Oxygen vacancy ordering and electron localization in reduced bulk CeO2 and its (111) surface

Trabajo presentado en el Meeting COST Action CM1104-Reducible oxide chemistry, structure and functions, celebrado en Upsala (Suecia) del 6 al 8 de noviembre de 2013.Ceria (CeO2) plays an important role in many technological applications such as catalysis and spintronics. The importance relies on its facile reducibility and the associated ability to release lattice oxygen, as well as its magnetic properties while doped with magnetic impurities and its good integrability to the present electronic devices. A particularly important aspect for the design of improved ceria-based materials is the fundamental knowledge of the preferred location for oxygen vacancies and the electrons left upon their formation, driving the Ce4+ → Ce3+ reduction. Yet, whether vacancies agglomerate or repel each other in the bulk and at the CeO2(111) surface, and whether vacancies are at surface or subsurface sites, is still under discussion. Moreover, for the (111) surface, it has been theoretically predicted [1] and experimentally confirmed [2] that near-surface vacancies are likely to be bound to Ce4+ ions rather than to Ce3+ as priorly predicted. In spite of this, atomistic models of reduced bulk CeO2 with the excess electrons localized on Ce ions which are nearest-neighbor to the vacancy are still being discussed [3,4], though similar predictions to the near-surface vacancies have recently been made for isolated bulk vacancies [5]. To this end, we apply density-functional theory (DFT) with the DFT+U approach to study reduced Co-doped [6] and undoped [7] bulk CeO2 and the CeO2(111) surface for varying vacancy concentrations[8]. We find that, in all cases, the preference for Ce3+ ions in sites not adjacent to the vacancy remains for all types of vacancy aggregates and concentrations, and that the interaction between vacancies is repulsive. A simple model which takes into account the vacancyvacancy distance and the Ce3+ coordination is proposed to predict the preferred ordering of the excess charge and vacancies in CeO2 bulk. This model could explain the way the Ce2O3 in the bixbyite phase forms by removing 1/4 of the oxygen atoms being second nearest-neighbor in the oxygen sublattice. On the other hand, in the CeO2(111) surface, it is shown that the most stable vacancy structure −under a wide range of reducing conditions− has all vacancies in the subsurface forming a (2×2) pattern with vacancies being third nearest-neighbor in the oxygen plane as suggested experimentally [9]. Vacancy-induced lattice relaxations effects are crucial for the interpretation of the findings.Peer Reviewe