Bonding and XPS chemical shifts in ZrSiO4 versus SiO2 and ZrO2:: Charge transfer and electrostatic effects -: art. no. 125117

The degree of ionic/covalent character in oxides has a great influence on the electronic structure and the material's properties. A simple phenomenological rule is currently used to predict the evolution of covalence/ ionicity in mixed oxides compared to the parent ones, and is also widely used to interpret the x-ray photoelectron spectroscopy (XPS) binding-energy shifts of the cations in terms of charge transfer. We test the validity of this simple rule and its application to XPS of mixed oxides with a prototypical system: zircon ZrSiO4 and parent oxides ZrO2 and SiO2. The ionic charges on Si, Zr, and O were extracted from the density functional theory in the local density approximation calculations in the plane-wave formalism. In agreement with the predictions of the phenomenological rule, the most ionic cation (Zr) becomes more ionic in ZrSiO4 than in ZrO2, while the more covalent one (Si) experiences a corresponding increase in covalence with respect to SiO2. The XPS chemical shifts of the O 1s, Si 2p, and Zr 3d(5/2) photoelectron lines in the three oxides were measured and the respective contributions of charge transfer and electrostatic effects (initial state), as well as extra-atomic relaxation effects (final state) evaluated. The validity of the phenomenological rule of mixed oxides used in x-ray electron spectroscopy as well as the opportunity to use the Ols binding-energy shifts to derive a scale of covalence in silicates is discussed.