Ordering Of Oxygen Vacancies Research Articles

Coupling between interfacial strain and oxygen vacancies at complex-oxides interfaces

The complex-oxides interfaces hold rich physics that have resulted in the emergence of various novel functional properties. While strain engineering has been widely used to induce many properties over the past decade, more recently the role of oxygen vacancies has increasingly drawn wider attention. In particular, research has revealed that there exists a strong coupling between strain and oxygen vacancy formation energy. This coupling can be used to alter oxygen vacancy concentration at interfaces, thereby opening another degree of freedom to control interfacial properties. In this review, we highlight recent works that have interrogated the connection between coupling and the emerging interfacial properties. The coupling has not only been used to selectively create oxygen vacancies at specific crystallographic oxygen sites but has also been used to manipulate ordering of oxygen vacancies near interfaces. In addition, recent studies have extended the existing connection between strain and octahedra distortion to oxygen vacancies, where the role of vacancies in the properties emerging due to octahedra distortion is now being unveiled. Finally, we discuss recent data-science efforts in the design and discovery of complex oxides and property prediction.

Breaking atomic-level ordering via biaxial strain in functional oxides: A DFT study

Oxygen vacancies are found to play a crucial role in inducing many functional properties at the heterointerfaces in complex oxides. Gaining better control over the properties requires an understanding of the atomic structure of oxygen vacancies at the heterointerfaces. In this paper, we elucidate the effects of the interfacial strain on the oxygen-vacancy ordering in fluorite δ-Bi2O3 and perovskite LaNiO2.5 using first-principles calculations. By applying biaxial strains, we find that the 〈110〉−〈111〉 oxygen vacancy order in δ-Bi2O3 is broken, resulting in a faster diffusion of oxygen ions. Similarly, the biaxial strain is used to leverage both ordered and disordered arrangements of vacancies in LaNiO2.5. Besides the vacancy order, we find that the biaxial strain can also be used to break the cation order in Gd2Ti2O7, where Gd and Ti antisites can be created on the cation sublattice, which leads to enhanced radiation tolerance and higher oxygen diffusivity. These results indicate that the biaxial strain that is commonly present at heterointerfaces can be used to gain control over both ordered and disordered arrangements of defects, potentially opening new opportunities to functionalize complex oxides.

Erratum: In-plane ordering of oxygen vacancies in a high- Tc cuprate superconductor with compressed Cu-O octahedrons: An automated cluster expansion study [Phys. Rev. Materials 4 , 044801 (2020)

Received 3 February 2021DOI:https://doi.org/10.1103/PhysRevMaterials.5.029901©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structureImpurities in superconductorsSuperconducting phase transitionVacanciesPhysical SystemsCupratesHigh-temperature superconductorsTechniquesDensity functional calculationsMethods in superconductivityCondensed Matter, Materials & Applied Physics

Exploring (1overline{1}2)-related ordered structure in oxidation-synthesized \u03b1-Fe2O3 nanowhiskers

Hematite (α-Fe2O3) nanowhiskers (NWs) synthesized via oxidation of iron-based substrates are a promising photoanode material for photoelectrochemical water splitting. Such synthesized α-Fe2O3 NWs have been found to contain ordered axial structures. Herein, we reveal that the known (1overline{1}2)-related ordered structure actually exists in bicrystalline α-Fe2O3 NWs instead of single-crystalline α-Fe2O3 NWs and that it is associated with another known (3overline{3}0)-related ordered structure. Through a spherical aberration (CS)-corrected high-resolution transmission electron microscopy (HR-TEM) investigation, the microstructural characteristic of the (1overline{1}2)-related ordered structure is verified to be periodic atomic column displacements serving as tensile strain accommodation. The HR-TEM observation are also supported by a monochromated O K-edge EELS analysis, which indicates that α-Fe2O3 NWs hosting the (1overline{1}2)-related ordered structure are indeed associated with lattice expansion. In sum, our microstructural study elucidates the root cause of the long-asserted relationship between the (1overline{1}2)-related ordered structure and oxygen vacancy ordering.

Re-entrance to a ferromagnetic insulator with oxygen-vacancy ordering in the La0.7Sr0.3MnO3/SrTiO3 superlattice

The ordering of oxygen vacancies induces the formation of a ferromagnetic charge-ordering (CO)/charge-density-wave (CDW) phase in La0.7Sr0.3MnO3, and the re-entrance to the ferromagnetic insulator CO/CDW could be induced by a high magnetic field.

