EXAFS study of n- and p-type Ba8Ga16Ge30

We report extended x-ray absorption fine-structure EXAFS studies of nand p-type Ba8Ga16Ge30 samples type-I clathrate at the Ga, Ge, and Ba K edges, to probe the local structure, particularly around the Ba atoms located inside 20and 24-atom cages Ba1 and Ba2 sites, respectively composed of Ga/Ge atoms. In agreement with diffraction analysis, we find Ba2 is off center, with a component in the bc plane 0.15 Å comparable to that found in diffraction. However, under the assumption of a stiff cage, we also require a significant a component. This suggests a coupling or attraction between the Ba2 atoms and the hexagonal rings at the top or bottom of the cage that encloses the Ba2 site. Further, changing the a component can change the number of shortest Ba2-Ga/Ge neighbors and hence the coupling of Ba2 to the surrounding cage. Within the cage structures which enclose both Ba sites, the Ga-Ga/Ge distances are slightly longer, while the Ge-Ga/Ge distances are slightly shorter than the average distance reported from diffraction. The longer Ga-Ga/Ge distances indicate that the Ba1 and Ba2 cages may be dimpled or distorted. At the second Ga/Ge distance, the local distortions in the Ba clathrate are smaller than those observed in the Eu clathrate, which likely plays a role in understanding the higher thermal conductivity of Ba clathrates compared to that of Eu clathrates. However, there is no clear difference in the EXAFS between the nand p-type materials for either the Ba, Ga, or Ge K-edge data, which would explain the difference in thermal conductivity between nand p-type materials. Finally, an average Einstein temperature for the shortest Ba2-Ga/Ge bonds is comparable to that for Ba1-Ga/Ge. This indicates a large effective spring constant for the closest Ga/Ge atoms to Ba2. We also develop a simple vibrational model to show explicitly the three types of vibration for Ba2 within the type-2 cage.