Electronic structure of hexagonal tungsten trioxide: XPS, XES, and XAS studies

X-Ray photoelectron (XPS), emission (XES) and absorption (XAS) spectroscopy methods were used to study the electronic structure of hexagonal tungsten trioxide, h-WO3. Its precursor, hexagonal hydrogen tungsten bronze, HxWO3, and the monoclinic form of tungsten trioxide, m-WO3, were also studied. For the mentioned compounds, both the XPS valence-band and core-level spectra, as well as the O Kα emission bands and the W LIII absorption edges were derived. It was established that, binding energies of both the W 4f and O 1s core-level electrons do not change when going from the tungsten trioxides to HxWO3. A high-energy shift of the inflection point of the XAS W LIII spectrum of the HxWO3 bronze, with respect to its position on the spectrum of pure metallic tungsten, was found to be close to those of the spectra of the two WO3 forms studied. Half-widths of both the XPS valence-band spectra and the O Kα bands increase somewhat in the sequence m-WO3→h-WO3→HxWO3. The formation of a near-Fermi sub-band, which is absent for both the modifications of WO3, was observed on the XPS valence-band spectrum of hexagonal HxWO3. The energy positions of the centers of gravity of the O Kα band remain constant for all the compounds studied.