Preparation and Photoelectrochemical Water Oxidation of Hematite Nanobelts Containing Highly Ordered Oxygen Vacancies

Preparation and Photoelectrochemical Water Oxidation of Hematite Nanobelts Containing Highly Ordered Oxygen Vacancies

Synthesis and structure of the perovskite related Sr4-xBaxNa1-yLiy(BO3)3 solid solution series and the related a site cation ordered (Sr/Ca)4Li(BO3)3 system

In this paper we revisit the borate systems, Sr4-xBaxNa(BO3)3 and illustrate their structural relationship with the perovskite structure. In these phases, the Na and B can be considered as forming an ordered arrangement on the perovskite small cation site, with the orientation of the oxygen atoms on the borate group such that the Na is octahedrally coordinated to O. In addition, we report the preparation of a new related solid solution series, Sr4-xBaxNa1-yLiy(BO3)3, showing a complete Sr/Ba solid solution series for y ​= ​0.5. Through further investigation into Ca doping into Sr4Li(BO3)3, we have identified a new A site (Ca/Sr) ordered variant of this structure with structure refinement indicating a composition, Sr2·2Ca1·8Li(BO3)3. Thus, these borate systems represent interesting perovskite-type variants, with large oxygen deficiencies compared to a traditional perovskite brought about by ordered oxygen vacancies, and whose existence is dependent on cation ordering features.

Effect of Heat Treatment on the Thermal Conductivity of Single Crystals of ZrO2-Based Solid Solutions Stabilized with Scandium and Yttrium Oxides

The effect of heat treatment at 1000°C for 400 h on the thermal conductivity of zirconium dioxide crystals stabilized with scandium oxide (ZrO2)1 – x(Sc2O3)x (x = 0.08–0.10) and simultaneously with scandium and yttrium oxides (ZrO2)1 – x – y(Sc2O3)x(Y2O3)y (x = 0.003–0.20, y = 0.02–0.025) has been studied. In the crystals of zirconium dioxide stabilized with scandium oxide, the most noticeable changes in the thermal conductivity are observed in the 9ScSZ crystals, in which the phase composition is changed and a marked content of the rhombohedral phase appears. These changes are less noticeable in the 8ScSZ crystals and they are mainly due to the ordering of oxygen vacancies and the changes in the microstructures of the samples, and there are no changes in the 10ScSZ crystals. The 10ScSZ crystals have the minimum electrical conductivity before and also after annealing, which is determined by the highest content of scandium oxide in the solid solution. Insignificant changes in the thermal conductivity are observed in crystals of partially stabilized zirconium dioxide co-alloyed with scandium and yttrium oxides. The cubic 8Sc2YSZ and 10Sc2YSZ crystals demonstrate only slight changes in the thermal conductivity, the character of the temperature dependence of the thermal conductivity, and the phase compositions. The alloying of the zirconia–based solid solutions with yttrium oxide simultaneously with scandium oxide enables one to increase the stability of their phase compositions and structurally dependent thermal and electrical physical characteristics.

Structural phase transitions and their effects on oxygen diffusion in Y2(Zr1-xTix)2O7 thin films

Rare earth zirconate pyrochlore compounds with their unique crystal structure have been extensively used as thermal barrier coatings, ionic conductors and buffer layer for high-temperature coated conductors. In this work, Y2(Zr1-xTix)2O7 (YZTO) thin films, as one kind of rare earth zirconates doped with Ti4+ ions, were employed to investigate the structural phase transition and its effect on oxygen diffusion in YZTO thin films. YZTO thin films were prepared on single-crystal silicon substrates by sol–gel method with adding a certain amount of Ti4+ ions into Y2Zr2O7 precursor solution. It was found that the crystal structure of YZTO thin films started gradually to transform from defective fluorite structure with disordered oxygen vacancies to pyrochlore structure with highly ordered oxygen vacancies as the doping ratio of Ti4+ ions was increased. Remarkably, in YZTO thin films, the higher the doping ratios of Ti4+ ions are, the stronger the ability to block oxygen diffusion will become.

Resistive switching behavior in epitaxial brownmillerite SrFeO2.5/Nb:SrTiO3 heterojunction

Epitaxial brownmillerite SrFeO2.5 thin films were deposited on conductive Nb-doped SrTiO3 single crystal (001) and (111)-oriented substrates using pulsed laser deposition, and their resistive switching behavior was investigated. In both the (001) and (111) device configuration, the bottom SrFeO2.5/Nb:SrTiO3 interface exhibited Ohmic behavior, while the top Au/BM-SrFeO2.5 interface displayed Schottky-like contact. Unfortunately, no resistive switching behavior was observed in the (001) devices, where the oxygen vacancy channels of SrFeO2.5 are ordered along the in-plane direction of the device. Conversely, the (111)-grown SrFeO2.5 thin films with out-of-plane oriented ordered oxygen vacancy channels displayed excellent resistive switching behavior with a high on/off ratio (∼104). The discrepancies between the (001) and (111) devices were explained in terms of their oxygen dynamics and corresponding topotactic phase transition in the SrFeO2.5 switching layer.

