Oxygen Occupancy Research Articles

Construction of Surface Ruoct─O─Cooct Units With Optimized Cooct Spin States for Enhanced Oxygen Reduction and Evolution.

The introduction of noble metal into spinel structure is an effective strategy to develop efficient oxygen evolution/reduction reaction (OER/ORR) catalysts. Herein, surface Cooct is substituted by Ruoct in Rux-Mn0.5Co2.5-xO4/NCNTs by ion-exchange, where presence of Ruoct─O─Cooct unit facilitates electron transfer. This strong electron coupling effect leads downward shift in d-band center and a narrowing of d-p bandgap. The increased charge density of Cooct bridged with Ruoct dioxygen optimizes adsorption of oxygen intermediates (*OH) and occupation of electrons in eg-orbital octahedral. The measured ORR/OER voltage difference is only 0.71V. The peak power density of assembled zinc-air battery reaches 148.8mW h cm-2, and energy density at 100mA cm-2 reaches 813.6mA h gZn -1, approaching a theoretical value of 820mA h gZn -1. The catalyst demonstrates stable operation for over 500h at 10mA cm-2 and over 200h at 50mA cm-2. This work provides new insights to guide fabrication of advanced oxygen electrocatalysts.

A novel route to synthesize B6O powder via reaction between CaB6 and B2O3

AbstractA novel approach was applied to synthesize B6O using CaB6 and B2O3 powders as reactants. CaB6 and B2O3 were reacted to generate B6O and CaB2O4 at 1573 K, and the by‐product CaB2O4 was removed by acid leaching. It was found that the samples prepared in this work were oxygen deficient (B6Ox, x ≈ .76–.77). The optimal molar ratio of CaB6 to B2O3 was determined to be 3:5, and excess B2O3 could not lead to a change in the value of oxygen occupancy. Scanning electron microscope images showed that regular holes similar to the morphologies of CaB6 particles were exhibited in the final samples, which could be explained by the diffusion of CaB6 during the reaction process.

Subsurface oxygen at transition metal surfaces: Its direct atom-resolved imaging and role in metal oxidation

Clarifying how oxygen is incorporated into the subsurface region of metal surfaces is a crucial step towards a robust understanding of the initial oxidation of metals. Due to lack of direct atomic-resolution imaging of subsurface oxygen, the atomistic mechanisms of oxygen-metal interactions and the role of subsurface oxygen remain elusive. Here, by state-of-the-art transmission electron microscopy, we demonstrate a direct atom-resolved imaging of subsurface oxygen that is dissolved in the subsurface region of the Fe (001) and Ti (0001) surfaces. It is found that subsurface oxygen occupies the octahedral sites of the Fe and Ti lattices, rather than the tetrahedral sites. First-principles calculations reveal that the octahedral-site occupancy of subsurface oxygen is energetically more favorable compared to the tetrahedral-site occupancy and will facilitate the process of metal oxidation by reducing the formation energy of metal oxides. Our findings clarify the atomistic mechanism of how oxygen attacks metal surfaces for the initiation of oxidation that occurs everywhere in nature.

Impact of the electrospinning synthesis route on the structural and electrocatalytic features of the LSCF (La0.6Sr0.4Co0.2Fe0.8O3–δ) perovskite for application in solid oxide fuel cells

In-house electrospun La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) nanofibers have been tested through synchrotron x-ray diffraction and electrochemical impedance spectroscopy (EIS) in the 823–1173 K range, namely in the operating window of intermediate-temperature solid oxide fuel cells. Identical tests have been carried out on commercial LSCF powders, as a control sample. The results demonstrate that the electrospinning manufacturing procedure influences the crystalline properties of the perovskite. The rhombohedral structure (R), stable at room temperature, is retained by nanofibers throughout the whole temperature range, while a rhombohedral to cubic transition (R→C) is detected in powders at ⁓1023 K as a discontinuity in the rhombohedral angle α, accompanied by an abrupt change in oxygen occupation and microstrain. EIS data have a single trend in the nanofibers Arrhenius plot, and two different ones in powders, separated by a discontinuity at the structural transition temperature. Thus, a striking parallel is demonstrated between the variation with temperature of crystallographic features and electrochemical performance. Interestingly, this parallel is found for both nanofiber and granular electrodes. This opens up questions and new perspectives in attributing activation energies derived from EIS tests of LSCF materials to electrochemical processes and/or crystal structure variations.

