Doping dependence of the electronic structure of Ba_1-xK_xBiO₃ studied by x-ray-absorption spectroscopy

We have performed x-ray-absorption spectroscopy (XAS) and x-ray-photoemission spectroscopy (XPS) studies of single crystal Ba_1-xK_xBiO₃ (BKBO) covering the whole composition range 0<∼x<∼0.60. Several features in the oxygen 1s core XAS spectra show systematic changes with x. Spectral weight around the absorption threshold increases with hole doping and shows a finite jump between x=0.30 and 0.40, which signals the metal-insulator transition. We have compared the obtained results with band-structure calculations. Comparison with the XAS results of BaPb_1-xBi_xO₃ has revealed quite different doping dependences between BKBO and BPBO. We have also observed systematic core-level shifts in the XPS spectra as well as in the XAS threshold as functions of x, which can be attributed to a chemical potential shift accompanying the hole doping. The observed chemical potential shift is found to be slower than that predicted by the rigid band model based on the band-structure calculations.