Improved Structural Properties in Homogeneously Doped Sm0.4Ce0.6O2-δ Epitaxial Thin Films: High Doping Effect on the Electronic Bands.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

Pressure effects on magnetism in Ca2Mn2O5 -type ferrites and manganites

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (with a ${d}^{5}$ electronic configuration) and ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$ configuration), exhibiting the ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ (${d}^{4}$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ (${d}^{5}$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\ensuremath{\approx}24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_{2}{\mathrm{Fe}}_{2}{\mathrm{O}}_{5}$ and shifts it to higher pressures.

Giant anisotropic magnetoresistance in oxygen-vacancy-ordered epitaxial La0.5Sr0.5CoO3−δ films

Anisotropic Magnetoresistance (AMR) is a property of all conductive ferromagnets, where the electrical resistivity depends on the relative orientation of the current and magnetization. In this work it is shown that the perovskite cobaltite La1-xSrxCoO3-d displays a giant form of AMR in epitaxial thin films, 20 times enhanced over bulk, and 10-100 times typical transition metals. The effect is ascribed to symmetry lowering due to spatial ordering of oxygen vacancies, pointing to a new route to enhance properties of complex oxides. The AMR values (up to 40%) are among the largest reported in the 160-year history of the effect.

Structure and water uptake in BaLnCo2O6−δ (Ln =La, Pr, Nd, Sm, Gd, Tb and Dy)

The structure of BaLnCo2O6-δ (Ln =La, Pr, Nd, Sm, Gd, Tb and Dy) was studied by the means of synchrotron radiation powder X-ray diffraction, neutron powder diffraction and Transmission Electron Microscopy (TEM), while water uptake properties were analysed with the use of thermogravimetry (TG) and water adsorption isotherms. The structure refinement revealed that the dominant phase in all compositions was orthorhombic with an ordering of the A-site cations along the c-axis and ordering of oxygen vacancies along the b-axis, which was also directly evidenced by TEM. It was shown that both unit cell volume and average Co-oxidation state at room temperature decrease linearly with decreasing Ln radius. TG water uptake experiments in humidified N2–O2 gas mixture at 300 °C revealed that among all compositions, only BaLaCo2O6-δ and BaGdCo2O6-δ exhibit significant water uptake. Surface water adsorption studies showed that the α, a normalised parameter reflecting the surface hydrophilicity, mostly independently of Ln radius was close to 0.5, which means that the surface is neither hydrophobic nor hydrophilic. The results indicated that water uptake observed at 300 °C is a bulk process, which cannot be described by standard hydration/hydrogenation reaction and it is related to the layered structure of the perovskite lattice and characteristic to La or Gd being present in the lattice.

Crystal structure, microstructure, and broadband electromagnetic response of Al3+-substituted Sr3YCo4O10+δ double perovskites

In this work, we investigate the structural, microstructural, and broadband dielectric and magnetic properties of Sr3YCo4–xAlxO10+δ (x = 0, 0.1, 0.3, 0.5, and 1.0; δ ≤ 1) double perovskite oxides in detail. The compounds were synthesized through solid-state reaction, and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The broadband dielectric and magnetic properties of the compounds were correlated qualitatively to the multiple valence state of the cobalt ions. The real part of permittivity was found to decrease with increase in Al3+ concentration from 117.3 (x = 0) to 37 (x = 1.0) (at 900 MHz), while the dielectric loss increased from 0.36 to 1.51. The real part of permeability decreased from 1.01 for x = 0 to 0.93 for x = 1.0 (at 900 MHz) accompanied by a decrease in the magnetic loss factor from 0.43 to 0.11. The broadband electromagnetic response was found to be in good agreement with the results obtained from XPS analysis. Impedance analysis revealed that the compounds exhibit a capacitive behavior, which can be attributed to the interfacial polarization due to the pore–grain interfaces. Further, room-temperature magnetic measurements revealed that the ferromagnetic nature of the parent compound diminished with the substitution of aluminum ions as a result of the reduction in oxygen vacancy ordering, which was also evident from FTIR analysis.

Microstructural characterization and electrochemical properties of Nd0.5Sr0.5Co0.5Mn0.5O3-δ as a novel cathode for SOFCs

The microstructure and electrochemical properties of a novel cathode Nd0.5Sr0.5Co0.5Mn0.5O3-δ were discussed in the paper. After calcination at 1000 °C, a perfect single crystal was obtained. Some diffraction streaks along [200] and [112] patterns in HR-TEM appeared for the ordering of oxygen vacancies, or as the overlap of B-site cations (Co2+/Co3+ and Mn2+/Mn3+) with stacking fault-derived scattering of ordering. Area-specific resistance (ASR) was 0.023 Ω · cm2 when Nd0.5Sr0.5Co0.5Mn0.5O3-δ was deposited on the electrolyte as the electrode at 700 °C in air. The maximum power density and the maximum OCV were 592.80 mW·cm−2 at 650 °C and 0.89 V at 550 °C for a single cell, respectively. Hence, the material Nd0.5Sr0.5Co0.5Mn0.5O3-δ could be considered as an air-electrode in Intermediate-temperature solid oxide fuel cells (IT-SOFC).

