Disordered Oxygen Vacancies Research Articles

Symmetry Breaking and Polarization Regulation in AgFeO2‐Based Photocathodes Through Oxygen Defect Engineering

AbstractPolar materials, with intrinsic polarization effects, present significant potential for photo(electro)catalysis. However, the available natural polar materials in this field are quite scarce, due to the requisite structural non‐centrosymmetry. Defect engineering emerges as a promising avenue for tuning material symmetry, yet achieving the transition from centrosymmetric to non‐centrosymmetric structures and optimizing associated polarization effects remains challenging. This study demonstrates symmetry breaking in centrosymmetric 3R‐delafossite AgFeO2 through ordered oxygen defects introduction, yielding substantial macroscopic polarization. The transition is achieved via annealing post‐treatment of co‐precipitation‐hydrothermal AgFeO2 samples, with precision in oxygen defects control by tailoring annealing conditions. Experimental characterizations reveal ordered interstitial oxygen and disordered oxygen vacancies. Density functional theory calculations indicate a higher propensity for the formation of disordered oxygen vacancies compared to ordered ones, while ordered interstitial oxygen is more easily formed than its disordered counterpart. Resultant macroscopic polarization enhances photoelectrochemical performance, with photocurrent density increasing from 0.79 to 2.95 µA cm−2. Coupling macroscopic and spin polarization via external electric and magnetic fields further enhances photocurrent density (≈18.44 µA cm−2). These findings provide reference cases and strategies for applying polarization effects in photo(electro)catalytic technology.

Chemical expansion of La3+ and Yb3+ incorporated Zr-doped ceria ceramics for concentrated solar energy-driven thermochemical production of fuels

Redox studies on Zr, La, Yb doped ceria (Ce0.9M3+/4+0.1O2-x, M3+/4+ = Zr4+, La3+, Yb3+ and CeZr0.05 M3+0.05O1.95, M3+ = La3+, Yb3+) ceramics were performed using dilatometry and mass change measurements to analyze the chemical and thermal volume changes depending on reduction state which can have dramatic effects on the long-term stability of these ceramics in Concentrated Solar Energy (CSE)–driven applications relevant to the synthesis of fuels. The reduction extents determined by both methods under vacuum (p = 2×10−5 mbar) are similar up to 1673 K. At higher temperatures, CeO2, which has a higher vapor pressure than the oxides of the dopant ions, evaporates. As a result, there is an accumulation of doping ions, which leads to a porous surface zone. Surface enrichment of dopants, especially that of Zr4+, is crucial for re-oxidation kinetics. Co-doping with trivalent doping ions such as La3+ and Yb3+ reduces the extent of selective evaporation materializing it into a thinner porous surface layer. Trivalent dopants and their additional oxygen vacancies also influence the chemical expansion/contraction of the Zr4+ doped ceria crystal lattice during the redox reaction and thus lead to a reduction of mechanical stresses. We assume that additional oxygen vacancies from trivalent dopant ions promote the disordering of intrinsic oxygen vacancies, while smaller tetravalent ions such as Zr4+ compensates this effect. This explains that doped ceria with trivalent and tetravalent ions show almost the same chemical expansion as pure ceria. The new results on alteration, thermal and chemical expansions/contractions at high temperature are particularly important for the development of metal oxide ceramic components exposed to cyclic reducing and oxidizing atmospheres at elevated temperatures achieved via CSE.

BaTa0.5Li0.5O2.5: a new highly oxygen deficient perovskite oxide-ion conductor

BaTa0.5Li0.5O2.5: a new highly oxygen deficient perovskite oxide-ion conductor with disordered oxygen vacancies is reported for the first time.

Effect of oxygen vacancies and cationic valence state on multiferroicity and magnetodielectric coupling in (1-x)BaTiO3.(x)LaFeO3 solid solution

