Thermal conductivity of thermoelectric clathrates

Transport properties of Ba8Ga16Ge30 single-crystalline samples prepared with an excess of Ga are presented. The excess Ga does not only produces p-type transport properties but also a thermal conductivity (kappa) that, unlike in any other Ba8Ga16Ge30 samples reported in the literature, is similar to that of the iso-structural Eu8Ga16Ge30 and Sr8Ga16Ge30. These observations disagree with the commonly made assumption that kappa of Eu8Ga16Ge30 and Sr8Ga16Ge30 at very low temperatures is determined by phonons scattered from guest atom tunneling states, since such states are believed to be absent in Ba8Ga16Ge30. Instead we propose that phonon charge carrier scattering must be considered in order to explain kappa at the low temperatures. The transport data also suggest that the resonant scattering, which dominates at intermediate temperatures, strongly depends on the charge carrier concentration.