First Principles Analysis of the Stability and Diffusion of Oxygen Vacancies in Metal Oxides (original) (raw)
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Rigorous characterization of oxygen vacancies in ionic oxides
Physical Review B, 2002
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.
Intrinsic Material Properties Dictating Oxygen Vacancy Formation Energetics in Metal Oxides
Oxygen vacancies (V O ) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E V ). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH f ), (ii) the midgap energy relative to the O 2p band center (E O 2p + (1/2)E g ), and (iii) atomic electronegativities reproduces calculated E V within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E V and a direct method to predict E V . We then predict the E V of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.
Despite the fundamental role oxygen vacancy formation energies play in a broad range of important energy applications, their relationships with the intrinsic bulk properties of solid oxides remain elusive. Our study of oxygen vacancy formation in La 1Àx Sr x BO 3 perovskites (B]Cr, Mn, Fe, Co, and Ni) conducted using modern, electronic structure theory and solid-state defect models demonstrates that a combination of two fundamental and intrinsic materials properties, the oxide enthalpy of formation and the minimum band gap energy, accurately correlate with oxygen vacancy formation energies. The energy to form a single, neutral oxygen vacancy decreases with both the oxide enthalpy of formation and the band gap energy in agreement with the relation of the former to metal-oxygen bond strengths and of the latter to the energy of the oxygen vacancy electron density redistribution. These findings extend our understanding of the nature of oxygen vacancy formation in complex oxides and provide a fundamental method for predicting oxygen vacancy formation energies using purely intrinsic bulk properties.
First-principles study of the structure and stability of oxygen defects in zinc oxide
Physical Review B, 2005
A comparative study on the structure and stability of oxygen defects in ZnO is presented. By means of first-principles calculations based on local density functional theory we investigate the oxygen vacancy and different interstitial configurations of oxygen in various charge states. Our results reveal that dumbbell-like structures are thermodynamically the most stable interstitial configurations for neutral and positive charge states due to the formation of a strongly covalent oxygen-oxygen bond. For negative charge states the system prefers a split-interstitial configuration with two oxygen atoms in almost symmetric positions with respect to the associated perfect lattice site. The calculated defect formation energies imply that interstitial oxygen atoms may provide both donor-and acceptor-like defects.
Electronic structure of oxygen vacancy in crystalline InGaO3(ZnO)m
Physica B: Condensed Matter, 2009
We perform first-principles theoretical calculations to investigate the defect properties of oxygen vacancy (V O) in crystalline InGaO 3 (ZnO) m (m ¼ 3). In a flat boundary structure, in which Ga atoms are located on a single plane, various configurations of V O exist. We find that neutral V O at a site near the In-O layer or in the ZnO area is energetically more favorable than those formed near and on the Ga-O layer. Although the defect levels vary with the type of metal ions in the neighborhood, V O defects act as deep donors, similar to that of bulk ZnO. Moreover, the O-vacancies exhibit the negative-U behavior, with the charge transition levels well below the conduction band minimum.
The Journal of Physical Chemistry C, 2013
Using a hybrid Hartree−Fock (HF)-DFT method combined with LCAO basis set and periodic supercell approach, the atomic, electronic structure and phonon properties of oxygen vacancies in ZnO and SrTiO 3 were calculated and compared. The important role of a ghost basis function centered at the vacant site and defect spin state for SrTiO 3 is discussed. It is shown that the use of hybrid functionals is vital for correct reproduction of defects basic properties. The Gibbs free energy of formation of oxygen vacancies and their considerable temperature dependence has been compared for the two oxides. These calculations were based on the polarizability model for the soft mode temperature behavior in SrTiO 3 . The supercell size effects in the Gibbs free energy of formation of oxygen vacancies in the two oxides are discussed. The major factors for the quite different behavior of the two oxides and the degree of electron delocalization nearby the oxygen vacancy have been identified.
$\ textit {Ab-initio} $ many-body calculations of the oxygen vacancy in ZnO
We have applied the many-body ab-initio diffusion quantum monte carlo (DMC) method to calculate the band gap of ZnO and to study the oxygen vacancy in this material. DMC calculations clearly rule out the oxygen vacancy as the source of the persistent n-type conductivity in ZnO. The DMC results were compared with Hartree-Fock, the Heyd-Scuseria-Ernzefhof (HSE) hybrid functional and other approximations of density functional theory (DFT). DMC predicts the band gap at 3.43(9) eV. DMC and HSE show that the thermodynamic transition levels of the oxygen vacancy are deep, between 1.8 and 2.5 eV from the valence-band maximum. The oxygen vacancy is unlikely to be the source of the persistent n-type conductivity in ZnO, confirming previous DFT calculations. Despite this agreement between DMC and HSE, we found two major differences: (i) the oxygen vacancy formation energy is about 1 eV higher in DMC than in HSE and other DFT approximations and (ii) DMC predicts a positive U behavior for the oxygen vacancy while HSE and other DFT approximations predict the opposite. These results are discussed in conjunction with recent experiments.
VOH and VOD centers in alkaline-earth oxides: An ab initio supercell study
Physical Review B, 2000
The V OH and V OD centers in MgO, CaO, and SrO have been investigated at the ab initio quantummechanical level, by using the CRYSTAL98 periodic program, the Hartree-Fock Hamiltonian, the supercell scheme, and a localized basis set. The defects, formally obtained by substituting a hydrogen atom for a cation, exhibit a large relaxation of H from the perfect lattice position towards one of the oxygens, and the formation of a strong O-H covalent bond, as documented by the calculated bond populations and O-H vibrational frequencies, which are in good agreement with infrared frequencies. The unpaired electron is fully localized on the oxygen opposite to H with respect to the M (M ϭMg, Ca, Sr͒ vacancy. The first evaluations of the Fermi contact, the anisotropic component of the hyperfine coupling tensor, and the electric-field gradient at the H atom are reported and compared successfully with electron paramagnetic resonance and electron-nuclear double resonance experimental data. The defect is also characterized in terms of charge-and spin-density maps, band structures, defect formation, and relaxation energies. The stability of the results as a function of the supercell size is documented.
Journal of the American Ceramic Society, 2005
The first-principles calculation of finite-temperature phase stability and thermodynamic properties of multicomponent oxides presents a significant challenge. The time scale on which substitutional disorder occurs prevents the use of standard simulation methods, and a correct description of entropic effects requires that excitation energies can be calculated accurately on the scale of k B T. A model is presented in which substitutional disorder is parameterized with a cluster expansion. The thermodynamics of this model can be easily obtained with lattice model statistical mechanics. The only input required to the procedure is a description of bonding in the system, which is used to calculate the energy of ordered ionic configurations. This method is applied to the CaO-MgO, Gd 2 O 3-ZrO 2 , CaO-ZrO 2 systems, and to Li x CoO 2 (x between 0 and 1) electrodes for rechargeable lithium batteries. In almost all cases, a correct description of the charge state of the ions is essential to obtain the proper mixing behavior. Only for a highly ionic material such as CaO-MgO does the charge state of the ions remain unvaried upon mixing. We find that approximate energy models that employ fixed charges will tend to overestimate the energy required for mixing, hence the order-disorder transition temperature. I.-O. Chen-contributing editor Manuscript No. 190965.