Local structure of Cu in Cs 8Na 16Cu 5Ge 131 type II clathrate

Abstract

We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Cu and Ge in the Cs 8Na 16Cu 5Ge 131 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 Cs 8Na 16Ge 136. The EXAFS data suggest that Cu either randomly substitutes for Ge on the clathrate framework or preferentially on the 96 g site but not preferentially on the 32 e or 8 a 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.