Sr}2\ensuremath{-}\mathrm{Ga}∕\mathrm{Ge}$ pair distributions within the $\mathrm{Eu}2∕\mathrm{Sr}2$ cage are strongly disordered even at low $T$, indicating considerable local disorder. This indicates that the off-center Eu or Sr atom is bonded to the side of the site 2 cage. This has two important implications for the thermal conductivity: it increases the coupling between the ``rattler'' vibrations and the lattice phonons and it introduces a symmetry-breaking large mass defect.