Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

Defects at transition metal (TM) and rare earth (RE) oxide surfaces, neutral oxygen vacancies in particular, play a major role in a variety of technological applications. This is the motivation of numerous studies of partially reduced oxide surfaces. We review, discuss, and compare theoretical data for structural and electronic properties and energetic quantities related to the formation of oxygen defects at TM and RE oxide surfaces using TiO2, ZrO2, V 2O5, and CeO2 as examples. Bulk defects, as far as relevant for comparison with the properties of reduced surfaces, are briefly reviewed. Special attention is given to the fate of the electrons left in the system upon vacancy formation and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the electronic structure of the partially reduced oxide systems.