Diffusion Of Oxygen Vacancies Research Articles

Reversible control of magnetism in La0.67Sr0.33MnO3 through chemically-induced oxygen migration

We demonstrate reversible control of magnetization and anisotropy in La0.67Sr0.33MnO3 films through interfacial oxygen migration. Gd metal capping layers deposited onto La0.67Sr0.33MnO3 leach oxygen from the film through a solid-state redox reaction to form porous Gd2O3. X-ray absorption and polarized neutron reflectometry measurements show Mn valence alterations consistent with high oxygen vacancy concentrations, resulting in suppressed magnetization and increased coercive fields. Effects of the oxygen migration are observed both at the interface and also throughout the majority of a 40 nm thick film, suggesting extensive diffusion of oxygen vacancies. After Gd-capped La0.67Sr0.33MnO3 is exposed to atmospheric oxygen for a prolonged period of time, oxygen diffuses through the Gd2O3 layer and the magnetization of the La0.67Sr0.33MnO3 returns to the uncapped value. These findings showcase perovskite heterostructures as ideal candidates for developing functional interfaces through chemically-induced oxygen migration.

Novel Vertical 3D Structure of TaOx-based RRAM with Self-localized Switching Region by Sidewall Electrode Oxidation.

A novel vertical 3D RRAM structure with greatly improved reliability behavior is proposed and experimentally demonstrated through basically compatible process featuring self-localized switching region by sidewall electrode oxidation. Compared with the conventional structure, due to the effective confinement of the switching region, the newly-proposed structure shows about two orders higher endurance (>108 without verification operation) and better retention (>180h@150 °C), as well as high uniformity. Corresponding model is put forward, on the base of which thorough theoretical analysis and calculations are conducted as well, demonstrating that, resulting from the physically-isolated switching from neighboring cells, the proposed structure exhibits dramatically improved reliability due to effective suppression of thermal effects and oxygen vacancies diffusion interference, indicating that this novel structure is very promising for future high density 3D RRAM application.

Synergistic Resistive Switching Mechanism of Oxygen Vacancies and Metal Interstitials in Ta2O5

Ta2O5 is extensively studied as a data-storage material for resistance random access memory (RRAM). The resistive switching (RS) in Ta2O5-based RRAM is generally believed to be due to the diffusion of oxygen vacancy (Vo) inside the oxide, while the role of metal interstitials is paid less attention. Here, on the basis of first-principles calculations, we show that the role of interstitial Ta (Tai) is competitive under the oxygen-poor condition and should also contribute to RS in Ta2O5. This is obvious by our calculated comparable energy barriers for the diffusion of Vo and Tai, which are 3.5 and 3.7 eV, respectively. Furthermore, the presence of electric field in working devices will enhance the migration of Tai due to its higher charge states compared to Vo. Meanwhile, Tai will introduce more defect states closer to the conduction bands and, thus, is more effective on tuning the electronic structure of Ta2O5. The present work unravels the contribution of Ta cations in RS of tantalum oxide-based RRAM, pre...

フェーズフィールド法による酸素空孔を有する非分極・分極チタン酸バリウム多結晶の圧電・誘電特性評価

This paper studies the dielectric and piezoelectric behavior of unpoled and poled barium titanate (BaTiO3) polycrystals with oxygen vacancies. A phase field model is employed for BaTiO3 polycrystals, coupled with the time-dependent Ginzburg-Landau theory and the oxygen vacancies diffusion, to demonstrate the interaction between oxygen vacancies and domain evolutions. To generate grain structures, the phase field model for grain growth is also used. The hysteresis loop and butterfly curve are predicted at room and high temperatures. The permittivity, and longitudinal and transverse piezoelectric constants of the BaTiO3 polycrystals are then examined for various grain sizes and oxygen vacancy densities.

