Electronic structure of high-$T_c$ ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ by x-ray photoelectron spectroscopy

We have investigated the electronic structure of ${\mathrm{Ba}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{K}}_{\mathrm{x}}$ ${\mathrm{BiO}}_3$ (0<x<0.4) by x-ray photoelectron spectroscopy. The Bi(4f) core levels show a normal valency of +3 as in ${\mathrm{Bi}}_2$ ${\mathrm{O}}_3$ and we find no evidence for the disproportionation of ${\mathrm{Bi}}^{4+}$ to ${\mathrm{Bi}}^{3+}$ and ${\mathrm{Bi}}^{5+}$. Instead, we find that ${\mathrm{BaBiO}}_3$ has mixed valent ${\mathrm{O}}^{2\mathrm{-}}$ and ${\mathrm{O}}^{1\mathrm{-}}$ ions. As the potassium doping is increased, the binding energy of the ${\mathrm{O}}^{2\mathrm{-}}$ ions in the O(1s) photoelectron spectra steadily decreases from 529.7 to 528.8 eV. The effect of lowering the O(1s) binding energy is to raise the O(2p) band towards $E_F$ and at the superconducting composition a finite density of O(2p) states is observed near $E_F$. Similarly, the Ba(5p) binding energies decrease with potassium doping, indicating increased metallicity. The behavior of the O(1s), Ba(5p), and the valence band resembles that of all the cuprate superconductors and we conclude that in all these oxide superconductors, a hole in the (filled) O(2p) band is the carrier responsible for superconductivity, which predicts hole conduction in the Ba-K-Bi-O and Ba-Pb-Bi-O systems.