Local structure of Cu in Cs8Na16Cu5Ge131 type II clathrate

We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Cu and Ge in the Cs8Na16Cu5Ge131 type II clathrate. We show that the local structure parameters for Ge (coordination number and distances) are consistent with those derived on the basis of XRD investigation of Cs8Na16Ge136. The EXAFS data suggest that Cu either randomly substitutes for Ge on the clathrate framework or preferentially on the 96g site but not preferentially on the 32e or 8a sites (Wyckoff notation). Furthermore, we find that the Cu–Ge distance is smaller than the Ge–Ge distance by 0.13 Å, indicating a local distortion around the Cu atoms. The estimated degrees of disorder for Cu–Ge and Ge–Ge interactions indicate the Cu–Ge clathrate framework to be relatively stiff, while those for Na–Ge and Cs–Ge interactions corroborate previous observations of strong thermal disorder of the alkali guests in these materials. Our XAS results offer insight into the site substitution of Cu in this material, information unattainable from X-ray diffraction due to the lack of scattering contrast between Cu and Ge.

Comparison of Fourier transforms of Cu K-edge and Ge K-edge EXAFS spectra for Cs8Na16Cu5Ge131 collected near the temperature of liquid nitrogen (LN). The data clearly show that the Cu–Ge distance is smaller than the Ge–Ge distance, indicating the presence of a local distortion around Cu.Figure optionsDownload as PowerPoint slide