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 TiO"2 and HfO"2. Based on soft X-ray photoelectron band edge spectra, and the occurrence of occupied band edge 4f states in Gd(Sc,Ti)O"3, complementary spectroscopic features have been confirmed in the pre-edge ( 545eV) regimes of O K edge spectra. Qualitatively similar spectral features have been obtained for thin films of HfO"2 and TiO"2, 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 d^2 state for a Ti monovacancy and a d^4 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.