A solid solution of (1-x)BaTiO3–(x)LaFeO3 (x = 0, 0.007, 0.015, 0.031, 0.062) has been investigated for room temperature multiferroicity and magnetodielectric effect. The incorporation of La and Fe ions in ferroelectric BaTiO3 leads to increased lattice disorder and generation of oxygen vacancies that induces magnetism. A detailed correlated study using XRD, XPS and Raman spectroscopy is being reported. A typical ferromagnetic and ferroelectric nature of each sample are confirmed by M−H and P–E hysteresis loops, respectively at room temperature. Temperature dependent dielectric studies reveal the decrease of transition temperature from 137 0C for undoped to room temperature for increasing substitution. The changes in the dielectric property with applied magnetic field shows the indications of magnetodielectric effect. For x = 0.015, existence of both proper ferroelectric without lossy properties and ferromagnetic properties in a single tetragonal phase makes it a perfect room temperature multiferroic material. For this sample, the magnetodielectric coupling is the strongest with Magnetocapacitance of ∼2.7% and Magneto loss of ∼0.8% at 10 kHz and 1 T. The multiferroicity and the magnetodielectric effect exhibited by this material has been correlated to the structural properties, electric and magnetic properties, changes in valence states, and oxygen vacancy (Ov).

Probing the Signature of Disordering and Delocalization of Oxygen Vacancies and Anti-site Defects in Doped LaAlO3 Solid Electrolytes

Probing the Signature of Disordering and Delocalization of Oxygen Vacancies and Anti-site Defects in Doped LaAlO₃ Solid Electrolytes

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.

Effect of fluorine doping on the ionic (O2−, H+) conductivity of oxygen-deficient complex oxides

The effect of fluorine doping on the transport properties (oxygen-ion and proton conductivity) of perovskite-related complex oxides Ba2In2O5, Ba4In2Zr2O11, and Ba4Ca2Nb2O11 with various concentrations and degrees of disordering of oxygen vacancies was analyzed. Regardless of the doping mechanism and concentrations and degrees of disordering of oxygen vacancies, the introduction of low concentrations of fluoride ions increases the ionic component of the conductivity due to the appearance of effects of electrostatic repulsion of ions of various nature in the anionic sublattice.

Surface-reaction induced structural oscillations in the subsurface

Surface and subsurface are commonly considered as separate entities because of the difference in the bonding environment and are often investigated separately due to the experimental challenges in differentiating the surface and subsurface effects. Using in-situ atomic-scale transmission electron microscopy to resolve the surface and subsurface at the same time, we show that the hydrogen–CuO surface reaction results in structural oscillations in deeper atomic layers via the cycles of ordering and disordering of oxygen vacancies in the subsurface. Together with atomistic calculations, we show that the structural oscillations in the subsurface are induced by the hydrogen oxidation-induced cyclic loss of oxygen from the oxide surface. These results demonstrate the propagation of the surface reaction dynamics into the deeper layers in inducing nonstoichiometry in the subsurface and have significant implications in modulating various chemical processes involving surface–subsurface mass transport such as heterogeneous catalysis, oxidation, corrosion and carburization.

Effect of Zr4+on the phase stability and oxygen permeation characteristics of SrCo0.8Fe0.2−yZryO3−δ (y ≤ 0.1) membranes

A stable and high oxygen permeable membrane system has been developed via partial replacement of iron with zirconium in SrCo0.8Fe0.2O3−δ compound. The initial powder series of composition SrCo0.8Fe0.2−yZryO3−δ (0 ≤ y ≤ 0.1), synthesized by an oxalate-based sol-gel route, is shown to exhibit a single perovskite-type cubic phase at 27 °C, transform to a brownmillerite (Ca2AlFeO5)-type orthorhombic structure, and finally assume a stable open cubic phase with disordered oxygen vacancies at temperatures determined by zirconium content, transitions being at 200 and 800 °C, respectively for the system y = 0.05. The highest JO2 of 2.54 × 10−6 mol cm−2 s−1 is realized in optimum composition of y = 0.05 at 1000 °C. The formation of cubic phase in Sr–Co–Fe–O system with abundance of disordered anion vacancies is crucial for both the high oxygen permeability and extended operational stability at elevated temperatures.

