Displacements Of Oxygen Ions Research Articles

Monitoring the formation of infinite-layer transition metal oxides through in situ atomic-resolution electron microscopy.

Infinite-layer transition metal oxides with two-dimensional oxygen coordination exhibit intriguing electronic and magnetic properties due to strong in-plane orbital hybridization. The synthesis of this distinctive structure has primarily relied on kinetically controlled reduction of oxygen-rich phases featuring three-dimensional polyhedral oxygen coordination. Here, using in situ atomic-resolution electron microscopy, we scrutinize the intricate atomic-scale mechanisms of oxygen conduction leading to the transformation of SrFeO2.5 to infinite-layer SrFeO2. The oxygen release is highly anisotropic and governed by the lattice reorientation aligning the fast diffusion channels towards the outlet, which is facilitated by cooperative yet shuffle displacements of iron and oxygen ions. Accompanied with the oxygen release, the three-dimensional to two-dimensional reconfiguration of oxygen is facilitated by the lattice flexibility of FeOx polyhedral layers, adopting multiple discrete transient states following the sequence determined by the least energy-costing pathways. Similar transformation mechanism may operate in cuprate and nickelate superconductors, which are isostructural with SrFeO2.

Electrical properties of Bi2.99(Li0.5Sm0.5)0.01Ti1-x(Mn1/3Ta2/3)xNbO9 high-temperature piezoelectric ceramics via oxygen vacancy defects tailoring

Tribismuth titanium niobium oxide (B3TiNbO9, BTNO) is one of the Aurivillius phases with the highest Curie temperature (Tc), but its piezoelectric coefficient (d33) is poor. When x = 0.045, high-temperature piezoelectric ceramics with d33 of 11.9 pC/N and Tc of 916.7 °C were prepared by (Mn1/3Ta2/3)(4+ω)+ modification of Ti4+ at B-site of Bi2.99(Li0.5Sm0.5)0.01Ti1-x(Mn1/3Ta2/3)xNbO9. The substitution will aggravate the distortion of BO6 octahedron due to the displacement of oxygen ions. The existence of Mn3+ in the dopant will act as an oxygen vacancy trap and then increase the resistivity. As a result, BTNO ceramics have enhanced piezoelectric properties as well as temperature and cycle stability. This work provides a theoretical basis for better high-temperature piezoelectric properties of A/B co-doped Aurivillius phase ceramics in the future.

Multiferroic order parameters in rhombic antiferromagnets RCrO3

Currently, active research is aimed at perovskite—based oxides, including rare earth orthochromites, which exhibit magnetoelectric properties owed to intrinsic magnetic interactions in external electric and magnetic fields. Due to a variety of structural instabilities and couplings in these materials, understanding the underlying magnetoelectric mechanisms is a challenge. In this paper, we explore magnetoelectric properties of the rare earth orthochromites in the framework of symmetry analysis. Our calculations show the presence in RCrO3 of electric dipole moments localized in the vicinity of Cr3+ ions. The electric dipole moments, appearing due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase, are arranged in an antiferroelectric mode. We have demonstrated the presence of electric dipole moments in the unit cell of RCrO3, localized in the vicinity of Cr3+ ions. The inversion symmetry breaks due to the displacements of oxygen ions from their highly symmetric positions in the parent perovskite phase, the electric dipoles become arranged in an antiferroelectric mode. We have introduced the basic distortive order parameters in consistence with the symmetry of RCrO3: the polar order parameters ( D , Q 2, Q 3, P ) and the axial order parameter Ω b and classified them according to the irreducible representations of the RCrO3 symmetry group (D 2h 16). We have determined the symmetry—allowed couplings between distortive, ferroelectric and magnetic orderings and found possible exchange-coupled magnetic and ferroelectric structures. The presented analysis makes it possible to explain experimentally observed polarization reversal and the concomitant reorientation of spins in a series of RCrO3 compounds and to predict the possible scenarios of phase transitions in RCrO3.

Thermodiffusion and ion transport in doped ceria by molecular dynamics simulations

We simulated thermodiffusion and ionic conductivity in doped ceria by molecular dynamics. Four approaches to estimate the ionic conductivity were applied using the auto-correlation function and collective displacement of oxygen ions, the tracer diffusion coefficient from the mean squared displacement, and the displacement in an electric field. The Green-Kubo method was applied to determine the mass-heat coupling cross-coefficients LOq and LqO of the Onsager transport matrix. This is the first calculation of these coefficients in an ionic compound. The simulated parameters are negative throughout the applied temperature range suggesting net transport of anions from a cold to a hot region in a temperature gradient. The results emphasize that the coupling between mass and heat transport in an oxygen ion electrolyte could have an impact for practical applications.

