Predicting point defect equilibria across oxide hetero-interfaces: model system of ZrO₂/Cr₂O₃

Abstract

We present a multi-scale approach to predict equilibrium defect concentrations across oxide/oxide hetero-interfaces. There are three factors that need to be taken into account simultaneously for computing defect redistribution around the hetero-interfaces: the variation of local bonding environment at the interface as epitomized in defect segregation energies, the band offset at the interface, and the equilibration of the chemical potentials of species and electrons via ionic and electronic drift-diffusion fluxes. By including these three factors from the level of first principles calculation, we build a continuum model for defect redistribution by concurrent solution of Poisson's equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. This model solves for and preserves the continuity of the electric displacement field throughout the interfacial core zone and the extended space charge zones. We implement this computational framework to a model hetero-interface between the monoclinic zirconium oxide, m-ZrO, and the chromium oxide CrO. This interface forms upon the oxidation of zirconium alloys containing chromium secondary phase particles. The model explains the beneficial effect of the oxidized Cr particles on the corrosion and hydrogen resistance of Zr alloys. Under oxygen rich conditions, the ZrO/CrO heterojunction depletes the oxygen vacancies and the sum of electrons and holes in the extended space charge zone in ZrO. This reduces the transport of oxygen and electrons thorough ZrO and slows down the metal oxidation rate. The enrichment of free electrons in the space charge zone is expected to decrease the hydrogen uptake through ZrO. Moreover, our analysis provides a clear anatomy of the components of interfacial electric properties; a zero-Kelvin defect-free contribution and a finite temperature defect contribution. The thorough analytical and numerical treatment presented here quantifies the rich coupling between defect chemistry, thermodynamics and electrostatics which can be used to design and control oxide hetero-interfaces.

Oxygen self-diffusion mechanisms in monoclinic ZrO2 revealed and quantified by density functional theory, random walk analysis, and kinetic Monte Carlo calculations

“…Third, none of these studies considers the contribution to oxygen diffusion under an electric field in an intrinsically formed space-charge layer at the oxide surface. Our study shows that the major contribution to the diffusion coefficient comes from charged defects, and the space-charge effect should be taken into consideration [32].…”

“…However, if a high concentration of extrinsic doping or a higher temperature and lower oxygen pressure regime is of interest, it will be important to include defect-defect interactions. Second, extended defects in the oxide structure can significantly change its transport properties, as we have demonstrated recently, for example, for secondary phase particles embedded in a ZrO 2 matrix [32]. The effects of surface, grain boundaries, and interfaces on oxygen diffusion are also worth exploring, and this is necessary for constructing accurate higher-level transport models as well as informing microstructure engineering for oxide materials.…”

“…The energies of each image were calculated using density functional theory (DFT) with Vienna Ab initio Simulation Package (VASP) [26][27][28][29] with a 2 × 2 × 2 supercell and a 2 × 2 × 2 k-point grid. The generalized gradient approximation (GGA) with a Perdew-Burke-Ernzerhof (PBE) functional [30,31] [24,16,32].…”

Anomalous diffusion along metal/ceramic interfaces

“…One of the earliest works that looked at the point defect formation energy near a metal/ceramic interface was carried out by Duffy et al 28 using a combination of first-principles and classical methods for Ag/MgO. More recently, there have been studies of oxygen vacancy formation energy 29 , 30 and oxygen Frenkel pair 31 formation energies at metal/oxide interfaces and oxide/oxide interfaces 32 , 33 . However, most such studies of metal/ceramic interfaces focused on the oxide side of the interface and on the electronic behavior of the contact rather than diffusion—the primary focus of the present work.…”

Polar or not polar? The interplay between reconstruction, Sr enrichment, and reduction at the La0.75Sr0.25MnO3 (001) surface

“…They support a high concentration of oxygen vacancies (under reducing conditions) or cation vacancies (under oxidizing conditions). Under SOFC operating conditions (high temperature, cathodic potential) the surface is assumed to be enriched in oxygen vacancies [24][25][26]51] with positive formal charge, as it has also been observed at surfaces and interfaces of binary oxides, such as doped CeO 2 [52][53][54][55][56][57] and ZrO 2 [53,58]. Accumulation and ordering of such defects have been shown to induce surface reconstructions in TiO 2 (110) [59].…”