Experimental and theoretical study of the electronic structure of HgO andTl2O3

Abstract

The electronic structures of HgO and ${\mathrm{Tl}}_{2}{\mathrm{O}}_{3}$ have been investigated by valence and core-level x-ray photoemission, x-ray absorption, and x-ray emission spectroscopies. Valence-band photoemission under Al $K\ensuremath{\alpha}$ excitation is dominated by the metal $5d$ partial density of states and thus provides a sensitive probe of shallow core mixing into the O $2p$ valence-band states. Conversely O $K$ shell emission is determined by the O $2p$ partial density of states and therefore allows the extent of corresponding mixing of O $2p$ character into the shallow core states to be measured. The experimental work is supported by band-structure calculations carried out within the framework of density-functional theory. It is shown that the bonding in HgO involves significant mixing between O $2p$ states and both Hg $6s$ and shallow core $5d$ states: the calculated O $2p$ partial density of states mirrors the O $K$ shell emission spectrum and reveals significant O $2p$ character within the shallow core Hg $5d$ states. There is, however, little direct on-site mixing between the Hg $6s$ and $5d$ states. In ${\mathrm{Tl}}_{2}{\mathrm{O}}_{3}$, the hybridization of the deeper metal $5d$ states with O $2p$ states is much less pronounced than in HgO. Moreover, the states at the bottom of what is conventionally regarded as the O $2p$ valence band are found in fact to have very strong Tl $6s$ atomic character. The photoemission spectrum of ${\mathrm{Tl}}_{2}{\mathrm{O}}_{3}$ shows a well-defined metallic Fermi edge: the shape of the structure around the photoemission onset suggests that the metallic nature of ${\mathrm{Tl}}_{2}{\mathrm{O}}_{3}$ arises from an occupation of states above the main valence-band edge, probably arising from oxygen vacancy defects. The conduction electrons of ${\mathrm{Tl}}_{2}{\mathrm{O}}_{3}$ are strongly perturbed by ionization of Tl core levels, giving rise to distinctive plasmon satellites in core x-ray photoemission spectroscopy.