Charge transfer and local structure in thermoelectric germanium clathrates

Germanium clathrates have recently received attention as potential highly efficient thermoelectric materials based on the phonon glass electron crystal (PGEC) concept. A combined EXAFS and XANES study has been performed in order to investigate the local structure and the degree of charge transfer between the guest atoms and the framework atoms. Analysis of the Sr, Ga, and Ge K-edge XANES spectra of a Sr₈Ga₁₆Ge₃₀ clathrate reveals that the atoms are close to neutrally charged and that the degree of charge transfer is low in agreement with recent theoretical predictions and charge density distribution measurements.