ELECTRONIC STRUCTURE OF HEXAGONAL AND ORTHORHOMBIC MODIFICATIONS OF TUNGSTEN TRIOXIDE

Abstract

The electronic structure of hexagonal and orthorhombic modifications of WO₃ (h-WO₃ and o-WO₃, respectively) has been studied from both theoretical and experimental perspectives. In particular, first-principles calculations of the band structure were performed for both forms of tungsten trioxide. It has been established that both WO₃ modifications are semiconductors with an energy band gap of 0.65 eV for the hexagonal modification and 0.49 eV for the orthorhombic one. In the orthorhombic structure, the energy gap is "quasi-direct" and is formed between k-points with coordinates (0 0 0) and (0 0 0.025), whereas the hexagonal WO₃ structure is a true semiconductor with an indirect bandgap, with the minimum of the conduction band and the maximum of the valence band located at k-points with coordinates (0 0 0) and (0.056 0 0.5), respectively. The main contribution to the valence band of the studied tungsten trioxide modifications comes from O2p states, predominantly in the upper part of the band, whereas the contribution from the W5d states is observed mainly in the lower and central parts of the band and is significantly smaller than that from the O2p states. The theoretical results are consistent with experimental studies of the hexagonal WO₃ modification, for which the valence band spectrum obtained by X-ray photoelectron spectroscopy and the X-ray band, reflecting the energy distributions of the W5d and O2p states, were measured and plotted on a common energy scale. Electronic states of W6s and W6p symmetry make a significantly smaller contribution to the valence band of the studied tungsten trioxide modifications compared to W5d states. Electronic states of O2s symmetry are not detected in the valence band of h-WO₃ and o-WO₃; however, these states form a rather intense band in the energy range of approximately –16.5 to –19.0 eV in the studied tungsten trioxide modifications. In the lower part of the conduction band of h-WO₃ and o-WO₃, unfilled W5d electronic states predominate, while the contribution from unfilled O2p states is somewhat smaller. The studied modifications of tungsten trioxide are characterized by hybridization of the O2p and W5d states, and the valence band width increases by 0.8 eV upon the transition from h-WO₃ to o-WO₃.