Thermal conductivity of thermoelectric clathrates

Transport properties of ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ 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 (\ensuremath{\kappa}) that, unlike in any other ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ samples reported in the literature, is similar to that of the iso-structural ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ These observations disagree with the commonly made assumption that \ensuremath{\kappa} of ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ at very low temperatures is determined by phonons scattered from guest atom tunneling states, since such states are believed to be absent in ${\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ Instead we propose that phonon charge carrier scattering must be considered in order to explain \ensuremath{\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.