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High-resolution photoemission and first-principles density-functional calculations were used to study the interaction of oxygen with ZrC(001) and VC(001) surfaces. Atomic oxygen is present on the carbide substrates after small doses of O 2 at room temperature. At 500 K , the oxidation of the surfaces is fast and clear features for Zr O x or V O x are seen in the O ( 1 s ) , Zr ( 3 d ) , and V ( 2 p 3 ∕ 2 ) core levels spectra, with an increase in the metal/carbon ratio of the samples. A big positive shift ( 1.3 – 1.6 eV ) was detected for the C 1 s core level in O ∕ Zr C ( 001 ) , indicating the existence of strong O ↔ C or C ↔ C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CZrZr hollow as the most stable site for the adsorption of O. Furthermore, the calculations also show that a C ↔ O exchange is exothermic on ZrC(001), and the displaced C atoms bond to CZrZr sites. In the O ∕ Zr C ( 001 ) interface, the surface C atoms play a major role in determining the behavior of the system. In contrast, the adsorption of oxygen induces very minor changes in the C ( 1 s ) spectrum of VC(001). The O ↔ V interactions are stronger than the O ↔ Zr interactions, and O ↔ C interactions do not play a dominant role in the O ∕ V C ( 001 ) interface. In this system, C ↔ O exchange is endothermic. VC(001) has a larger density of metal d states near the Fermi level than ZrC(001), but the rate of oxidation of VC(001) is slower. Therefore the O ∕ Zr C ( 001 ) and O ∕ V C ( 001 ) systems illustrate two different types of pathways for the oxidation of carbide surfaces.

has been addressed with quantum chemicalcalculations on large embedded clusters. We find a charge ordered state for the crystal structure reported byRadaelli et al. [Phys. Rev. B 55, 3015 (1997)] and Zener polaron formation in the crystal structure withequivalent Mn sites proposed by Daoud-Aladineet al. [Phys. Rev. Lett. 89, 097205 (2002)]. Important O toMn charge transfer effects are observed for the Zener polaron.DOI: 10.1103/PhysRevB.70.235104 PACS number(s): 75.47.Lx, 71.15.Ap, 71.27.1aI. INTRODUCTION

Charged and neutral oxygen vacancies in the bulk and on perfect and defective surfaces of MgO are characterized as quantum-mechanical subsystems chemically bonded to the host lattice and containing most of the charge left by the removed oxygens. Attractors of the electron density appear inside the vacancy, a necessary condition for the existence of a subsystem according to the atoms in molecules theory. The analysis of the electron localization function also shows attractors at the vacancy sites, which are associated to a localization basin shared with the valence domain of the nearest oxygens. This polyatomic superanion exhibits chemical trends guided by the formal charge and the coordination of the vacancy. The topological approach is shown to be essential to understand and predict the nature and chemical reactivity of these objects. There is not a vacancy but a coreless pseudoanion that behaves as an activated host oxygen.

The nature of the chemical bond in three titanium oxides of different crystal structure and different formal oxidation state has been studied by means of the ab initio cluster-model approach. The covalent and ionic contributions to the bond have been measured from different theoretical techniques. All the analysis is consistent with an increasing of covalence in the TiO, ${\mathrm{Ti}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{TiO}}_{2}$ series as expected from chemical intuition. Moreover, the use of the ab initio cluster-model approach combined with different theoretical techniques has permitted us to quantify the degree of ionic character, showing that while TiO can approximately be described as an ionic compound, ${\mathrm{TiO}}_{2}$ is better viewed as a rather covalent oxide.