First-principles study of the effect of oxygen vacancy and strain on the phase transition temperature of VO2

The calculated oxygen-vacancy diffusion barrier indicates that the existence of oxygen-vacancy could stabilize the rutile phase at a low temperature.

Surface vacancies concentration of CeO2(1 1 1) using kinetic Monte Carlo simulations

Kinetic Monte Carlo simulations (kMC) are useful tools for the investigation of the dynamics of surface properties. Within this method we investigate the oxygen vacancy concentration of at ultra high vacuum conditions (UHV). In order to achieve first principles calculations the input for the simulations, i.e. energy barriers for the microscopic processes, we use density functional theory (DFT) results from literature. We investigate the possibility of ad- and desorption of oxygen on ceria as well as the diffusion of oxygen vacancies to and from the subsurface. In particular, we focus on the vacancy surface concentration as well as on the ratio of the number of subsurface vacancies to the number of vacancies at the surface. The comparison of our dynamically obtained results to the experimental findings leads to several issues. In conclusion, we can claim a substantial incompatibility of the experimental results and the dynamical calculation using DFT inputs.

On the Origin of Low-Resistance State Retention Failure in HfO2-Based RRAM and Impact of Doping/Alloying

We study in detail the impact of alloying HfO2 with Al (Hf1– x Al2 x O2+ x ) on the oxide-based resistive random access memory (RRAM) (OxRRAM) thermal stability through material characterization, electrical measurements, and atomistic simulation. Indeed, migration of oxygen atoms inside the dielectric is at the heart of OxRRAM operations. Hence, we performed comprehensive diffusion barrier calculations in HfO2, Hf1– x Al2 x O2+ x , and Hf1– x Ti x O2 relative to the oxygen vacancy ( $V_{o}$ ) movement involved in low-resistance state ( $R_{\mathrm{\scriptscriptstyle ON}}$ ) thermal stability. Calculations are performed at the best level using ab initio techniques. This paper provides an insight on the improved $R_{\mathrm{\scriptscriptstyle ON}}$ stability of our Hf1– x Al2 x O2+ x -based RRAM devices and predicts the degraded retention of Hf1– x Ti x O2-based RRAM measured in the literature. Our theoretical calculations link the origin of $R_{\mathrm{\scriptscriptstyle ON}}$ retention failure to the lateral diffusion of oxygen vacancies at the constriction/tip of the conductive filament in HfO2-based RRAM.

Utilizing multiple state variables to improve the dynamic range of analog switching in a memristor

Memristors and memristive systems have been extensively studied for data storage and computing applications such as neuromorphic systems. To act as synapses in neuromorphic systems, the memristor needs to exhibit analog resistive switching (RS) behavior with incremental conductance change. In this study, we show that the dynamic range of the analog RS behavior can be significantly enhanced in a tantalum-oxide-based memristor. By controlling different state variables enabled by different physical effects during the RS process, the gradual filament expansion stage can be selectively enhanced without strongly affecting the abrupt filament length growth stage. Detailed physics-based modeling further verified the observed experimental effects and revealed the roles of oxygen vacancy drift and diffusion processes, and how the diffusion process can be selectively enhanced during the filament expansion stage. These findings lead to more desirable and reliable memristor behaviors for analog computing applications. Additionally, the ability to selectively control different internal physical processes demonstrated in the current study provides guidance for continued device optimization of memristor devices in general.

Analysis of hematite re-oxidation in the chemical looping process

Very little attention has been dedicated to the carrier re-oxidation in chemical looping systems. The work presented in this paper is for the re-oxidation of partially reduced hematite from a cyclic chemical looping fixed bed process. The underlying purpose of this work is to develop engineering rates and mechanisms for the re-oxidation of partially reduced hematite that can be included in CFD models for a chemical looping process. To this end, experiments were run using nominally 1000g of hematite material in a fixed bed reactor cycling between reduction and re-oxidation. The cyclic processing began with the reduction step then proceeded to the oxidation step repeating this analysis for several cycles ranging from 5 to 10. The re-oxidation process was conducted at temperatures ranging from 745°C to 825°C and oxygen concentrations ranging between 9% and 11%. The reduction was carried out at the same temperature as the re-oxidation step at various CH4 concentrations from 5% to 9%. In this paper, cyclic induced variations in performance are presented as well as the kinetic parameters for the first cycle. The re-oxidation of the depleted hematite occurs through a 2 step parallel process in which oxygen reacts to fill the surface of each grain within the particles and then migrates through oxygen vacancy diffusion to the depleted cores of each grain.