Mn-rich SmBaCo0.5Mn1.5O5+δ double perovskite cathode material for SOFCs

SmBaCo0.5Mn1.5O5+δ oxide with Sm-Ba cation-ordered perovskite-type structure is synthesized and examined in relation to whole RBaCo0.5Mn1.5O5+δ series (R: selected rare earth elements). Presence of Sm and 3:1 ratio of Mn to Co allows to balance physicochemical properties of the composition, with moderate thermal expansion coefficient value of 18.70(1)·10−6 K−1 in 300–900 °C range, high concentration of disordered oxygen vacancies in 600–900 °C range (δ = 0.16 at 900 °C), and good transport properties with electrical conductivity reaching 33 S cm−1 at 900 °C in air. Consequently, the compound enables to manufacture catalytically-active cathode, with good electrochemical performance measured for the electrolyte-supported laboratory-scale solid oxide fuel cell with Ni-Gd1.9Ce0.1O2-δ|La0.4Ce0.6O2-δ|La0.8Sr0.2Ga0.8Mg0.2O3-δ|SmBaCo0.5Mn1.5O5+δ configuration, for which 1060 mW cm−2 power density is observed at 900 °C. Furthermore, the tested symmetrical SmBaCo0.5Mn1.5O5+δ|La0.8Sr0.2Ga0.8Mg0.2O3-δ|SmBaCo0.5Mn1.5O5+δ cell delivers 377 mW cm−2 power density at 850 °C, which is a promising result.

AC Magnetic Susceptibility of Y3Ba5Cu8O18 Substituted by Nd3+ and Ca2+ Ions

Effect of Nd3+ and Ca2+ ion substitutions in Y3Ba5Cu8O18 (Y-358) was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical resistivity, and ac magnetic susceptibility measurements. Superconducting samples of type Y3 − xNdxBa5 − yCayCu8O18 (with 0 ≤ x, y ≤ 0.4) were synthesized by the standard solid-state reaction technique. XRD patterns revealed that all the samples were mainly composed of Y-358 phase with orthorhombic structure. SEM micrographs showed that Nd3+ and Ca2+ ion substitutions had the ability to alter the grain morphology of Y-358 to irregular randomly oriented grains with smaller average grain sizes. The temperature-dependent electrical resistivity curves indicated that the superconducting transition temperature Tc was decreased from 96.51 to 87.41 K as the substitution contents were increased from 0 to 0.4. Detailed AC magnetic susceptibility measurements were done using a quantum design physical property measurement system (PPMS) at different dc fields (B = 0.5–20 kG) and frequency of 1 kHz. A comparison between electrical resistivity and AC magnetic susceptibility data was performed. It was found that Tcs obtained from AC magnetic susceptibility were much lower than those obtained from the resistivity measurements, for all studied samples. This was discussed in terms of filamentary paths that could produce sharp pronounced drops in diamagnetism. Generally, the depression in Tc was attributed to the decrease in relative volume fraction of Y-358 or due to trapping of mobile holes or other mechanism connected with oxygen vacancy disorder. Werthamer–Helfand–Hohenberg (WHH) theory was employed to study the temperature dependence of the upper critical magnetic field Bc2(T) for the samples. The relative importance of both orbital and paramagnetic effects in the suppression of superconductivity was described with the aid of Maki parameter (κM). The estimated values of the upper critical magnetic field at 0 K, Bc2(0), were found to decrease from 37.3 to 13.5 T as Nd3+ and Ca2+ contents were increased from 0 to 0.4. This decrement was attributed to the weak electron–phonon coupling and to the orbital as well as spin pair breaking mechanisms. Furthermore, Ginzburg–Landau coherence length ξGL of Y3 − xNdxBa5 − yCayCu8O18 was found to increase with the increase of the Nd3+ and Ca2+ contents. Therefore, Nd3+ and Ca2+ ions play an important role in the pair-breaking mechanism and in suppressing superconductivity of the Y-358 phase.

Effect of Sol-Gel Synthesized BiFeO<sub>3</sub> Nanoparticle Addition in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–δ</sub> (Y123) Superconductor Synthesized by Standard Solid State Reaction Method

The effects of the multiforric BiFO3 nanoparticles addition on the structure and superconducting properties of YBa2Cu3O7–δ (Y123) with different concentrations were systematically investigated using X-ray diffraction (XRD), field emission scanning electron micrograph (FESEM), EDX and four point probe measurement. It was found that the added samples were predominant by Y-123 phase beside small amount of Y-211 and unreacted BiFeO3 secondary phases. Samples with less (wt.%) BFO added YBCO precursor powder preserved the orthorhombic structure similar to the pure YBCO, while samples with higher wt% addition show orthorhombic-to-tetragonal transition tendency. The samples became more porous and their grain size slightly decreased with addition of BiFeO3. The addition of nanoBiFeO3 disturbed the grain growth of Y123, thus resulting in the degradation of superconducting properties of the samples. The superconducting transition temperature (Tc onset) of samples decreased from 92 K for x=0.0 to 44 K for x=10.0 wt. %, which could be attributable to oxygen vacancy disorder.

