Arrangement Of Oxygen Vacancies Research Articles

Pseudocapacitive Energy Storage and Electrocatalytic Hydrogen-Evolution Activity of Defect-Ordered Perovskites SrxCa3–xGaMn2O8 (x = 0 and 1)

The diverse range of possible arrangements of oxygen vacancies in oxygen-deficient perovskites results in a variety of structure types and fascinating electrochemical properties. Here, we report Ca...

Resonant X-ray diffraction studies on modulation structures of high temperature superconducting sample Sr<sub>2</sub>CuO<sub>3.4</sub>

<sec>Sr<sub>2</sub>CuO<sub>3+<i>δ</i></sub> is cuprate, a high temperature superconducting (HTS) material that has a single copper oxide layer and a relatively high critical temperature. Its structure is simple and contains fewer atoms, but there are many modulation structures in it. A lot of studies have pointed out that the modulation structure is related to its superconductivity. In order to further study the relationship between modulated structure and superconductivity in Sr<sub>2</sub>CuO<sub>3.4</sub> HTS sample, a new explanation for the formation mechanism of modulation structure is proposed in this paper. </sec><sec>The synchrotron radiation resonant X-ray diffraction (RXD) technique is used to detect the variation of modulation structure near the absorption edge of Cu atom. Cu<sup>2+</sup>, Cu<sup>3+</sup> valence order is detected and used to explain the formation mechanism of modulation structure in Sr<sub>2</sub>CuO<sub>3.4</sub> high temperature superconducting sample. The energy values of incident light are selected to be 8.52, 8.95, 8.98, 9.05, 9.5, and 10.0 keV near the edge of Cu <i>K</i>. The energy resolution is about 1.5 eV. The detector used in the experiment is Mar165 CCD surface detector. The distance from the detector to the sample is about 315 mm. The two-dimensional diffraction pattern recorded by the CCD plane detector is processed by Fit2D software to obtain the diffraction integral intensity. In addition, the energy calibration for each of the copper foil samples is carried out prior to the start of the experiment and in the process of varying energy value.</sec><sec>The experimental results show that the Bragg diffraction peaks corresponding to <i>T</i><sub>c</sub> = 48 K and the modulation structures of <i>Fmmm</i> and <i>Pmmm</i> are visible and calibrated. The intensity of the corresponding (2/5, 4/5, 0) diffraction peak of <i>Fmmm</i> is energy-dependent near the Cu <i>K</i> edge and first increases and then decreases abruptly near the absorption edge. This indicates that a stable ordered arrangement structure of Cu<sup>2+</sup> and Cu<sup>3+</sup> is formed at this time. The weak diffraction signal of this ordered arrangement structure confirms the fact that the copper-O bonding is stronger.</sec><sec>The experiments indicate that oxygen vacancies occupy both the apical position and the CuO<sub>2</sub> plane. The ordering arrangement of oxygen vacancies results in the ordering of copper ions with variable valence. The Cu<sup>2+</sup>, Cu<sup>3+</sup> valence order is related to the superconductivity of Sr<sub>2</sub>CuO<sub>3.4</sub>.</sec>

Dynamics and Thermochemistry of Oxygen Uptake by a Mixed Ce–Pr Oxide

The dynamics of oxygen uptake by mixed Ce0.55Pr0.45O2–x oxide is studied in a pulsed oxygen supply mode using in situ high-temperature heat flow differential scanning calorimetry. It is stated that the oxidation proceeds in two regimes: a fast one at the beginning of the oxidation process, and a slow one, which is controlled by the diffusion of oxygen through the bulk of the solid at the later stages of the process. Analysis of the shape of calorimetric profiles reveals some processes, accompanied by heat release, that occur in the sample in the absence of oxygen in the gas phase. These could be due to both the redistribution of consumed oxygen in the oxide lattice and the lattice relaxation associated with the transformation of phases with different arrangements of oxygen vacancies in them. The heat effect (which diminishes from ~60 to ~40 kJ/mol in the course of oxygen uptake) associated with the oxidation of the reduced form of mixed Ce–Pr oxide, corresponds to the oxidation of praseodymium ions from (3+) to (4+).