Oxygen-redox reactions in LiCoO2 cathode without O–O bonding during charge-discharge

Summary Oxygen activity in highly delithiated LiCoO2 is critical to fully utilizing the energy density of this high-tap-density cathode but still lacks a clear understanding. In this work, we combined the results of several experimental techniques, especially resonant inelastic X-ray scattering (RIXS) and neutron pair distribution function (NPDF) analysis, together with theoretical calculations to study this topic. Our results conclude that oxygen redox takes place globally in the lattice, rather than forming localized dimerization as previously thought. RIXS results directly reveal the reversible oxygen redox, and NPDF results show that the O–O pair distance is considerably shortened in the highly delithiated LiCoO2. Theoretical calculations indicate that no O–O bonding is formed in LiCoO2, in sharp contrast to the lithium-rich system in which O–O bonding does form. These results provide the rationale for achieving a reversible deep delithiation and high energy density for LiCoO2-based electrodes.

Giant Thermal Enhancement of the Electric Polarization in Ferrimagnetic BiFe_{1-x}Co_{x}O_{3} Solid Solutions near Room Temperature.

Thermal excitations typically reduce the electric polarization in ferroelectric materials. Here, we show by means of first-principles calculations that multiferroic BiFe_{1-x}Co_{x}O_{3} solid solutions with 0.25≤x≤0.50 (BFCO) represent a noteworthy exception to this behavior. In particular, we find that, at room temperature and for moderate pressures of 0.1-1.0GPa, depending on the composition, the electric polarization of bulk BFCO increases by ∼150%. The origin of such an exceptional behavior is a phase transformation involving a low-T rhombohedral (R) phase and a high-T supertetragonal (T) phase. Both R and T phases are ferrimagnetic near room temperature with an approximate net magnetization of 0.13 μ_{B} per formula unit. Contrary to what occurs in either bulk BiFeO_{3} or BiCoO_{3}, the T phase is stabilized over the R by increasing temperature due to its higher vibrational entropy. This extraordinary T-induced R→T phase transition is originated by polar phonon modes that involve concerted displacements of transition-metal and oxygen ions.

Surfactant assisted synthesis of rare earth Dy3+ substituted MnFe2O4 nanoparticles

In this study, we substituted the magnetic Fe3+ ions with highly paramagnetic rare earth Dy3+ cations in MnFe2O4 spinel ferrite nanoparticles to investigate the physical, structural, spectral and electrical properties in detail. Dysprosium substituted inverse spinel ferrite nanoparticles (DyxMnFe2-xO4) (0 ≤ x ≥ 0.16) were prepared via facile optimized wet chemical approach i.e. micro-emulsion route. Substituted and unsubstituted manganese ferrite nanoparticles were annealed at 400 °C for 4 h under vacuum. X-ray diffraction (XRD) analysis proved the spinel structure of all compositions of DyxMnFe2-xO4 nanoparticles. Various physical parameters were calculated from XRD data. Quantitative measurements of cations distribution were carried out to estimate the distribution of divalent metal cations (Mn2+) and trivalent metal cations (Fe3+ and Dy3+) between A-sites (tetrahedral) and B-sites (octahedral). Oxygen parameter ‘u’ was calculated to study the displacement of oxygen ions in tetrahedral sites along (111) direction. Fourier transform infrared spectra (FTIR) were recorded to study the two significant absorption bands corresponding to A-site and B-site. Scanning electron microscopic (SEM) analysis was carried out to study the morphology of DyxMnFe2-xO4 nanoparticles. After structural and morphological investigations, all compositions of DyxMnFe2-xO4 nanoparticles were subjected to dielectric parameters measurements. The dielectric parameters like dielectric constant (real and imaginary part) and tangent loss were investigated as a function of frequency from 1 MHz to 3 GHz at room temperature.

