Electronic structure reconstruction by trimer formation in CsW2O6 studied by x-ray photoelectron spectroscopy

We have studied the electronic structure of $\mathrm{Cs}{\mathrm{W}}_{2}{\mathrm{O}}_{6}$ across its metal-insulator transition by means of hard/soft x-ray photoelectron spectroscopy. In the high-temperature metallic phase, the W $5d$ band exhibits a clear Fermi edge. The W $4{f}_{7/2}$ and $4{f}_{5/2}$ core-level peaks are accompanied by shoulders on their lower binding energy side. The shoulder and main peaks, respectively, are attributed to the well- and poorly screened final states where the screening is due to the W $5d$ electronic states in the vicinity of the chemical potential. In going from 300 to 180 K (across the metal-insulator transition), a band gap of about 0.2 eV is created at the Fermi level. The origin of the band gap can be assigned to the trimerization of the W sites. The W $4f$ and $5d$ spectra exclude the possibility of charge disproportionation and suggest moderate electronic correlation effects.