Oxygen Ion Dynamics Research Articles

Out-of-equilibrium charge redistribution in a copper-oxide based superconductor by time-resolved X-ray photoelectron spectroscopy

Charge-transfer excitations are of paramount importance for understanding the electronic structure of copper-oxide based high-temperature superconductors. In this study, we investigate the response of a Bi2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}Sr2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}CaCu2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}O8+δ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{\ extrm{8}+ \\delta }$$\\end{document} crystal to the charge redistribution induced by an infrared ultrashort pulse. Element-selective time-resolved core-level photoelectron spectroscopy with a high energy resolution allows disentangling the dynamics of oxygen ions with different coordination and bonds thanks to their different chemical shifts. Our experiment shows that the O 1s component arising from the Cu–O planes is significantly perturbed by the infrared light pulse. Conversely, the apical oxygen, also coordinated with Sr ions in the Sr-O planes, remains unaffected. This result highlights the peculiar behavior of the electronic structure of the Cu–O planes. It also unlocks the way to study the out-of-equilibrium electronic structure of copper-oxide-based high-temperature superconductors by identifying the O 1s core-level emission originating from the oxygen ions in the Cu–O planes. This ability could be critical to gain information about the strongly-correlated electron ultrafast dynamical mechanisms in the Cu–O plane in the normal and superconducting phases.

Direct Observation of Oxygen Ion Dynamics in a WO3‐x based Second‐Order Memristor with Dendritic Integration Functions

AbstractDirect observation of oxygen dynamics in an oxide‐based second‐order memristor can provide the valid evidence to clarify the memristive mechanism, however, which is still limited for now. In this study, the migration and diffusion of oxygen ions in the region of Pt/WO3‐x Schottky interface are observed in the WO3‐x second‐order memristor by using the technique of in situ transmission electron microscopy (TEM) and the electron energy loss spectroscopy. Interestingly, the coexistence of memristive and memcapacitive switching can be implemented in this memristor. Combined with the analysis of depth‐profile X‐ray photoelectron spectroscopy (XPS), an interface‐barrier‐modulation second‐order memristive model is proposed based on the above results. Notably, temporally correlative oxygen dynamics in the memristor offers the platform to integrate signals from multiple inputs, enabling the realization of the dendritic functions of synchronous and asynchronous integration for the application of logic operations with fault‐tolerance capability and associative learning. These findings provide the experimental evidence to in‐depth understanding of oxygen dynamics and switching mechanism in second‐order memristor, which can support the optimization of memristive performance and the achievement of biorealistic synaptic functions.

Shock Induced Strong Substorms and Super Substorms: Preconditions and Associated Oxygen Ion Dynamics

It is well known that the interaction between interplanetary (IP) shocks and the Earth’s magnetosphere would generate/excite various types of geomagnetic phenomena. Progresses have been made on the Earth’s magnetospheric response to solar wind forcing in recent years in the aspects associated with magnetospheric substorms. Strong substorms and super substorms could be triggered externally by sudden changes of solar wind dynamic pressures. When a strong substorms (AE > 1000 nT) or super substorms (AE > 2000 nT) occurs, singly charged oxygen ions escaped from the Earth’s ionosphere are found to be a dominated ion population in the magnetotail and in the inner magnetosphere—ring current region. The products of a strong substorms or super substorms- plasmoid, burst bulk flows are also found to contain significant oxygen ions, even substorm injections can be dominated by oxygen ions. Thus, the magnetospheric dynamic must consider the contributions from the heavy oxygen ions. Also, the IP shock induced super substorms associated electromagnetic pulses (dB/dt) would shift the energetic particle (injections) inward and accelerate existing population significantly. Extensive attempts have also been made to understand how the solar wind energy couples with the magnetosphere to excite magnetospheric substorms. The statistical analysis shows that strong substorms (AE > 1000 nT) and super substorms (AE > 2000 nT) triggered by interplanetary shocks are most likely to occur under the southward interplanetary magnetic field (IMF) and fast solar wind pre-conditions. In addition, strong substorms after the IP shock arrival are more likely to occur when IMF points toward (away from) the Sun around spring (autumn) equinox, which can be ascribed to the Russell-McPherron effect. Thus, the southward IMF precondition of an interplanetary shock and the Russell-McPherron effect can be considered as precursors of a strong substorm and/or super substorm triggered by IP shocks. Moreover, the average duration of CME sheath region which is just behind the interplanetary shock are found to be about 7 hours. This indicates that southward IMF compressed by shock could last at least 7 hours long in the downstream of the interplanetary shock (sheath region) if a southward IMF pre-condition is present, which explains why the largest substorm often occur in the CME sheath.

