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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Spectroscopic properties of oxygen vacancies in LaAlO 3

Oxygen vacancies in LaAlO 3 (LAO) play an important role in the formation of the two-dimensional electron gas observed at the LaAlO 3 /SrTiO 3 interface and affect the performance of MOSFETs using LAO as a gate dielectric. However, their spectroscopic properties are still poorly understood, which hampers their experimental identification. Here we predict the absorption spectra and ESR parameters of oxygen vacancies in LAO using periodic and embedded cluster methods and density functional theory (DFT). The structure, charge distribution, and spectroscopic properties of the neutral (V 0 O) and charged (V + O and V 2+ O ) oxygen vacancies in cubic and rhombohedral LaAlO 3 are investigated. The highest intensity optical transitions [calculated using time-dependent DFT (TDDFT)], from the oxygen vacancy states to the conduction-band states have onsets at 3.5 and 4.2 eV for V 0 O and 3.6 eV for V + O in rhombohedral LAO and 3.3 and 4.0 eV for V 0 O and 3.4 eV for V + O in cubic LAO, respectively. Also reported are the isotropic g value (2.004026) and hyperfine coupling constants of V + O , which are compared to the experimental data obtained using electron spin resonance (ESR) spectroscopy, and accurately predict both the position and the width (3 mT) of its ESR signature. These results may further facilitate the experimental identification of oxygen vacancies in LAO and help to establish their role at the LAO/STO interfaces and in nanodevices using LAO.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Tuning the electronic structure of SrTiO{sub 3}/SrFeO{sub 3−x} superlattices via composition and vacancy control

Using density functional theory-based calculations, we explore the effects of oxygen vacancies and epitaxial layering on the atomic, magnetic, and electronic structure of (SrTiO{sub 3}){sub n}(SrFeO{sub 3−x}){sub 1} superlattices. While structures without oxygen vacancies (x = 0) possess small or non-existent band gaps and ferromagnetic ordering in their iron layers, those with large vacancy concentrations (x = 0.5) have much larger gaps and antiferromagnetic ordering. Though the computed gaps depend numerically on the delicate energetic balance of vacancy ordering and on the value of Hubbard U{sub eff} used in the calculations, we demonstrate that changes in layering can tune the band gaps of these superlattices below that of SrTiO{sub 3} (3.2 eV) by raising their valence band maxima. This suggests the possibility that these superlattices could absorb in the solar spectrum, and could serve as water-splitting photocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Creation And Ordering Of Oxygen Vacancies At WO3-d And Perovskite Interfaces

Changes in structure and composition resulting from oxygen deficiency can strongly impact the physical and chemical properties of transition metal oxides. However, it is extremely difficult to accurately characterize the oxygen deficiency levels and pinpoint how these materials properties change in response. In this paper, the effects of substrate strain and oxidizing power on the creation and distribution of oxygen defects in WO3 thin films are investigated in detail. An 18O2 isotope labeled time-of-flight secondary ion mass spectrometry (ToF-SIMS) study reveals that WO3-films grown on STO display a significantly larger oxygen vacancy gradient along the growth direction. This result is corroborated by scanning transmission electron microscopy (STEM) images which reveal a large number of defects present close to the interface to accommodate interfacial strain, leading to ordering of oxygen vacancies and formation of semi-aligned Magnéli phases. The strain is gradually released and tetragonal phase with much better crystallinity is observed at the film/vacuum interface. The changes in structure resulting from oxygen defect creation are shown to have a direct impact on the electronic and optical properties of the films.

Zhang, Kelvin H.↗
DOE OSTI2018

Iron Vacancies Accommodate Uranyl Incorporation into Hematite

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material depend on the local coordination environment of incorporated uranium. While measurements of uranium coordination have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate EXAFS spectra which were fitted to experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite lattice accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing lattice sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics over thermodynamics in determining the state of an impurity incorporated in a host phase under hydrothermal conditions.

McBriarty, Martin E.↗
DOE OSTI2018

Initial Reduction of the PdO(101) Surface: Role of Oxygen Vacancy Formation Kinetics

Here, we investigated the initial thermal reduction of the PdO(101) surface using density functional theory (DFT) calculations. The defect-free PdO(101) surface consists of parallel rows of undercoordinated Pd and O atoms (Pd cus and O cus , respectively). DFT was used to map out the elementary processes of O cus vacancy formation, oxygen surface atom diffusion along the Pd cus rows, and various O 2 formation pathways. Because oxygen vacancies form during the reduction of the surface, the elementary processes were examined with and without the presence of adjacent O cus vacancies. DFT calculations show that the presence of oxygen vacancies strongly affects the barriers of adjacent surface processes. Barriers for O vacancy formation are reduced by nearly 50% with an adjacent O vacancy. The barrier for O 2 formation along the Pd cus row drops from 1.54 eV on the defect-free PdO(101) surface to 0.81 eV in the presence of an O cus vacancy dimer. Yet, the presence of O cus vacancies also leads to more strongly bound O 2 on the Pd cus row. The strong neighbor effect of O vacancies on the PdO(101) surface will favor the growth of O vacancy chains and increase the rate of thermal reduction of the surface, both features that have been observed in recent ultrahigh vacuum isothermal reduction experiments of the PdO(101) surface.

