Role of Ag doping in Ba8Si46 compounds

The silicon clathrate compound Ba8Si46 shows superconductivity below the critical temperature (Tc) of 8K, and the Tc decreases monotonically with doping Ag. In order to reveal effects of Ag doping on the electronic states, we have applied soft x-ray photoemission spectroscopy to Ag-doped silicon clathrate compounds Ba8AgxSi46-x (x=0,1,3,6). The valence band photoemission spectra show that a Ba 5d-derived state at the Fermi level (EF), which is prominently observed in Ba8Si46, decreases with increasing Ag content. The reduction in the peak intensity at EF with increasing Ag content is therefore in accord with the decrease of Tc in Ba8AgxSi46-x. Band structure calculation using local-density approximation reproduces the observed valence band spectra of x=0 and 6. The Si 2p and Ba 4d core-level photoemission spectra demonstrate that the valence electron of Si is attracted to the Ag site in x=1 and the 5d electron of Ba inside the Si24 cage is further donated to Ag in x≥3. Hence, Ag doping leads to the reduction of the peak at EF.