Effects of atom- and phase-scale compressive stress on fracture toughness in yttrium-doped lanthanum zirconate solid solutions

The poor fracture toughness of cubic pyrochlores has severely blocked their wide application. To toughen them, a series of yttrium dopants have been introduced to lanthanum zirconate, creating atomic or phase-scale compressive stress dependent on the concentration of dopants. We find bigger substitutional dopants, rather than smaller ones, can toughen cubic ceramics, attributing to the dominance of an imposed atomic compressive stress to surroundings over the local one near normal point defects. For special point defects such as oxygen vacancies or interstitials, the local stress in their vicinity could become dominant given an adequate concentration. Despite of the pyrochlore/fluorite intermixture possessing evidently reduced grains, it only exhibits intermediate fracture toughness, suggesting negligible grain refining effects on toughening. Further, a random distribution of atomic compressive stress induced by disordered oxygen vacancies is beneficial to toughening. This research highlights the atomic compressive stress on toughening, providing guidance for future design of novel thermal barrier coatings.

57Fe Mössbauer study into oxygen vacancy disorder in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

A 57Fe Mössbauer study of mixed ionic electronic conducting (Ba0.5Sr0.5)(Co0.857Fe0.08Fe0.12)O3−δ (BSCF) has been made on samples of different oxygen content. The room-temperature spectra are found to depend strongly on oxygen stoichiometry. They reveal two magnetically and chemically different environments of the iron probes whose charge states and oxygen coordination are discussed. Analysis of spectral intensities provides confirmation of the close similarity of formation energies of different types of oxygen vacancies in BSCF. It is also shown that the distribution of oxygen vacancies on their sites of different coordination is far from random.

Magnetic properties of (Nd,Ca)(Ba,La)Co2O5+δ tuned by the site-selected charge doping, oxygen disorder, and hydrostatic pressure

Comprehensive study of magnetic properties of the layer-ordered perovskites $\mathrm{N}{\mathrm{d}}_{1--x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{B}{\mathrm{a}}_{1--y}\mathrm{L}{\mathrm{a}}_{y}\mathrm{C}{\mathrm{o}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ is presented as a function of the site-selected charge doping $\mathrm{[}c=(x--y)/2+\ensuremath{\delta}\ensuremath{-}0.5,x\ensuremath{\le}0.2$ and $y\ensuremath{\le}0.1,0.07l\ensuremath{\delta}l0.84]$, oxygen disorder, and hydrostatic pressure $Pl10\phantom{\rule{0.16em}{0ex}}\mathrm{kbar}$. The single-phase oxygen-ordered orthorhombic phase exhibiting complex ferrimagnetic, antiferromagnetic, and metal-insulator phase transitions was found for a narrow oxygen range around $\ensuremath{\delta}\ensuremath{\sim}0.5\ifmmode\pm\else\textpm\fi{}0.1$. Significant difference between impact of hole $(cg0)$ and electron $(cl0)$ doping was observed depending on the site of cation substitution. Gradual enhancement of the Curie temperature ${T}_{\mathrm{C}}$ was observed over the whole range of $c$ to be unaffected by the local oxygen vacancy disorder. Maximum of the N\'eel temperature ${T}_{\mathrm{N}}$ at $c=0$ was found rapidly disappearing at $c=0.05$ for Ca/Nd substitution while it was maintained for La/Ba substitution, indicating that the oxygen vacancy disorder, especially for $\ensuremath{\delta}g0.5$, has a larger effect on antiferromagnetic phase than the charge doping. The temperature of metal-insulator transition ${T}_{\mathrm{MIT}}$ was found practically unchanged by either charge doping or disorder. The application of hydrostatic pressure slightly suppressed ${T}_{\mathrm{C}}$ and increased ${T}_{\mathrm{N}}$ by stabilization of the antiferromagnetic phase with the largest observed value of $\mathrm{d}{T}_{\mathrm{N}}/\mathrm{d}P=5.75\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{kbar}$. Complex magnetic behavior affected by hydrostatic pressure was accounted for by ferro- and antiferromagnetic interactions resulting from the charge separation and spin transitions.

