Experimental and theoretical study of the electronic structure of HgO and Tl2O3

The electronic structures of HgO and Tl2O3 have been investigated by valence and core-level x-ray photoemission, x-ray absorption, and x-ray emission spectroscopies. Valence-band photoemission under Al Kα 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 Tl2O3, 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 Tl2O3 shows a well-defined metallic Fermi edge: the shape of the structure around the photoemission onset suggests that the metallic nature of Tl2O3 arises from an occupation of states above the main valence-band edge, probably arising from oxygen vacancy defects. The conduction electrons of Tl2O3 are strongly perturbed by ionization of Tl core levels, giving rise to distinctive plasmon satellites in core x-ray photoemission spectroscopy.