Non-equilibrium oxidation states of zirconium during early stages of metal oxidation

The chemical state of Zr during the initial, self-limiting stage of oxidation on single crystal zirconium (0001), with oxide thickness on the order of 1 nm, was probed by synchrotron x-ray photoelectron spectroscopy. Quantitative analysis of the Zr 3d spectrum by the spectrum reconstruction method demonstrated the formation of Zr¹⁺, Zr²⁺, and Zr³⁺ as non-equilibrium oxidation states, in addition to Zr⁴⁺ in the stoichiometric ZrO₂. This finding resolves the long-debated question of whether it is possible to form any valence states between Zr⁰ and Zr⁴⁺ at the metal-oxide interface. The presence of local strong electric fields and the minimization of interfacial energy are assessed and demonstrated as mechanisms that can drive the formation of these non-equilibrium valence states of Zr.