Peierls or Jahn-Teller effect in endohedrally doped silicon clathrates:: An EXAFS study

The effect of doping silicon clathrate structures with sodium atoms has been investigated experimentally by x-ray photoemission and x-ray absorption spectroscopies. Both techniques reveal a poor screening of the sodium atoms inside the silicon cages. We also discuss the sodium state that is intermediate between the metal-like and the atomlike ones. These results are in agreement with theoretical predictions of Demkov et al. and Smelyansky and Tse. In addition, the fine analysis of the extended x-ray absorption fine structure region reveals unambiguously a large displacement (0.9 +/- 0.2 Angstrom) of the sodium atom with respect to the center of the silicon cage. This displacement is higher than the one predicted by a simple Jahn-Teller effect and is discussed in terms of sodium pairing. The formation of dimers with covalent bonding is compared to the Peierls distortion in a monodimensional network.