A new approach to modeling TiO2−x-based memristors using molecular dynamics simulation

This study aims to investigate the oxygen ion migration in defect rutile titanium dioxide (TiO2−x) in the presence of oxygen and titanium vacancies using a new approach based on molecular dynamic simulation. In this approach, the force field models along with Buckingham and Columbic potentials are used. For this purpose, the simulation is conducted in two different phases. In the first phase, the effect of temperature on the mean square displacements of oxygen ions is examined; besides, the diffusion and ionic conductivity of oxygen are studied in the absence of the electrical field. The results reveal that in the temperature range of 1100–1900 K, the oxygen vacancies tend to form clusters. These clusters consist of two oxygen vacancies. In the second phase, the memristor is applied to the model and hysteresis loops are studied in five cycles. The results of the second phase show that the new model correctly predicts the migration behavior of the oxygen ions in the memristor configuration. In conclusion, compared to the models using the Langevin equation method, the proposed model provides information which is closer to reality. Furthermore, the mean square displacements of the oxygen ions versus time are studied in the presence and absence of the electrical field at 300 K, and the results indicate greater diffusion in the presence of the electrical field.

Insight into the structure and functional application of Mg-doped Na0.5Bi0.5TiO3 electrolyte for solid oxide fuel cells

Na0.5Bi0.5TiO3 (NBT) and its acceptor-doped perovskites, as lead-free piezoelectric materials before, have been found to be excellent oxygen-ion conductors with potential applications in intermediate-temperature solid oxide fuel cells (SOFCs). Among them, the Bi deficient and B-site Mg doped sodium bismuth titanate (Na0.5Bi0.49Ti0.98Mg0.02O2.965, NBTM) exhibits a remarkable oxygen ionic conductivity. Here, we report a structural analysis on NBT and NBTM ceramics using high-temperature in situ neutron diffraction (NPD) to gain insight into the effects of Mg doping on ionic conductivity and structure. Both perovskite oxides exhibit two consecutive phase transformations from R3c to P4bm at ∼400 °C and from P4bm to Pm3¯m at ∼600 °C. The effects of Bi deficiency and Mg substitution on oxygen vacancy concentration and ion displacements are revealed. In addition, a NBTM supported single cell consisting of Ag-NBTM cathode and Ni-NBTM anode is fabricated to evaluate the possibility of its application in intermediate-temperature SOFCs. The structural stability of NBTM is still a major concern that hinders its utilization at high temperature and under reducing atmosphere.

Transport of oxygen ions in Er doped La2Mo2O9 oxide ion conductors: Correlation with microscopic length scales

We report oxygen ion transport in La2−xErxMo2O9 (0.05 ≤ x ≤ 0.25) oxide ion conductors. We have measured conductivity and dielectric spectra at different temperatures in a wide frequency range. The mean square displacement and spatial extent of non-random sub-diffusive regions are estimated from the conductivity spectra and dielectric spectra, respectively, using linear response theory. The composition dependence of the conductivity is observed to be similar to that of the spatial extent of non-random sub-diffusive regions. The behavior of the composition dependence of the mean square displacement of oxygen ions is opposite to that of the conductivity. The attempt frequency estimated from the analysis of the electric modulus agrees well with that obtained from the Raman spectra analysis. The full Rietveld refinement of X-ray diffraction data of the samples is performed to estimate the distance between different oxygen lattice sites. The results obtained from such analysis confirm the ion hopping within the spatial extent of non-random sub-diffusive regions.

Synthesis and photochemical properties of ferrotitanate In4FeTi3O13.5 with layer structure

In4FeTi3O13.5 (InTi0.75Fe0.25O3.375) semiconductor was prepared via sol-gel citrate-complexation synthesis. This ferrotitanate derives from a solid-solution with InFeO3:In2Ti2O7=2:3. Phase formation and crystal structure of the sample were confirmed via XRD Rietveld refinement. Structural analyses indicated that there were two dimensional layers in the structure. The mutual repulsion in the layers induces great displacements of oxygen ions. The optical properties of In4FeTi3O13.5 nanoparticles were investigated. The direct allowed band gap (2.56eV) shows a characteristic charge-transfer (CT) transitions of (O2p+Fe3d)→(Ti/Fe)3d in visible-light region. The band structure and energy positions were discussed. In4FeTi3O13.5 nanoparticles are demonstrated to be efficient for the photodegradation of Rhodamine B (RhB) under visible light irradiation (λ>420nm). The photocatalytic activities were attributed to the special layer structure and the catalytic mediators of multivalent Ti4+/3+ and Fe3+/2+ confirmed by XPS measurements.