Electronic Structure of Oxygen Vacancies in the Orthorhombic Noncentrosymmetric Phase Hf0.5Zr0.5O2

The electronic structure of stoichiometric and oxygen-depleted Hf0.5Zr0.5O2 in the orthorhombic noncentrosymmetric phase has been studied by X-ray photoelectron spectroscopy and quantum-chemical simulation based on the density functional theory. It has been established that the ion-etching-induced peak in the photoelectron emission spectrum with the energy above the top of the o-Hf0.5Zr0.5O2 valence band is due to oxygen vacancies. A method of estimating the density of oxygen vacancies from the comparison of the experimental and theoretical photoelectron spectra of the valence band has been proposed. It has been established that oxygen polyvacancies in o-Hf0.5Zr0.5O2 are not formed: the energetically favorable spatial arrangement of oxygen vacancies in a crystal corresponds to noninteracting oxygen vacancies distant from each other.

Stabilizing a disordered structural modification of barium indate by means of heterogenous doping

The effect inert additions of complex oxide Ba2InNbO6 have on the structure and properties of solid electrolyte Ba2In2O5 is studied. It is shown that composite samples of (1 − x) Ba2In2O5 · xBa2InNbO6 acquire improved electrical properties, due to the stabilization of a highly conductive modification of barium indate characterized by a disordered arrangement of oxygen vacancies.

Metallic filament formation by aligned oxygen vacancies in ZnO-based resistive switches

The electronic structure of ZnO with defects of oxygen vacancies were investigated by using first-principles methods. Some structure models were constructed in order to investigate the effects of the distribution of oxygen vacancies on the electronic properties of ZnO. By analyzing the calculated results, we found that only the aligned oxygen vacancies can form the conducting channel in ZnO, and the transformation of the oxygen vacancy from charged state to neutral state is consistent with the energetics rule of the forming aligned oxygen vacancies. As for the heterojunction of Pt/ZnO/Pt, the oxygen vacancies near the interface of Pt/ZnO depress the local Schottky barrier effectively, and the aligned oxygen vacancies in ZnO form a conducting filament connecting two Pt electrodes. The metallic filament formation in Pt/ZnO/Pt resistive switching cells should be closely related to the carrier injection from Pt electrode into ZnO and the arrangement of oxygen vacancies in ZnO slab.

Role of oxygen vacancies in TiO2-based resistive switches

The effect of oxygen vacancies in TiO2 on the electronic properties of Pt/TiO2/Pt heterojunctions were investigated using first-principles methods. The arrangement of oxygen vacancies in TiO2 slabs is a crucial factor for conducting filament formation. Extended defect induced by the aligned neutral oxygen vacancies results in defect states being located at the Fermi level of the Pt electrodes in Pt/TiO2/Pt heterostructures, and produces a conducting filament connecting two Pt electrodes in the TiO2 film via the oxygen-deficient Ti ions. As oxygen vacancies in intrinsic state, the O-deficient Ti ions, which remain in a high valence state, like those in perfect TiO2, do not contribute to the conducting filament formation. Electronic structure analysis suggests that metallic filament formation in Pt/TiO2/Pt resistive switching cells might be closely related to the charge state and the arrangement of oxygen vacancies in TiO2 films.

Synthesis, structure, and electric properties of oxygen-deficient perovskites Ba3In2ZrO8 and Ba4In2Zr2O11

