Electronic and atomic structure of Ba8Ga16Ge30-xSix Type I Clathrates: A Ge and Ga K-XAFS Study
Azzam Mansour, Joshua B. Martin, Winnie Wong-Ng, George S. Nolas
X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba8Ga16Ge30-xSix type I clathrates (x=7.5, 9.1, 10.7, 13.4). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution with the greatest changes occurring for the specimen with the highest measured power factor (2/ρ, where is the Seebeck coefficient, ρ is the electrical resistivity). Our experimental results provide a strong correlation with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in and consequently in the power factor, according to the Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge indicating that Si resides preferentially next to Ga. Both the Ge-Ga/Ge and Ga-Ge/Ga coordination distances remain relatively unchanged (2.51 Å) regardless of the degree of Si substitution. Furthermore, The Ge-Si and Ga-Si coordination distances remained relatively unchanged at 2.41 and 2.45 Å, respectively, regardless of the degree of Si substitution. Understanding the underlying physics of this structure-property relationship could indicate additional routes for tuning the electronic properties of clathrates for thermoelectric applications.
, Martin, J.
, Wong-Ng, W.
and Nolas, G.
Electronic and atomic structure of Ba8Ga16Ge30-xSix Type I Clathrates: A Ge and Ga K-XAFS Study, Journal of Physics D-Applied Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=911270
(Accessed May 28, 2023)