Ion migration in crystalline and amorphous HfOX

The migration of ions in HfOx was investigated by means of large-scale, classical molecular-dynamics simulations over the temperature range 1000≤T/K≤2000. Amorphous HfOx was studied in both stoichiometric and oxygen-deficient forms (i.e., with x = 2 and x = 1.9875); oxygen-deficient cubic and monoclinic phases were also studied. The mean square displacement of oxygen ions was found to evolve linearly as a function of time for the crystalline phases, as expected, but displayed significant negative deviations from linear behavior for the amorphous phases, that is, the behavior was sub-diffusive. That oxygen-ion migration was observed for the stoichiometric amorphous phase argues strongly against applying the traditional model of vacancy-mediated migration in crystals to amorphous HfO2. In addition, cation migration, whilst not observed for the crystalline phases (as no cation defects were present), was observed for both amorphous phases. In order to obtain activation enthalpies of migration, the residence times of the migrating ions were analyzed. The analysis reveals four activation enthalpies for the two amorphous phases: 0.29 eV, 0.46 eV, and 0.66 eV (values very close to those obtained for the monoclinic structure) plus a higher enthalpy of at least 0.85 eV. In comparison, the cubic phase is characterized by a single value of 0.43 eV. Simple kinetic Monte Carlo simulations suggest that the sub-diffusive behavior arises from nanoscale confinement of the migrating ions.

Highly dispersed silica-supported ceria–zirconia nanocomposites: Preparation and characterization

Ternary CeO2–ZrO2/SiO2 nanocomposite oxides were synthesized by a liquid-phase method and subjected to thermal treatment at 550 °C. The physicochemical characterization of the obtained nanosized systems was carried out by various techniques, namely: X-ray diffraction (XRD), nitrogen adsorption–desorption, Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The XRD measurements revealed the presence of cubic phase Zr0.4Ce0.6O2 in CZS1 (CeO2:ZrO2:SiO2 = 3:10:87) and CZS2 (CeO2:ZrO2:SiO2 = 10:10:80) samples. The crystallinity of this phase slightly increased with increasing ceria content in nanocomposites. Raman spectra indicated the presence of oxygen vacancies in the case of CZS2 and an absence displacement of oxygen ions from their normal lattice positions in both of the samples. XPS data indicated a significant quantity of Ce3+ ions present in the composites, which is greater in the CZS2 composite. In the case of CZS systems the TEM–HRTEM results showed well-dispersed nanosized Zr–Ce–oxides over the surface of amorphous SiO2. All the characterization techniques revealed that silica does not form any compounds with the dispersed Zr–Ce-oxides. The prepared materials were tested in catalytic reaction of CO oxidation in the presence of H2 and it was shown that CZS2 system had better performance than CZS1 one.

Phase composition, structure and properties of (ZrO2)1−x−y(Sc2O3)x(Y2O3)y solid solution crystals (x=0.08–0.11; y=0.01–0.02) grown by directional crystallization of the melt

For the first time crystals of the (ZrO2)1−x−y(Sc2O3)x(Y2O3)y solid solutions (x=0.08–0.11; y=0.01–0.02) have been grown by directional melt crystallization. We have determined the range of melt compositions for which growth from the melt produces of the (ZrO2)1−x−y(Sc2O3)x(Y2O3)y solid solution single crystals. The single-phase optically transparent single crystals following composition were grown: (ZrO2)0.9(Sc2O3)0.08(Y2O3)0.02; (ZrO2)0.89(Sc2O3)0.09(Y2O3)0.02; (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01; (ZrO2)0.88(Sc2O3)0.10(Y2O3)0.02. Comprehensive study of the crystal structure by using XRD, transmission electron microscopy, and Raman spectroscopy revealed that the all single crystals, which is identified by XRD data as cubic one, in fact have t″ tetragonal structure, which forms by small displacement of oxygen ions along the c-axis. Data on the phase stability of the crystals during mechanical crushing were obtained. The electrical conductivity was measured as a function of temperature by electrochemical impedance spectroscopy. It is established that (ZrO2)0.89(Sc2O3)0.10(Y2O3)0.01 crystals have the highest conductivity (0.168S/cm at 1173K).

First-Charge Instabilities of Layered-Layered Lithium-Ion-Battery Materials: Relationship of Oxygen Ion Displacement to Mn Migration

Layered-layered composites with composition xLi2MnO3•(1-x)LiMO2 enable high capacity and energy density Li-ion batteries, but suffer from degradation with cycling. Evidence of atomic instabilities during the first charge are discussed based X-ray absorption spectroscopy, first principles simulation, and existing literature. EXAFS measurements on 0.3Li2MnO3•0.7LiMn0.5Ni0.5O2 during the first charge were analyzed to determine the coordination number deficit of Mn and Ni.