Oxygen Inhomogeneity and Reversibility in Single Crystal LaNiO3−δ

LaNiO3−δ single crystals have been obtained via high pressure floating zone growth under 149 bar of oxygen pressure. We find a radial gradient in the magnetic properties of specimens extracted from the as-grown boule, which we correlate with the appearance of ordered oxygen vacancy structures. This radial oxygen inhomogeneity has been characterized systematically using a combination of magnetization and X-ray scattering measurements. We establish the presence of rhombohedral ( R 3 ¯ c ), oxygen stoichiometric specimens at the periphery of the boule and the presence of a dilute concentration of ordered oxygen-deficient orthorhombic La2Ni2O5 in the center. Furthermore, we demonstrate that the as-grown, oxygen-deficient central regions of the crystal can be annealed under high oxygen pressure, without loss of crystallinity, into fully oxygenated LaNiO3, recovering magnetic properties that are characteristic of stoichiometric specimens from the exterior region of the crystal. Thus, single crystals of LaNiO3−δ possess oxygen content that can be reversibly modified under oxidizing and reducing conditions.

Graphene mediated resistive switching and thermoelectric behavior in lanthanum cobaltate

Multifunctional materials are of utmost importance owing to the combination of several functionalities into a single device. We present the multifunctional properties, namely, thermoelectric (TE) for energy harvesting and resistive switching (RS) for memory storage in the LaCoO3 (LCO)–graphene nanocomposite system. The existence of individual phases of LCO and graphene is confirmed from x-ray diffraction and electron microscopy techniques. The x-ray photoelectron spectroscopy measurement reveals the formation of oxygen vacancies in the nanocomposite with the addition of graphene. The bipolar resistive switching behavior observed in a LCO–graphene nanocomposite is explained using space charge limited conduction mechanism and is found to evolve from a trap-limited to a trap-free region with an increase in the graphene volume fraction, which eventually ascribed to the ordering of oxygen vacancies in the nanocomposite system. The endurance plot of the nanocomposite shows a stable RS behavior for consecutive 1000 cycles. On the other hand, the creation of oxygen vacancies in the nanocomposite leads to change in the configurational entropy of charge states of cobalt, which tunes the Seebeck coefficient (α). The decrease in α leads to an increase in the hopping hole concentration (nh) estimated using the classical Heikes formula and is consistent with the increase in the electrical conductivity. The increase in oxygen vacancies leads to point-defect scattering, which further reduces the thermal conductivity of the nanocomposite. The optimized TE parameters show a figure-of-merit of 0.004±0.00048 at 300 K. The simultaneous observation of RS and TE properties in LCO–graphene nanocomposite provides new directions for multifunctional materials.

Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite

The polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal–insulator or paramagnet–ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO3 of 40 nm could be transformed to AFM SrCoO2.5 with ordered oxygen vacancy planes either vertical (V-SrCoO2.5) or parallel (P-SrCoO2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO2.5 are along [010] and , respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO3 and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.

Synergistic effect of B site co-doping with Co and Ce in bifunctional oxygen electrocatalysis by oxygen deficient brownmillerite Ba2In2O5

Specificity in oxygen reduction and evolution reactions is pivotal in bifunctional catalysts in advanced energy devices which are expected to be active in a wide potential window and stable in both electrochemically reducing and oxidising atmospheres. State of the art noble metal catalysts for these reactions are fraught with issues whereas, metal oxides are expected to be stable in large potential window and can be tuned to have better bifunctional activity. Catalytically active metals, redox centres, oxygen vacancies etc. are some of the features of structured oxides which can be exploited in oxygen electrocatalysis. Brownmillerite family of compounds with general formula, A2B2O5 has high concentrations of ordered oxygen vacancy in a layer and can be promising bifunctional oxygen electrocatalysts, by appropriate B site doping. Ba2In2O5 is a well-studied system for its oxide ion conductivity and proximity of active B sites to O vacancies provide good O adsorption sites which are catalytically active. Here, cobalt and cerium are co-doped in Ba2In2O5, anticipating a synergistic effect of Ce(III)/Ce(IV) redox centre in ORR and Co activity in OER along with O vacancies providing adsorption sites. We have studied the structural changes associated with the doping by Rietveld refinement of the XRD patterns and correlated the structure to bifunctional oxygen electrocatalytic activity which is found to enhance on increasing the Co and Ce content.