Perovskite phases Ba3In2ZrO8 and Ba4In2Zr2O11 with the nominal concentration of structural oxygen vacancies 1/9 and 1/12, respectively, were synthesized by solid-phase and solution methods. X-ray diffraction showed cubic symmetry of both phases with the unit cell parameter a = 0.4193(2) and 0.4204(3) nm, respectively. The absence of superstructural lines resulted in the conclusion on statistical arrangement of oxygen vacancies. Thermogravimetry and mass spectrometry proved that both phases can reversibly absorb water from gas phase (pH2O = 2 × 10−2 atm) with observed correlation between the concentration of oxygen vacancies and amount of absorbed water. The total water amount was up to 0.9 mol per formula unit or, if recalculated for perovskite unit ABO3, 0.3 and 0.23 mol H2O, respectively. The temperature curves of coductivity in the atmosphere with various partial water vapor pressures (pH2O = 3 × 10−5 and 2 × 10−2 atm) showed significantly higher conductivity and lower activation energy (0.52 eV) in humid atmosphere due to proton transfer. The proton conductivity is up to 5 × 10−4 Ohm−1 cm−1 at 300°C for Ba3In2ZrO8 specimen. IR spectrometry showed that protons in the structure exist primarily in OH-groups.

Effect of strain and oxygen vacancies on the structure of 180° ferroelectric domain walls in PbTiO3

Abstract In this paper, we study the effect of normal and shear strains and oxygen vacancies on the structure of 180° ferroelectric domain walls in PbTiO3. It is known that oxygen vacancies move to the domain walls and pin them. Hence, we assume a periodic arrangement of oxygen vacancies on both Pb-centered and Ti-centered domain walls in PbTiO3. We use a semi-analytic anharmonic lattice statics method for obtaining the relaxed configurations using a shell potential. In agreement with recent ab initio calculations, we observe that a Pb-centered domain wall with oxygen vacancies is not stable even under strain. Our semi-analytic calculations for PbTiO3 show that oxygen vacancies affect the structure of 180° domain walls significantly but do not have a considerable effect on the thickness of domain walls; they broaden the domain walls by about 50%. We also study the effect of normal and shear strains on both perfect and defective 180° domain walls. We observe that normal and shear strains affect the structure but do not change the domain wall thickness.

Oxygen-vacancy ordering in lanthanide-doped ceria: Dopant-type dependence and structure model

Studies of electron energy loss spectroscopy and selected area electron diffraction (SAED) were systematically performed on 15 and 25 at. % lanthanide (Ln)-doped ceria samples (Ln=Sm, Gd, Dy, and Yb), through which the local ordering of oxygen vacancies that develops with increase in doping level was confirmed in the sequence of (Gd,Sm)>Dy>Yb. Furthermore, a monotone correlation between the development of the ordering and the degradation of ionic conductivity with increasing the doping concentration from 15 to 25 at. % was observed. Based on the analysis of SAED patterns, a structural model for the ordering of oxygen vacancies has been constructed, in which the arrangement of oxygen vacancies is similar to that in C-type Ln2O3 oxides and the 110 pairs of the vacancies are preferred. Then, the factors that can influence the formation of the ordering are discussed.

Structure and physical properties of layered double perovskite NdBaCo2O5.50 + δ (δ ≈ 0.25)

Magnetic, electric, and elastic properties of the crystal and magnetic structure of double layered perovskite NdBaCo2O5.50 + δ are studied by the neutron diffraction method at various temperatures. The data are analyzed using two models of crystal structure. In the first model, the sample consists of two crystal-structure phases with ordered and disordered arrangements of oxygen vacancies. In the second model, a new crystal-structure phase is formed in this compound, which is characterized by ordering of oxygen vacancies in the plane of the rare-earth ion in the 1c crystallographic position (0, 0, 1/2) of space group Pmmm. Two crystal-structure models correspond to different types of magnetic ordering (a mixture of a ferromagnetic phase and a G-type antiferromagnetic phase is presumed in the two-phase crystal-structure model, while a canted antiferromagnetic structure is presumed in the one-phase crystal structure model). The behavior of electric and elastic parameters is better described in the first model, while neutron diffraction studies are in better agreement with the second model.

