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 t he electronic structure and the material's properties. A simple phenomenolo gical rule is currently used to predict the evolution of covalence/ ionicit y in mixed oxides compared to the parent ones, and is also widely used to i nterpret the x-ray photoelectron spectroscopy (XPS) binding-energy shifts o f the cations in terms of charge transfer. We test the validity of this sim ple rule and its application to XPS of mixed oxides with a prototypical sys tem: 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 agreemen t 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 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 o f charge transfer and electrostatic effects (initial state), as well as ext ra-atomic relaxation effects (final state) evaluated. The validity of the p henomenological 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 s cale of covalence in silicates is discussed.