Theoretical study on the element distribution characteristics and the effects of oxygen in TiZrHfNb high entropy alloys

Short range order and oxygen both have great effects on the mechanical properties like yield strength and ductility of refractory high entropy alloys. In this work, we studied the element distribution characteristics and oxygen occupation properties in TiZrHfNb high-entropy alloy based on first principles, molecular dynamics and Monte Carlo calculations. Two thousands of TiZrHfNb crystal models were firstly calculated based on first principles calculations to analyze the element distribution characteristics. The Warren-Cowley parameters were then evaluated, the Nb-Nb and Ti-Zr atomic pairs tend to form in TiZrHfNb. Hybrid of molecular dynamics and Monte Carlo calculations at 300 K and 1000 K are further carried out to study the possible short range order and phase transition in TiZrHfNb. The element distributions are not uniform, and the transition from body-centered cubic to hexagonal close packed structures are observed. The oxygen atoms are then placed into the enriched regions of Ti, Zr, Hf, and Nb in high-entropy alloy, respectively. The ab initio molecular dynamics calculations of these four configurations are carried out and the diffusion coefficients are evaluated. The oxygen owns the largest diffusion coefficient in the Nb-rich zone. Furthermore, the tensile simulations reveal the great effects of element distributions and oxygen occupations on the mechanical properties of TiZrHfNb. These findings will help us to understand the short range order characteristic and oxygen occupation effects on the phase stability and to regulate the mechanical properties of TiZrHfNb high-entropy alloy.

Origin of high stability, enhanced specific capacity, and low Li diffusion energy in boron doped Li3V2(PO4)3

Li3V2(PO4)3 (LVP) is a well-known cathode material of Li-ion batteries, whereas the specific capacity is limited by the maximum Lithium (Li) de-intercalation probability associated with vacancy energy. The reason behind this limitation is not known yet, which needs to be address, and the modification needs to be done in the electronic structure to get a more specific capacity. In this work, using First-principles calculation, we have found that Li1 (the tetrahedra geometry) is having a higher Li vacancy energy as compared to Li2 and Li3, which limits the Li de-intercalation below x = 1.8, (where x is Li concentration). The change in bandgap of LVP structure after boron (B) substitution is directly correlated to the change in the oxygen (O) occupancy due to B substitution. As of comparing formation energy plot of pristine LVP and B substituted LVBP structure, we have found that in case of LVBP structure formation of x > 2 Li de-intercalation is possible, which help to enhance the specific capacity up to 205 mAh/g. Overall, we identify the cause of the less specific capacity of LVP and provide the solution by suggestion B doping, which also provides less Li diffusion barrier energy.

Effect of vacancy defects and hydrogen, oxygen and nitrogen atomic occupation on the mechanical and electronic properties of C40-type VSi2

To improve the inherent brittle and mechanical properties of VSi2, the effect of vacancy defects and Hydrogen, Oxygen and Nitrogen atomic occupation on the physical properties of C40-type VSi2 silicide have been explored using first-principles methods. Two vacancy models (V-Va, Si-Va) and six atom occupation models (H-V, H-Si, O-V, O-Si, N-V, N-Si) are selected for assessment. According to the results of thermodynamic parameters, C40-type VSi2 with different models exhibits the structural stability. Due to vacancy defects and atomic occupations, the deformation resistance and elastic hardness of the original C40-VSi2 decreased. B/G (1.845) and Poisson's ratio v (0.27) of H occupation for V-Va site are larger than the critical points. The presence of vacancy defects and atomic occupations causes VSi2 to achieve the brittle-to-ductile transition. The Young's modulus anisotropy of these silicides was described by observing the three-dimensional surface structure and two-dimensional projection maps. The density of state graphs demonstrated that these binary silicides display metallic properties. Moreover, the phase stability of different VSi2 can be further explained by the electronic structure.

