Ultrasound versus thermal conductivity in Ge clathrates

In this paper, we compare the results of the thermal conductivity measurements in the polycrystal Sr 8 Ga 16 Ge 30 with those of the ultrasonic data in the single crystal at low temperatures (T < 1 K). The thermal conductivity calculated from the ultrasound data is one order of magnitude higher than the measured value in the polycrystal. There are several possible explanations: the anisotropy of the crystal, the tunneling properties of the Sr atom might depend on crystallographic direction, and the possible existence of two-level systems (TLSs) on the grain surfaces. Our estimation of TLSs density of states at the grain boundaries gives the value P surface = 10 28 erg -1 cm -2 , which is extremely high. More studies are necessary to discriminate between these possible explanations i.e. measurements of the thermal conductivity in a single crystal would be very interesting.