Correlation between Cr3+ Luminescence and Oxygen Vacancy Disorder in Strontium Titanate under MeV Ion Irradiation

Strontium titanate (SrTiO3), a model system with a strongly correlated electronic structure, has attracted much attention recently because of its outstanding physicochemical properties and considerable potentials for technological applications. The capability to control oxygen vacancy profiles and their effect on valence states of cations will increase significantly the functionality of devices based on transition metal oxides. This work presents new insights into the near-infrared luminescence emission of Cr3+ centers in stoichiometric SrTiO3 induced using 3 MeV protons at temperatures of 100 K, 170 K, and room temperature. The study covers a wide spectral range, including near-infrared, visible, and near-UV regions. Our main purpose is to investigate the role of the oxygen vacancies introduced by energetic charged particles on the shape and yield of induced luminescence spectra, in particular to explore the interplay between the Cr3+ luminescence at 1.55 eV and oxygen disorder. A clear correlation is fo...

Melting of Oxygen Vacancy Order at Oxide-Heterostructure Interface.

Modifications in oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. Although lots of attention has been paid to slight modifications in the rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies have been challenging. Here, we show that a significant modification in the oxygen coordination environments-melting of oxygen vacancy order-is induced at the heterointerface between SrFeO2.5 (SFO) and DyScO3 (DSO). When an oxygen-deficient perovskite (brownmillerite structure) SrFeO2.5 film grows epitaxially on a perovskite DyScO3 substrate, both FeO6 octahedra and FeO4 tetrahedra in the (101)-oriented SrFeO2.5 thin film connect to ScO6 octahedra in DyScO3. As a consequence of accommodating a structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2 nm thick heterointerface region. The stabilized heterointerface structure consists of Fe3+ octahedra with an oxygen vacancy disorder. The melting of the oxygen vacancy order, which in bulk SrFeO2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.

The effect of Sr2 + substitution on the electrical conductivity and chemical stability for cubic perovskite Ba1 − xSrxFe0.9In0.1O3 − δ

The effect of Sr2+ substitution on the electrical conductivity and chemical stability for Ba1−xSrxFe0.9In0.1O3−δ prepared by the Pechini method was investigated. The specimens could be indexed as cubic perovskite structure with disordered oxygen vacancies (0≤x≤0.8). To evaluate the properties of Ba1−xSrxFe0.9In0.1O3−δ, thermogravimetry–differential thermal analysis and electrical conductivity measurements were performed under N2 flow. These experiments revealed that the major charge carrier shifted from oxide ions to electron holes by the partial substitution of Sr2+ into the Ba2+-site. The substitution resulted in an enhanced electrical conductivity; this may be caused by an increase in the mobility of electron holes.

Effects of Oxygen Vacancy Order–Disorder Phase Transition on Air Separation by Perovskite Sorbents

Perovskite oxide sorbents offer promise for adsorption-based separation of air to produce an oxygen enriched stream. The oxygen desorption rate however should be further improved. This work studies the oxygen sorption properties of three La1–xSrxCo1–yFeyO3-δ (LSCF) perovskite oxides with and without oxygen vacancy disorder–order phase transition. The LSCF with lower iron content [La0.1Sr0.9Co0.9Fe0.1O3-δ (LSCF1991)] shows the highest sorption capacity in 500–800 °C. The material with disorder–order phase transition during desorption also exhibits enhanced oxygen desorption rate during the TGA measurement. However, in the fixed-bed desorption process, no obvious rate enhancement resulting from the disorder–order phase transition in oxygen desorption is observed for the LSCF1991 sorbent. The LSCF1991 sample in the fixed-bed quenched at 600 °C after 2 h of desorption shows occurrence of disorder–order phase transition for the perovskite sorbent in the fixed-bed environment. The difference in oxygen desorptio...

Correlation between native defects and morphological, structural and optical properties of ZnO nanostructures

ZnO nanostructures are synthesized using chemical vapor deposition method. Well-distributed ZnO nanowires, nanogrenades and nanoislands are obtained by altering the temperature of joining the reactant gas O2 from 550 to 570 and 600 °C, respectively. The structural properties are derived from the X-ray diffraction patterns and Raman spectra. It is indicated that all the samples are c-oriented wurtzite ZnO. Photoluminescence spectra also indicate that ZnO nanogrenades present fewer native defects because of their lower relative intensities of visible to ultraviolet emissions (1.1) than that of ZnO nanowires (3.1) and nanoislands (1.9). The visible emission of photoluminescence spectrum is divided into green and yellow luminescence which are attributed to defects of oxygen vacancy and surface disorder, respectively. And it is found different proportions of oxygen vacancy and surface disorder of variform ZnO nanostructures.