Electronic structure reconstruction by trimer formation in CsW2 O6 studied by x-ray photoelectron spectroscopy

We have studied the electronic structure of CsW2O6 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 4f7/2 and 4f5/2 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.