Molecular dynamics study on the nature of ferroelasticity and piezoconductivity of lanthanum cobaltite

Abstract Lanthanum cobaltite systems are known to exhibit unique characteristics under mechanical stress such as ferroelasticity and piezoelasticity. In the present study, molecular dynamics simulation on LaCoO3 was performed to investigate the nature of these characteristics. Under uniaxial compression, the material can accommodate the highest stress in the c direction, i.e., hexagonal c-axis, and this orientation preference is related to displacement of oxygen-ions due to rhombohedrally-distorted CoO6 octahedron. The elastic deformation before ferroelastic domain switching process, where domains are re-oriented to make the hexagonal c-axis parallel to the stress axis, could be attributed to a change in the Co O bond length, whereas the elastic deformation after the switching process could be mostly due to the tilt of CoO6 octahedron. The general lattice deformation (Co Co) proceeds continuously during the whole deformation process. The Co O Co bond angle shows a linear correlation to stress, which could be related to the piezoconductivity.

Amorphization of oxides under irradiation with fast neutrons

A variant of the solid-state radiation amorphization as a result of accumulation of the critical concentration of defects in the crystal has been considered using the example of oxides with the garnet and perovskite structures irradiated by fast neutrons. It has been shown that such defects can be antisite defects, the formation of which leads to considerable static displacements from the equilibrium sites of nearest ions and, consequently, to the loss of stability of the crystalline structure. The dependences of the root-mean-square displacements of oxygen ions on the concentration of the antisite defects are constructed based on the analysis of the experimental data. It has been established that the so-called critical concentrations of antisite defects, at which the spontaneous amorphization occurs, differ for oxides with the garnet and perovskite structures. As the criterion of the spontaneous radiation amorphization, it is proposed to consider the critical static displacement of the ions, which is identical for studied oxides and equal to ∼0.28 A, or ∼0.14 in fractions of interatomic distances, which is close to the well-known Lindemann melting criterion.

Thermophysical properties of rare-earth stannates: Effect of pyrochlore structure

Compounds with an A2B2O7-type pyrochlore structure have been identified as a class of materials with low thermal conductivity. To ascertain the effect of the pyrochlore structure, the thermophysical properties of rare-earth stannates which constitute the longest isostructural series with this unique crystal structure were measured and the thermal conduction behavior was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory. The results show that the contraction of rare-earth ions and the accompanying deviation from the ideal pyrochlore structure result in a simultaneous increase in the Young’s modulus and the thermal expansion coefficient. Scattering due to the strain field fluctuation caused by the systemic displacement of 48f oxygen ions rather than scattering from the so-called 8a site “oxygen vacancies” and thermal defects predominates in reducing the phonon mean free path and consequently the thermal conductivity at relative low temperature. At a certain critical temperature, the thermal conduction behavior of the stannates undergoes a transition to a minimum phonon mean free path mechanism, which is responsible for the remarkable variation in temperature dependence of thermal conductivity across the compositions.

Computer Aided Design of the Reversible Solid Oxide Fuel Cell Air Electrode

This paper adopted molecular dynamics technique to simulate the nano-scale transport phenomena inside air electrode of the URSOFC. A molecular model consist of 135 atoms has been built as the simulation target. Born-Meyer-Buckingham potential function is used and a perovskite type structure is built. Simulation results demonstrate hopping mechanism of oxygen ions inside air electrode by calculating mean square displacement of oxygen ions. Also, the effect of the operating temperature for the ionic conductivity is able to be predicted by the present study.

Insight into the grain boundary effect on the ionic transport of yttria-stabilized zirconia at elevated temperatures from a molecular modeling perspective

A molecular dynamics (MD) simulation is used to reveal the grain boundary effect on the ionic transport of yttria-stabilized zirconia (YSZ). The oxygen ion displacements and diffusivities of the ideal and grain boundary-inserted YSZ models are analyzed at elevated temperatures. An optimized Y 2O 3 concentration within YSZ for the best ionic conductivity is achieved by balancing the trade-off between the increased vacancies and the decreased accessible free space. The mass transfer resistance of the grain boundary in YSZ can be more easily found at higher temperatures by observing the oxygen ion diffusivities or traveling trajectories. At lower temperatures, the grain interior and the grain boundary control the ionic transport. In contrast, the grain boundary effect on the diffusion barrier is gradually eliminated at elevated temperatures. The modeled results in this work agree well with previous experimental data.