Oxygen Diffusion in SrTiO3 and Related Perovskite Oxides

The transport of oxygen ions plays a central role in determining the performance or the degradation of perovskite‐type oxides in applications as diverse as ceramic capacitors, solid electrolytes, memristive devices and all‐oxide electronics. In this Feature Article, experimental data reported in the literature for oxygen diffusion in SrTiO3 and in related titanate, zirconate and cerate perovskite oxides [CaTiO3, BaTiO3, (Na,Bi)TiO3, Pb(Ti,Zr)O3, CaZrO3, SrZrO3, BaZrO3, SrCeO3, BaCeO3] are reviewed. The two related aims are to draw attention to discrepancies and to identify reliable diffusion data. The physical limit to the diffusivity of oxygen vacancies in a perovskite oxide is also considered.

Role of Strain Ans Sr-Doping for the Anisotropic Oxygen Diffusion in LaMnO3

A high ionic conductivity is desireable property for cathode materials forsolid oxide fuel cells. LaMnO3 is a well studied candidate for this applica-tion. Hence, it is important to understand the oxygen transport mecha-nism. We find increase in oxygen vacancy formation energy and diffusionunder compressive and tensile strain as well as under Sr-doping using thedensity functional theory and nudged-elastic band calculations. The oxy-gen vacancy sites and diffusion paths are studied. We investigate the effectof Sr-doping on the anisotropy of the diffusion. Modified strengths ofthe Mn and O bonding as well as volume change are identified as fac-tors that affect the formation/diffusion of vacancies. Furhermore, tensilestrain facilitates enhanced oxygen vacancy diffusion along the [011]/[0 ̄11] direction. The weakening of the Mn- O bonds as well as the distortion ofthe MnO6 octahedra hinder diffusion along the [110]/[1 ̄ 10}direction.

Effect of Cation Ordering on Oxygen Vacancy Diffusion Pathways in Double Perovskites

Perovskite structured oxides (ABO3) are attractive for a number of technological applications, including as superionics because of the high oxygen conductivities they exhibit. Double perovskites (AA′BB′O6) provide even more flexibility for tailoring properties. Using accelerated molecular dynamics, we examine the role of cation ordering on oxygen vacancy mobility in one model double perovskite SrLaTiAlO6. We find that the mobility of the vacancy is very sensitive to the cation ordering, with a migration energy that varies from 0.6 to 2.7 eV. In the extreme cases, the mobility is both higher and lower than either of the two end member single perovskites. Further, the nature of oxygen vacancy diffusion, whether one-dimensional, two-dimensional, or three-dimensional, also varies with cation ordering. We correlate the dependence of oxygen mobility on cation structure to the distribution of Ti4+ cations, which provide unfavorable environments for the positively charged oxygen vacancy. Our results demonstrate the potential of using tailored double perovskite structures to precisely control the behavior of oxygen vacancies in these materials.

Phase field simulation on the electromechanical response of poled barium titanate polycrystals with oxygen vacancies

This work investigates the electromechanical response of poled barium titanate (BaTiO3) polycrystals with oxygen vacancies at room temperature. A phase field model is developed for ferroelectric polycrystals, coupled with the time-dependent Ginzburg-Landau theory and the oxygen vacancies diffusion, to demonstrate the interaction between oxygen vacancies and domain evolutions. To generate grain structures, the phase field model for grain growth is also employed. The hysteresis loop (polarization vs electric field curve) and butterfly loop (strain vs electric field curve) are computed, and the effects of grain size and oxygen vacancy density on the dielectric and piezoelectric properties of the BaTiO3 polycrystals are discussed.

