On the involvement of the shallow core 5d level in the bonding in HgO

Oxygen K shell X-ray emission spectroscopy has been used to demonstrate the covalent involvement of shallow core 5d electrons in the bonding in HgO. The extent of core hybridisation with the O-2p levels is much more pronounced in HgO than in ZnO or CdO and shows an inverse correlation with the atomic binding energy of the shallow core state. Band structure calculations confirm the importance of mixing between Hg-5d and O-2p states in HgO. However, the assumption that direct intra-atomic mixing between Hg-6s and Hg-5d orbitals determines the linear stereochemistry in HgO is shown to be incorrect. © 2004 Elsevier B.V. All rights reserved

The band structure of WO3 and non-rigid-band behaviour in Na0.67WO3 derived from soft x-ray spectroscopy and density functional theory.

The electronic structure of single-crystal WO3 and Na0.67WO3 (a sodium-tungsten bronze) has been measured using soft x-ray absorption and resonant soft x-ray emission oxygen K-edge spectroscopies. The spectral features show clear differences in energy and intensity between WO3 and Na0.67WO3. The x-ray emission spectrum of metallic Na0.67WO3 terminates in a distinct Fermi edge. The rigid-band model fails to explain the electronic structure of Na0.67WO3 in terms of a simple addition of electrons to the conduction band of WO3. Instead, Na bonding and Na 3s-O 2p hybridization need to be considered for the sodium-tungsten bronze, along with occupation of the bottom of the conduction band. Furthermore, the anisotropy in the band structure of monoclinic γ-WO3 revealed by the experimental spectra with orbital-resolved geometry is explained via density functional theory calculations. For γ-WO3 itself, good agreement is found between the experimental O K-edge spectra and the theoretical partial density of states of O 2p orbitals. Indirect and direct bandgaps of insulating WO3 are determined from extrapolating separations between spectral leading edges and accounting for the core-hole energy shift in the absorption process. The O 2p non-bonding states show upward band dispersion as a function of incident photon energy for both compounds, which is explained using the calculated band structure and experimental geometry