Bonding and XPS chemical shifts in ${\mathrm{ZrSiO}}_4$ versus ${\mathrm{SiO}}_2$ and ${\mathrm{ZrO}}_2:$ Charge transfer and electrostatic effects

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 ${\mathrm{ZrSiO}}_4$ and parent oxides ${\mathrm{ZrO}}_2$ and ${\mathrm{SiO}}_2.$ 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 ${\mathrm{ZrSiO}}_4$ than in ${\mathrm{ZrO}}_2,$ while the more covalent one (Si) experiences a corresponding increase in covalence with respect to ${\mathrm{SiO}}_2.$ 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 $\mathrm{O}1s$ binding-energy shifts to derive a scale of covalence in silicates is discussed.