Mechanism of electric conductivity change in BaCo0.7Fe0.2Nb0.1O3 − δ at intermediate temperature

In this paper, the change mechanism of electric conductance behavior of BaCo0.7Fe0.2Nb0.1O3−δ (BCFN) materials at the temperature range 550–650°C is studied. The ionic conductivity separated by the electrode-blocking method exhibits a gradual increase at this temperature range. However, the electronic conductivity shows a steady curve. According to the characterization of thermogravimetry (TG) and O2-Temperature Programmed Desorption (O2-TPD), the desorption of oxygen and the formation of oxygen vacancies also occur at this temperature range. Using the oxygen vacancy diffusion coefficients Dchem (obtained by Electric Conductivity Relaxation (ECR)) and the oxygen vacancy content (obtained by TG), the theoretical ionic conductivity is calculated by Einstein–Nernst equation. The theoretical value is in good agreement with the experimental results, which means that the increase of BCFN ionic conductivity can be explained by the mostly ionized oxygen vacancies. The defect reaction of BCFN suggests that there's consumption of 4+ charge B sites ions accompanied with the formation of oxygen vacancies, which will stabilize the electrical conductivity according to the Zener double exchange effects. Besides, the X-ray photoelectron spectroscopic (XPS) results suggest that the B sites ions are also greatly influenced by the formation of oxygen vacancies. This may increase the exchange energy in the Zener double exchange effect and stabilize the electrical conductivity.

Defect Chemical Models for Temperature Dependence of Oxygen Stoichiometry and Electrical Conductivities of Ba0.5Sr0.5Co0.8Fe0.2O3-δ

Ba0.5Sr0.5Co0.8Fe0.2O3-δ is a well-known cathode material of intermediate-temperature solid oxide fuel cell. To elucidate its electrical and electrochemical properties, three chemical defect models were adopted in this work for fitting the data of oxygen stoichiometry and electrical conductivities as a function of temperature. Reasonable values of concentrations and mobility of charge carriers, and were obtained with the cluster defect model. The p-type electronic conductivities and oxygen ionic conductivities were separated from the total electrical conductivities. The temperature-dependence of oxygen vacancy diffusion coefficient of BSCF were also obtained, and the results were comparable with the data measured experimentally.

Reaction fronts formation during oxygen chemical diffusion in oxides

We present a theoretical study of the impact of oxygen diffusion in oxide crystals on the dynamic changes of metal dopants in the ionic state, and on the crystal conduction type. The analysis invokes ambipolar diffusion for all participating species: oxygen vacancies, metal dopant, holes and electrons. The species dynamics are demonstrated for an oxide of 8eV energy gap, electron and hole diffusivities being 105 times larger than the oxygen vacancy one, and the dopant energy level being 1.0eV above or 1.0eV below the mid-gap. We obtain linkage between the oxygen chemical diffusivity and the occurrence of a reaction front. The necessary condition is the existence of a sufficiently long interval, where a power dependence of the oxygen chemical diffusivity on the ambient oxygen pressure exists. In our demonstrated system, prominent reaction fronts occur at very low and at very high ambient oxygen pressures. The fronts relate to the reversible MMe×+e′→←MMe′ reaction at the low pressures, and to the reversible MMe′+h•→←MMe× reaction at high pressures, where M marks the dopant metal, and Me marks an oxide lattice constituent metal. In the mid-pressure interval, the oxygen chemical diffusivity is approximately constant, being several orders of magnitude smaller than the oxygen vacancy diffusivity.