Defects in solids↗
DOE OSTI2015

Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations

We study the chemical expansion for neutral and charged oxygen vacancies in fluorite, rocksalt, perovskite and pyrochlores materials using first principles calculations. We show that the neutral oxygen vacancy leads to lattice expansion whereas the charged vacancy leads to lattice contraction. In addition, we show that there is a window of strain within which an oxygen vacancy is stable; beyond that range, the vacancy can become unstable. Using CeO2|ZrO2 interface structure as an example, we show that the concentration of oxygen vacancies can be manipulated via strain, and the vacancies can be preferentially stabilized. These results could serve as guiding principles in predicting oxygen vacancy stability in strained systems and in the design of vacancy stabilized materials.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2021

Origin, nature, and the dynamic behavior of nanoscale vacancy clusters in Ni-rich layered oxide cathodes

Oxygen and transition metal (TM) vacancies in transition metal oxides (TMOs) play an important role in materials functionalities. For TMO cathodes of Li-ion batteries, vacancies can be introduced, either deliberately or inevitably, during materials synthesis or upon electrochemical cycling. Effects of vacancies on the electrochemical properties of cathodes critically depend on the dynamic characteristics of vacancies during the battery cycling. However, a fundamental understanding of such characteristics in the layer structured cathode remains elusive. Here, using scanning transmission electron microscopy (STEM), we reveal a cycling-induced aggregation behavior of oxygen and TM vacancies in a Ni-rich layer structured cathode of secondary particles. We discover that during the initial charging, vacancies aggregate to form visible vacancy-rich nanoregions (vacancy clusters) firstly at the outer layer of the secondary particle, and then the vacancy-cluster populated region extends to the inner part of particle upon fully charged. The spatial distinction of preferential nucleation of vacancy clusters at the outer layer indicates that the vacancy migration and clustering is associated with Li migration flux, which is larger at the outer layer, as all the Li from the inner part has to go through the outer layer. With extended cycling (> 50), these vacancy clusters become immobilized. We further reveal that the generation of these vacancy clusters is correlated to the material-synthesis conditions. Our findings solve a long-standing puzzle on the origin, nature, and behavior of the commonly visible vacancy clusters in the NMC cathode, providing insights on correlation between properties and dynamic behaviors of atomic-scale defects in layered oxide cathodes.

Li-ion batteries, layered oxide cathodes, NMC, cat↗
DOE OSTI2014

Understanding the presence of vacancy clusters in ZnO from a kinetic perspective

Vacancy clusters have been observed in ZnO, but their physical origin remains elusive. This is because under thermal equilibrium clusters typically have higher formation energies. Combining first-principles calculation and kinetic Monte Carlo (kMC) simulation, we determine the roles of non-equilibrium kinetics on the vacancy clustering. We find that clustering starts with the formation of Zn and O vacancy pairs (VZn-Vo), which further grow by attracting additional mono-vacancies. At this stage, vacancy diffusivity becomes crucial: due to the larger diffusivity of VZn compared to VO, more VZn-abundant clusters are formed than VO-abundant clusters. The large dissociation energies, e.g., 3 eV for (VZn-Vo), suggest that, once formed, it is difficult for the clusters to dissociate. As such, thermal annealing will increase the size of the clusters by promoting mono-vacancy diffusion, in agreement with experiment.

Ab initio calculations↗
DOE OSTI2017

Vacancies and Vacancy-Mediated Self Diffusion in Cr 2 O 3 : A First-Principles Study

Charged and neutral vacancies and vacancy mediated self diffusion in alpha-Cr2O3 were investigated using first principles density functional theory (DFT) and periodic supercell formalism. The vacancy formation energies of charged defects were calculated using the electrostatic finite-size corrections to account for electrostatic interactions between supercells and the corrections for the bandgap underestimation in DFT. Calculations predict that neutral oxygen (O) vacancies are predominant in chromium (Cr)-rich conditions and Cr vacancies with -2 charge state are the dominant defects in O-rich conditions. The charge transition levels of both O and Cr vacancies are deep within the bandgap indicating the stability of these defects. Transport calculations indicate that vacancy mediated diffusion along the basal plane has lower energy barriers for both O and Cr ions. The most favorable vacancy mediated self diffusion processes correspond to the diffusion of Cr ion in 3+ charge state and O ion in 2- state, respectively. Our calculations reveal that Cr triple defects comprised of Cr in octahedral interstitial sites with two adjacent Cr vacancies along the c-axis have a lower formation energy compared to that of charged Cr vacancies. The formation of such triple defects facilitate Cr self diffusion along the c-axis.

