Off-center phonon scattering sites in Eu8Ga16Ge30 and Sr8Ga16Ge30

We present a detailed extended x-ray absorption fine structure (EXAFS) 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 (0.445±0.020 A) in the large cage called the Eu2 site. The results for Sr are similar, but ∼75% are off-center 0.40±0.05 A 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 (80±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 pairs). 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