Crystallographic and magnetic structures of Y 0.8Sr 2.2Mn 2GaO 8−δ: a new vacancy-ordered perovskite structure

The anion-deficient perovskite Y 0.8Sr 2.2Mn 2GaO 8−δ (where δ∼0.1) has been synthesised and the crystal and magnetic structures determined by Rietveld analysis of neutron powder diffraction (NPD) data. The material has body-centred tetragonal symmetry (I4/mmm, a=7.6373(3) Å and c=15.6636(10) Å) and consists of alternating layers of octahedral and tetrahedral polyhedra, the layers being perpendicular to [001]. The octahedral layers are preferentially occupied by manganese and the tetrahedral layers are a mixture of manganese and gallium. The precise cation distribution depends critically on preparative conditions. An unusual structural feature of these materials is the arrangement of oxygen vacancies in the tetrahedral layers: in the basic structure, isolated squares of corner-linked tetrahedra are formed instead of the chains that are observed in brownmillerite phases. Additional oxide ions in this layer probably allow the Mn ions to achieve distorted square pyramidal coordination. Low temperature NPD and magnetisation data indicate antiferromagnetic ordering below 100 K.

States of H +-containing species and proton migration forms in hydrated niobates and tantalates of alkaline-earth metals with a perovskite-related structure

Hydrated niobates and tantalates of alkaline-earth metals Sr 6Ta 2O 11, Sr 6Nb 2O 11, and Ba 4Ca 2Nb 2O 11 with a perovskite-related structure have been investigated by using wide-line 1H nuclear magnetic resonance (NMR), thermogravimetry (TG), conductivity measurements, and IR spectroscopy. TG investigations show that these phases can incorporate up to ∼1 mol of water, which corresponds to the theoretical limit of hydration. A wide-line 1H-NMR study indicates that the proton signal consists of three components, because protons were present in the form of (i) relatively isolated OH groups, (ii) paired OH groups bound with the same metal atom, and (iii) closely spaced OH groups belonging to different octahedra and localized within a defect cluster. The niobates and the tantalates have different ratios of those groups that may be explained by different coordination tendencies of niobium and tantalum and as a consequence a different arrangement of oxygen vacancies. A dehydration scheme may be proposed from a comparison analysis of the TG and 1H-NMR data for partially hydrated samples.

Transport properties of the LaSrNiO4 ± δ ceramics

The temperature dependence of electrical resistivity in LaSrNiO4 ± δ ceramics synthesized using various techniques and subjected to heat treatment is studied. The occurrence of a metal-semiconductor transition is shown to be accounted for by the Anderson carrier localization originating from the random arrangement of oxygen vacancies.

Oxygen vacancy ordering and the incommensurate structure of mullite

A description of the disordered arrangement of oxygen vacancies in a 2(Al2O3)·SiO2 mullite is presented. This description is consistent with the incommensurate diffraction maxima observed in the 1/2 c* reciprocal section and with many broader diffuse diffraction features that have been measured in other reciprocal sections. The structure was modelled using Monte Carlo techniques where the oxygen vacancies were allowed to interact via a set of pair energies. Cations in adjacent tetrahedrally coordinated sites were given displacements that depended only on the local arrangement of these oxygen vacancies. In addition to the 1/2〈110〉 and [110] repulsive interactions which are required to satisfy cation bonding requirements, it was found that unequal vacancy repulsive interactions were required along 〈100〉 and 〈010〉. Attractive vacancy-vacancy interactions along 1/2〈112〉 and 〈001〉 were also necessary but in the latter case the magnitude of the interaction is such that the probability of having an 〈001〉 vacancy pair was near that of a random vacancy distribution.

Superstructure of a NASICON-related compound NH 4Ti 2P 3O 12

Ion-pathways in a NASICON-related compound NH 4Ti 2P 3O 12 (hexagonal, a = 0.832 and c = 2.34 nm) are directly observed by means of high-resolution transmission electron microscopy. When the composition changes to Ti 4P 6O 23 due to the deammoniation as well as dehydration of heating at 770°C, a supercell with the dimensions of A ⋍ 2a and C ⋍ 2c is constructed. A model of the superstructure is proposed based on the arrangement of oxygen vacancies as well as the induced distortion of the TiO 6 octahedra, retaining the three-dimensional network in the starting structure.