Long- and short-range orders in 10-component compositionally complex ceramics

Neutron diffraction and total scattering are combined to investigate a series of single-phase 10-component compositionally complex fluorite-based oxides, [(Pr 0.375 Nd 0.375 Yb 0.25 ) 2 (Ti 0.5 Hf 0.25 Zr 0.25 ) 2 O 7 ] 1- x [(DyHoErNb)O 7 ] x , denoted as 10CCFBO x Nb . A long-range order-disorder transition (ODT) occurs at x = 0.81 ± 0.01 from the ordered pyrochlore to disordered defect fluorite. In contrast to ternary oxides, this ODT occurs abruptly without an observable two-phase region; moreover, the phase stability in 10CCFBOs deviates from the well-established criteria for simpler oxides. Rietveld refinements of neutron diffraction patterns suggest that this ODT occurs via the migration oxygen anions from the position 48 f to 8 a , with a small final jump at the ODT; however, the 8a oxygen occupancy changes gradually (without an observable discontinuous jump). We further discover diffuse scattering in Nb-rich compositions, which suggests the presence of short-range order. Using small-box modelling, four compositions near ODT ( x = 0.75, 0.8, 0.85 and 1) can be better fitted by C 222 1 weberite ordering for the local polyhedral structure at nanoscale. Interestingly, 10CCFBO 0.75Nb and 10CCFBO 0.8Nb possess both long-range pyrochlore order and short-range weberite-type order, which can be understood from severe local distortion of the pyrochlore polyhedral structure. Thus, weberite-type short-range order emerges before the ODT, coexisting and interacting with long-range pyrochlore order. After the ODT, the long-range pyrochlore order vanishes but the short-range weberite-type order persists in the long-range disordered defect fluorite structure. Notably, a drop in the thermal conductivity coincides with emergence of the short-range order, instead of the long-range ODT. A Short Text: Neutron diffraction and total scattering were used to characterize long- and short-range orders in 10-component compositionally complex ceramics. An unusual long-range order-disorder transition (ODT) from disordered defect fluorite to ordered pyrochlore occurs, which competes with emergence of weberite-type short-range ordering. Interestingly, reduced thermal conductivity coincides with the rise of short-range ordering, instead of occurrence of the long-range ODT. • Neutron diffraction and total scattering are combined to investigate 10-component oxides. • A long-range order-disorder transition (ODT) from pyrochlore to fluorite is observed. • This long-range ODT occurs abruptly without an observable two-phase region. • Long-range pyrochlore order and short-range weberite-type order can co-exist and interact. • A drop in the thermal conductivity is coincident with emergence of the short-range order.

Resolving the polar interface of infinite-layer nickelate thin films.

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between Nd1-xSrxNiO2 and SrTiO3. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O3 layer. Density functional theory calculations with a Hubbard U term show how the observed structure alleviates the polar discontinuity. We explore the effects of oxygen occupancy, hole doping and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the non-trivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

Understanding Structural Incorporation of Oxygen Vacancies in Perovskite Cobaltite Films and Potential Consequences for Electrocatalysis

Owing to their excellent mixed-ionic and electronic conductivity, fast oxygen kinetics, and cost efficiency, layered oxygen-deficient perovskite oxides hold great potential as highly efficient cathodes for solid oxide fuel cells and anodes for water oxidation. Under working conditions, cation ordering is believed to substantially enhance oxygen diffusion while maintaining structural stability owing to the formation of double perovskite (DP), thus attracting extensive research attention. In contrast, the incorporation of oxygen vacancies and the associated vacancy ordering have rarely been studied at the atomic scale, despite their decisive roles in regulating the electronic and spin structures as well as in differentiating the crystal structure from DP. Here, atomic-resolution transmission electron microscopy is used to directly image oxygen vacancies and measure their concentration in (Pr,Ba)CoO3-δ films grown on SrTiO3 substrates. We find that accompanied by the presence of oxygen vacancy ordering at Co–O planes, the A–O (A = Pr/Ba) planes also exhibit a breathing-like lattice modulation. Specifically, as confirmed by first-principle calculations, the AO–AO interplanar spacings are found to be linearly correlated with the vacancy concentration in the enclosing Co–O planes. On this basis, potential consequences of oxygen occupancy for the catalytic properties of structurally pure PBCO phases are discussed. Through establishing a simple correlation of oxygen concentration with the easily achievable lattice measurement, our results pave a way for better understanding the structure–performance relationship of oxygen-deficient complex cobaltites used for electrocatalysis.

