EXAFS Study of N- And P-Type Ba(8)Ga(16)Ge(30)
We report extended x-ray absorption fine-structure (EXAFS) studies of n- and p-type Ba{sub 8}Ga{sub 16}Ge{sub 30} samples (type-I clathrate) at the Ga, Ge, and Ba K edges, to probe the local structure, particularly around the Ba atoms located inside 20- and 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 {angstrom}) 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 n- and p-type materials for either the Ba, Ga, or Ge K-edge data, which would explain the difference in thermal conductivity between n- and 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.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 953132
- Report Number(s):
- SLAC-REPRINT-2009-152; TRN: US200914%%205
- Journal Information:
- Phys. Rev. B78:014111,2008, Vol. 78, Issue 1
- Country of Publication:
- United States
- Language:
- English
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