Off-center phonon scattering sites in Eu8Ga16Ge30 and Sr8Ga16Ge30

We present a detailed extended x-ray absorption fine structure sEXAFSd analysis of the thermoelectric clathrates Eu8Ga16Ge30 and Sr8Ga16Ge30, 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 Ga/Ge lattice is quite stiff with a high correlated Debye temperature ,400 K. Eu is on-center in the site 1 cage but off-center s0.445±0.020 Åd in the large cage called the Eu2 site. The results for Sr are similar, but ,75% are off-center 0.40±0.05 Å and ,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 s80±10 and 100±10 K for Eu1 and Sr1, respectively, and 95±10 and 125±10 K for the shortest Eu2-Ga/Ge and Sr2-Ga/Ge pairsd. In contrast, the more distant Eu2/Sr2-Ga/Ge pair distributions within the Eu2/Sr2 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.