Introducing C phase in additively manufactured Ti-6Al-4V: A new oxygen-stabilized face-centred cubic solid solution with improved mechanical properties

An oxygen-rich face-centred cubic (FCC) Ti phase was engineered in the microstructure of a Ti-6Al-4V alloy via additive manufacturing using laser powder bed fusion. Designated 'C', this oxygen-rich FCC phase has a lattice parameter of 0.406 nm and exhibits an orientation relationship with the parent α′ phase as follows: (0001)α′//{111}C, and 〈12¯10〉α′ //〈11¯0〉C. We propose that the formation of the C phase is facilitated by the combined effect of thermal gradients, deformation induced by the martensitic transformation, and local O enrichment. This enables an in-situ phase transformation from the hexagonal close-packed α′ phase to the C phase at elevated temperatures. Our density functional theory calculations indicate that oxygen occupancy in the octahedral interstices of the FCC structure is energetically preferred to corresponding sites in the α′ phase. The in-situ mechanical testing results indicate that the presence of the FCC phase significantly increases the local yield strength from 1.2 GPa for samples with only the α′ phase to 1.9 GPa for samples comprising approximately equal volume fractions of the α′ and FCC phases. No loss of ductility was reported, demonstrating great potential for strengthening and work hardening. We discuss the formation mechanism of the FCC phase and a pathway for future microstructural design of titanium alloys by additive manufacturing.

Crystalline structure, oxygen defects and textural properties of the CeO2/SBA-15 nanomaterials

In this study, a series of CeO2/SBA-15 nanomaterials were synthesized by varying the CeO2 content. These materials were characterized by different techniques to determine their textural, surface chemical and crystalline properties. Crystallite size and oxygen defects were determined by Microanalysis Using X-Ray Diffraction (MAUD) software based on the Rietveld refinement method. The results showed that the crystallite size slightly increased from 5.6 to 8.4 nm and the surface area decreased from 620 to 370 m2/g as CeO2 content increased from 5 to 10, 15 to 20wt%. The oxygen occupancy that was associated with oxygen defects was found in the crystalline structure of CeO2. The defect concentration was between 6.18 and 14.42%, depending on the ceria content and dispersion. This study confirmed that the CeO2/SBA-15 nanomaterials have larger surface area, highly dispersed CeO2 nanoparticles, and a big number of oxygen defects in the crystalline structure, which can be used in different catalytic reactions.

Structural Stability and Phase Transitions in Zeolite A: An In Situ High Pressure-High Temperature Investigation.

The high pressure-high temperature structural stability of Zeolite A (ZA) has been studied using the X-ray diffraction (XRD) method. Structural studies at high temperatures show a reduction in the oxygen occupancy, belonging to the water molecule, indicating thermal dehydration and subsequent expulsion of water molecules from the pores of the structure. ZA does not undergo structural phase transition with temperature. However, structural transitions are observed in in situ XRD studies at high pressure and high temperature. At 1.3 GPa and 300 °C, the cubic ZA concomitantly transformed to cubic sodalite (SOD) and tetragonal zeolite NaP (ZNP). This transition was completely forbidden at 2.7 GPa, where a temperature-induced amorphization was favored at 250 °C. The thermal studies at higher pressure reveal the marginal influence of pressure on the thermal expansion coefficients of hydrated ZA. Pressure evolution of the high pressure-high temperature phases indicates no further phase transitions up to 5.9 GPa. The equation of state fit to the pressure-volume data of these phases show that ZNP is less compressible, followed by SOD and ZA. In contrast to the behavior at 0.1 MPa, SOD shows a pressure-induced negative thermal expansion (NTE) at 5.9 GPa. On the other hand, the positive thermal expansion (PTE) observed along the direction of c axis is compensated by the NTE along the a axis leading to a negligible volume thermal expansion for the ZNP structure. The bulk moduli and thermal expansion coefficients of all of the observed phases are reported. The outcomes of this study have been consolidated as a pressure-temperature phase diagram, which provides an insight into the technological and industrial applications of ZA at extreme conditions.

