Role of Ag doping in Ba8Si46 compounds

The silicon clathrate compound ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ shows superconductivity below the critical temperature $({T}_{c})$ of $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the ${T}_{c}$ 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 ${\mathrm{Ba}}_{8}{\mathrm{Ag}}_{x}{\mathrm{Si}}_{46\ensuremath{-}x}$ $(x=0,1,3,6)$. The valence band photoemission spectra show that a Ba $5d$-derived state at the Fermi level $({E}_{F})$, which is prominently observed in ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$, decreases with increasing Ag content. The reduction in the peak intensity at ${E}_{F}$ with increasing Ag content is therefore in accord with the decrease of ${T}_{c}$ in ${\mathrm{Ba}}_{8}{\mathrm{Ag}}_{x}{\mathrm{Si}}_{46\ensuremath{-}x}$. 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 ${\mathrm{Si}}_{24}$ cage is further donated to Ag in $x\ensuremath{\geqslant}3$. Hence, Ag doping leads to the reduction of the peak at ${E}_{F}$.