On the initial stage of reduction of hexagonal tungsten trioxide, h-WO 3 , the nonstoichiometric h-WO 2.8 phase was synthesized. The X-ray powder diffraction analysis and subsequent refinement using the Rietveld Full Profile Matching & Integrated Intensities Refinement of X-ray and/or Neutron Data Programs (FullProf Version 3.5 Dec97-LLB-JRC) indicate that the crystal structure of the h-WO 2.8 phase is ascribed to the UO 3 structure type with lattice parameters a ) 0.3625 nm and c ) 0.3780 nm. The X-ray photoelectron spectroscopy (XPS) method was applied to study the electronic structure of the nonstoichiometric tungsten trioxide. The XPS valence band and core-level spectra of the h-WO 2.8 phase and, for comparison, h-WO 3 were derived. The formation of an additional near-Fermi subband, which is absent on the XPS valence-band spectrum of hexagonal tungsten trioxide, was observed on the spectrum of the nonstoichiometric hexagonal WO 2.8 compound. Half-widths of the XPS valence band as well as both W 4f and O 1s core-level spectra increase somewhat when going from h-WO 3 to h-WO 2.8 .

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