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 similar to400 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 similar to75% are off-center 0.40+/-0.05 A and similar to25% 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.