Analysis of the nature of electrical conductivity in nominally undoped LiNbO3 crystals

The temperature dependence of electrical conductivity and admittance of two nominally undoped lithium niobate crystals of congruent composition has been investigated in the temperature range of 293–450 K and simulated in the range of 350–700 K. It is shown that the ion conductivity at T < 450 K is mainly determined by OH− groups; however, some other mechanisms may manifest themselves at higher temperatures; a likely one is diffusion of oxygen vacancies, which leads to an increase in the “averaged” conductivity activation energy.

Two components for one resistivity in LaVO3/SrTiO3 heterostructure

A series of 100 nm LaVO3 thin films have been synthesized on (0 0 1)-oriented SrTiO3 substrates using the pulsed laser deposition technique, and the effects of growth temperature are analyzed. Transport properties reveal a large electronic mobility and a non-linear Hall effect at low temperature. In addition, a cross-over from a semiconducting state at high-temperature to a metallic state at low-temperature is observed, with a clear enhancement of the metallic character as the growth temperature increases. Optical absorption measurements combined with the two-bands analysis of the Hall effect show that the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3 substrate. These results allow us to understand that the film/substrate heterostructure behaves as an original semiconducting-metallic parallel resistor, and electronic transport properties are consistently explained.

Multifunctional AlPO4 coating for improving electrochemical properties of low-cost Li[Li0.2Fe0.1Ni0.15Mn0.55]O2 cathode materials for lithium-ion batteries.

Layered Li-rich, Fe- and Mn-based cathode material, Li[Li0.2Fe0.1Ni0.15Mn0.55]O2, has been successfully synthesized by a coprecipitation method and further modified with different coating amounts of AlPO4 (3, 5, and 7 wt %). The effects of AlPO4 coating on the structure, morphology and electrochemical properties of these materials are investigated systematically. XRD results show that the pristine sample is obtained with typical Li-rich layered structure and trace amount of Li3PO4 phase are observed for the coated samples. The morphology observations reveal that all the samples show spherical particles (3-4 μm in diameter) with hierarchical structure, composed of nanoplates and nanoparticles. XPS analysis confirms the existence of AlPO4 and Li3PO4 phases at the surface. The electrochemical performance results indicate that the sample coated with 5 wt % AlPO4 exhibits the highest reversible capacity (220.4 mA h g(-1) after 50 cycles at 0.1C), best cycling performance (capacity retention of 74.4% after 50 cycles at 0.1C) and rate capability (175.3 mA h g(-1) at 1C, and 120.2 mA h g(-1) at 10C after 100 cycles) among all the samples. Cycle voltammograms show good reversibility of the coated samples. EIS analysis reveals that charge transfer resistance after coating is much lower than that of the pristine sample. The excellent electrochemical performances can be attributed to the effects of multifunctional AlPO4 coating layer, including the suppression of surface side reaction and oxygen vacancies diffusion, the acceleration of lithium ions transport as well as the lower electrochemical resistance. Our research provides a new insight of surface modification on low-cost Li-rich material to achieve high energy as the next-generation cathode of lithium-ion batteries.

Phase Transitions Of The Ferroelectric Na0.5Bi0.5TiO3 By Dielectric And Internal Friction Measurements

This work focuses on the high temperature dielectric and mechanical spectroscopic properties of lead free relaxor Sodium Bismuth Titanate (NBT) ceramics, fabricated by conventional ceramic double sintering method. Systematic measurements of dielectric and mechanical properties have been performed as a function of temperature. A sequence of phase transitions has been studied by both dielectric and anelastic measurements. Three internal friction peaks were observed near 350, 200 and 120 °C. The 350 °C-peak corresponds to a transition associated with the tetragonal (P4bm) to rhombohedral (R3c) phase, and the 200 °C-peak is related to the ferroelectric to antiferroelectric phase transition. The 120 °C-peak could be ascribed to the interaction between the domain walls and the diffusion of oxygen vacancies in the domains. Copyright © 2015 VBRI Press.