Structural analysis and oxygen defects in the partially oxygen-reduced T′-Pr2-xCexCuO4+δ with x = 0 and 0.15 powders at the normal state

Structural analysis and observation of the oxygen migration of Pr2-xCexCuO4+δ (PCCO) with Ce-doping concentration, x, of 0 and 0.15 powders are briefly studied through neutron diffraction (ND) patterns and x-ray absorption spectroscopy (XAS) spectrum in the extended x-ray absorption fine structure (EXAFS) range. The PCCO powders are synthesized using the chemical dissolved technique and time-dependent oxygen-reduction annealing in flowing Ar atmosphere. All powders show a stable PCCO phase with tetragonal T′-type crystal structure (space group of I4/mmm) for both the as-prepared and partially oxygen-reduced PCCO powders at room temperature. Occupancy of the apical oxygen, O(3), is detected higher in the doped powders due to Ce-doping effect and generally increases after the shorter-time oxygen-reduction annealing. The decrease of O(3) occupancy is observed for the longer-time reduction annealing process indicating a trace of oxygen migration and recovery of oxygen defect on CuO2 plane in the T′-structure. This study is essential to elucidate the mechanism of oxygen removal in the electron-doped cuprates.

Modulating oxygen electronic orbital occupancy of Cr-based MXenes via transition metal adsorbing for optimal HER activity

Modulating the surface electronic properties of the 2D MXenes is of significant importance to boost their hydrogen evolution reaction (HER) activity. Herein, a series of transition metal adatoms are employed to tune the surface electronic properties of Cr-based MXenes with oxygen function group for realizing impressive HER performance. The Results show that the charge of surface oxygen atoms, which is affected by both the host metal atoms and the adsorbed transition metal atoms, play critical role in the adsorption strength of hydrogen. The optimal performance is achieved by depositing Cr atom on the Cr2TiC2O2 MXene, which results in the adsorption free energy of hydrogen very close to zero (0.03 eV). Systematic electronic structure analyses confirm that the charge transfer from the adsorbed transition metals to the neighboring surface oxygen atoms could tune the orbital occupancy of oxygen and their adsorption strength to hydrogen atom and therefore the HER activity. These findings and concepts may be useful for the design of advanced MXene-based HER catalysts.

Possible Defect Recovery in T’-Pr<sub>2-<i>x</i></sub>Ce<i><sub>x</sub></i>CuO<sub>4</sub> with <i>x</i> = 0.10 Nanoparticles Analyzed by Neutron Diffraction

We report the possible existence of defect recovery and the magnetic behavior in the T’-Pr2-xCexCuO4 (T’-PCCO) with x = 0.10 nanoparticles through the partially reduction annealing process. The powders of T’-PCCO nanoparticles were synthesized by using the chemically dissolved method followed by partially reduction annealing at 700 °C for 5 h in argon atmosphere. The high-resolution neutron powder diffraction (HRPD) technique has been employed to study the nuclear structure, vacancy, and the magnetic properties of the T’-PCCO nanoparticles. It is found that there is an increase of oxygen occupancy at the in-plane oxygen, O(1), and the apical oxygen, O(3), which signifies the decrease of the number of the vacancy on their sites. Meanwhile, the out plane oxygen, O(2), seems to be unchanged in the partially reduced samples. The Fourier difference profile shows an enhancement of the neutron scattering density at all the critical sites of O(1), O(3), and the Cu site. This may lead to the idea of the defect recovery affecting the whole magnetic moments which is responsible for the absence of weak ferromagnetism in the T’-PCCO nanoparticles.

