Spectroscopic differentiation between O-atom vacancy and divacancy defects, respectively, in TiO₂ and HfO₂ by X-ray absorption spectroscopy

Defect state features have been detected in second derivative O K edge spectra for thin films of nano-crystalline TiO₂ and HfO₂. Based on soft X-ray photoelectron band edge spectra, and the occurrence of occupied band edge 4f states in Gd(Sc,Ti)O₃, complementary spectroscopic features have been confirmed in the pre-edge (<530 eV) and vacuum continuum (>545 eV) regimes of O K edge spectra. Qualitatively similar spectral features have been obtained for thin films of HfO₂ and TiO₂, and these have been assigned to defect states associated with vacancies. The two electrons/removed O-atom are not distributed uniformly over the TM atoms defining the vacancy geometry, but instead are localized in equivalent d-states: a d² state for a Ti monovacancy and a d⁴ state for a Hf divacancy. This new model for electronic structure provides an unambiguous way to differentiate between monovacancy and divacancy arrangements, as well as immobile (or fixed) and mobile vacancies.