Medasani, Bharat (ORCID:0000000220734162)↗
DOE OSTI2015

Vacancy–Vacancy Interaction Induced Oxygen Diffusivity Enhancement in Undoped Nonstoichiometric Ceria

In this paper, molecular dynamics simulations and molecular static calculations have been used to systematically study oxygen vacancy transport in undoped nonstoichiometric ceria. A strong oxygen diffusivity enhancement appears in the vacancy concentration range of 2–4% over the temperature range from 1000 to 2000 K. An Arrhenius ion diffusion mechanism by vacancy hopping along the (100) direction is unambiguously identified, and an increasing trend of both the oxygen migration barrier and the prefactor with increasing vacancy concentration is observed. Within the framework of classical diffusion theory, a weak concentration dependence of the prefactor in oxygen vacancy migration is shown to be crucial for explaining the unusual fast oxygen ion migration in the low concentration range and consequently the appearance of a maximum in oxygen diffusivity. Finally, a representative (100) direction interaction model is constructed to identify long-range vacancy–vacancy interaction as the structural origin of the positive correlation between oxygen migration barrier and vacancy concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Direct observation of oxygen-vacancy-enhanced polarization in a SrTiO{sub 3}-buffered ferroelectric BaTiO{sub 3} film on GaAs

The integration of functional oxide thin-films on compound semiconductors can lead to a class of reconfigurable spin-based optoelectronic devices if defect-free, fully reversible active layers are stabilized. However, previous first-principles calculations predicted that SrTiO{sub 3} thin films grown on Si exhibit pinned ferroelectric behavior that is not switchable, due to the presence of interfacial vacancies. Meanwhile, piezoresponse force microscopy measurements have demonstrated ferroelectricity in BaTiO{sub 3} grown on semiconductor substrates. The presence of interfacial oxygen vacancies in such complex-oxide/semiconductor systems remains unexplored, and their effect on ferroelectricity is controversial. Here, we use a combination of aberration-corrected scanning transmission electron microscopy and first-principles density functional theory modeling to examine the role of interfacial oxygen vacancies on the ferroelectric polarization of a BaTiO{sub 3} thin film grown on GaAs. We demonstrate that interfacial oxygen vacancies enhance the polar discontinuity (and thus the single domain, out-of-plane polarization pinning in BaTiO{sub 3}), and propose that the presence of surface charge screening allows the formation of switchable domains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Influence of Hydrogen Isotopes on Vacancy Formation and Antisite Defect Diffusion in Palladium and Vanadium Metals

Density functional theory calculations are performed to study the interaction between hydrogen isotopes, vacancy, and antisite defects in Pd and V. Various defect configurations and defect-defect distances are systematically explored. Binding energies and migration energy barriers are calculated and tabulated. The study provides atomistic data for subsequent mesoscale simulations of tritium, vacancy, and antisite diffusion. In Pd, a strong binding is found between a vacancy and tritium (0.16 eV). This tritium binding increases the thermal concentration of vacancies by a factor of ~10 at 500 ?C. The increase in vacancy concentration enhances V antisite diffusion in Pd by a factor of ~6 at 500 ?C. The influence of tritium is even stronger in V, with a tritium and vacancy binding energy of 0.38 eV. Such a strong binding increases the thermal concentration of vacancies by a factor of ~300 at 500 ?C. The increase in vacancy concentration enhances Pd antisite diffusion in V by a factor of ~640 at 500 ?C. Vanadium and Pd exhibit a strong driving force to intermix with a formation energy of -1.57 eV for V antisite in Pd and -1.05 eV for Pd antisite in V. The results suggest vanadium diffusion into Pd is energetically stronger than the reverse. Zero-point-energy corrections are taken into account and calculations for hydrogen and tritium are presented.

Setyawan, Wahyu↗
DOE OSTI2019

Injection of oxygen vacancies in the bulk lattice of layered cathodes

Surfaces, interfaces, and grain boundaries are, classically, known to be sinks at which defects generated within bulk lattice will preferentially migrate to and get annihilated, therefore rejuvenating a crystalline lattice to restore the intended functionalities.1, 2, 3 Here we report an inverse case by which the defects generated at the particle surface are continuously pumped into the bulk lattice and consequentially compromising the intended functionalities. We discovered that during the operation of a rechargeable battery, oxygen vacancies generated at cathode particle surfaces migrate toward inside of a perfect lattice in lithium-rich layered cathodes, featuring electrochemical driven surface vacancy creation, injection and condensation phenomenon and revealing an unrecognized surface process induced bulk lattice degradation featured with void formation and lattice reorganization. This process is associated with a high-cutoff voltage at which the anionic redox is activated. Coupling with calculations, we substantiate that triggering of anionic redox leads to sharply lowering of both formation energy of oxygen vacancy and migration barrier of oxidized oxide ions, therefore enabling migration of oxygen vacancy via oxidized oxide ion and leading to the injection of oxygen vacancy generated at the particle surface into the bulk lattice. This work unveils unpredicted behavior of oxidized oxide ion, leading to essential insight for designing of high capacity cathode for better batteries through coupled cationic and anionic redox dynamics in layered cathodes. In a broad term, this work provides insight on the intimate coupling between interfacial process and bulk lattice behavior for governing electrode functionality.

Yan, Pengfei↗