The vacancy defects and oxygen atoms occupation effects on mechanical and electronic properties of Mo5Si3 silicides

Improving brittle behavior and mechanical properties is still a big challenge for high-temperature structural materials. By means of first-principles calculations, in this paper, we systematically investigate the effect of vacancy and oxygen occupation on the elastic properties and brittle-or-ductile behavior on Mo5Si3. Four vacancies (Si–Va1, Si–Va2, Mo–Va1, Mo–Va2) and oxygen occupation models (O–Mo1, O–Mo2, O–Si1, O–Si2) are selected for research. It is found that Mo–Va2 vacancy has the stronger structural stability in the ground state in comparison with other vacancies. Besides, the deformation resistance and hardness of the parent Mo5Si3 are weakened due to the introduction of different vacancy defects and oxygen occupation. The ratio of B/G indicates that oxygen atoms occupation and vacancy defects result in brittle-to-ductile transition for Mo5Si3. These vacancies and the oxygen atoms occupation change the localized hybridization between Mo–Si and Mo–Mo atoms. The weaker O–Mo bond is a contributing factor for the excellent ductile behavior in the O-Si2 model for Mo5Si3.

Investigation of Structural and Thermal Evolution in Novel Layered Perovskite NdSrMn2O5+δ via Neutron Powder Diffraction and Thermogravimetric Analysis

Neutron diffraction is one of the best methods for structural analysis of a complex, layered perovskite material with low symmetry by accurately detecting the oxygen positions through octahedral tilting. In this research, the crystal structure of NdSrMn2O5+δ was identified through X-ray diffraction (XRD) and neutron powder diffraction (NPD) at room temperature (RT), which indicated the formation of a layered structure in orthorhombic symmetry in the Pmmm (no. 47) space group. Rietveld refinement of the neutron diffraction data has confirmed the orthorhombic symmetry with unit cell parameters (a = 3.8367 (1) Å, b = 3.8643 (2) Å, and c = 7.7126 (1) Å), atomic positions, and oxygen occupancy. Thermogravimetric analysis revealed the total weight loss of about 0.10% for 20–950°C temperature, which occurred mainly to create oxygen vacancies at high temperatures. Rietveld analyses concurred with the XRD and neutron data allowing correlation of occupancy factors of the oxygen sites.

Effects of Ru doping and of oxygen vacancies on the optical properties in α-Fe2O3 powders

Fe2−xRuxO3 (0 ≤ x≤ 0.08) powders were prepared by solid-state method. X-ray diffraction patterns revealed that the Fe2O3 powders crystallize in a hexagonal structure typical of α-Fe2O3 phase (hematite). The crystallite size was found to decrease from 141 to 84 nm with increasing Ru content from 0 to 0.08, respectively. UV–visible absorption study was performed at room temperature. The direct energy band gap of un-doped α-Fe2O3 was 2.22 eV and found to increase with increasing Ru concentration to reach 2.45 eV for x = 0.08. This increase in band gap was attributed to the decrease in oxygen vacancies with increasing Ru as indicated by calculations of the oxygen occupancy rate using Rietveld refinement. Other properties such as the absorption coefficient, refractive index, the extinction coefficient, real and imaginary dielectric constant, real and imaginary electrical conductivity were calculated using spectrophotometric measurements.

Effect of Substitution Sr Cations on the Structure in the Gd<sub>1</sub>(Ba<sub>2</sub><sub>−<i>x</i></sub>Sr<sub><i>x</i></sub>)Cu<sub>3</sub>O<sub>7−<i>δ</i></sub> Phases

In an effort to improve the performance of superconductors in the field and high temperatures it is important to study the superconducting mechanism. For this reason, the cation substitution can be conducted. One of the high Tc superconductors Gd1Ba2Cu3O7−δ phase with Sr substitution has been synthesized, i.e. Gd1(Ba2−xSrx)Cu3O7−δ compound. The sample was synthesized by using a solid-state reaction method with a wet mixing, sintered for 12 hours at temperature 900°C. The synthesis results are characterized by using XRD. The results of Match-3 software analysis showed high (higher 85%) Gd1Ba2Cu3O7−δ phase was formed. The Sr substitution causes changes to the structure, i.e. the lattice parameters a, b and c, where the orthorhombicity tends to decrease with increasing Sr content. Refinement results show that based on the oxygen occupancy, the total oxygen content tends to increase.