Strong phonon charge carrier coupling in thermoelectric clathrates

The origin of the very low and glasslike lattice thermal conductivity (kappa(L)) in highly crystalline clathrates has been intensely discussed. Guest atom tunneling, guest atom resonant scattering, disorder scattering, and soft potentials have been proposed as possible explanations. However, none of these models can explain the fundamental difference in kappa(L) between n- and p-type Ba8Ga16Ge30. Here we provide a comprehensive explanation through analysis of the physical properties of a range of Ba8In16-xGe30-y square(x+y) and Ba8Ni6-xGe40+x samples. We show that kappa(L) is intimately linked to the charge carriers, and propose that the glasslike kappa(L) is a consequence of strong phonon charge carrier coupling. In general the influence of charge carriers on kappa(L) has been studied only in relatively lightly doped and structurally ordered semiconducting materials. Clathrates constitute a class of materials with relatively large charge carrier concentrations in combination with structural disorder. This opens up the possibility of studying the influence of charge carriers on kappa(L) in the limit where kappa(L) and kappa(e) are of equal magnitude.