Topotactic Phase Transition Driving Memristive Behavior.

Redox-based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen-ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen-ion transport properties for resistive-switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X-ray absorption spectromicroscopy, it is demonstrated that the reversible redox-based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5 , and the conductive perovskite phase, SrFeO3 , gives rise to the resistive-switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric-field-induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111)-grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive-switching mechanism in SrFeOx -based memristive devices within the framework of metal-insulator topotactic phase transitions.

Correlation between structure and oxygen ion dynamics in Y substituted La2Mo2O9 ionic conductors

We have measured the ac conductivity and dielectric spectra for La2-xYxMo2O9 (0.05 ≤ x ≤ 0.20) in wide temperature and frequency ranges. We have obtained the mean square displacement and the spatial extent of nonrandom sub-diffusive regions for oxygen ions by analyzing the ac conductivity and dielectric spectra respectively. The structure of the samples has been studied using X-ray photoelectron spectra, X-ray and electron diffraction. It is observed that the composition dependence of these characteristic lengths is correlated with the distance between oxygen ions obtained from structural studies.

Observing Local Oxygen Interstitial Diffusion in Donor-Doped Ceria by 17O NMR Relaxometry

The local structure and oxygen ion dynamics in pure CeO2 and CeO2 doped with Ta and Gd is studied by 17O nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy, and conductivity measurements. 17O magic-angle spinning nuclear magnetic resonance spectra of CeO2 doped with Ta show two oxygen environments. A major peak at 876 ppm represents the oxygen ions on the regular crystal lattice site with four Ce4+ neighbors. A second small peak at 751 ppm (1–2% area fraction) represents point defects that can be ascribed to interstitial oxygen ions. The concentration of these point defects increases with the dopant concentration. Oxygen ion dynamics was studied by variable-temperature spin–lattice relaxation time measurements on pure and Ta-doped CeO2 powders. The obtained results reveal a fast local jump process in Ta-doped CeO2 with a jump rate of about 2.5·108 s–1 at 350 K. This local hopping process is assigned to interstitial oxygen ions trapped by tantalum dopand ions.

Ion Dynamics in Li2CO3 Studied by Solid-State NMR and First-Principles Calculations

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO2 looping, where the material reacts reversibly with CO2 to form Li2CO3, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li2CO3 has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li2CO3 over the temperature range of 293–973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li2CO3. Importantly, we have shown that these pro...

Spatially-resolved mapping of history-dependent coupled electrochemical and electronical behaviors of electroresistive NiO.

Bias-induced oxygen ion dynamics underpins a broad spectrum of electroresistive and memristive phenomena in oxide materials. Although widely studied by device-level and local voltage-current spectroscopies, the relationship between electroresistive phenomena, local electrochemical behaviors, and microstructures remains elusive. Here, the interplay between history-dependent electronic transport and electrochemical phenomena in a NiO single crystalline thin film with a number of well-defined defect types is explored on the nanometer scale using an atomic force microscopy-based technique. A variety of electrochemically-active regions were observed and spatially resolved relationship between the electronic and electrochemical phenomena was revealed. The regions with pronounced electroresistive activity were further correlated with defects identified by scanning transmission electron microscopy. Using fully coupled mechanical-electrochemical modeling, we illustrate that the spatial distribution of strain plays an important role in electrochemical and electroresistive phenomena. These studies illustrate an approach for simultaneous mapping of the electronic and ionic transport on a single defective structure level such as dislocations or interfaces, and pave the way for creating libraries of defect-specific electrochemical responses.

Conduction and relaxation mechanisms in bismuth doped La2Mo2O9 ionic conductors

Oxygen ion conduction and relaxation mechanisms in bismuth substituted La2Mo2O9 have been reported in this paper. The phase transition observed in La2Mo2O9 is suppressed in bismuth substituted samples, and the cubic β form is stabilized at room temperature. A transition of the dc anionic conductivity from low temperature Arrhenius type to high temperature Arrhenius type has been observed in La2Mo2O9. However, for bismuth doped samples the dc anionic conductivity shows a transition from Arrhenius behavior at low temperatures to Vogel-Tamman-Fulcher in the high temperature regime. The frequency dependence of the ac conductivity shows a power law behavior. The ac conductivity remains constant in low frequency region but shows dispersion at the crossover frequency. The crossover frequency shifted towards higher value with the increase of temperature. The dimensionality of the conduction space does not change with bismuth doping. The scaling behavior of the conductivity spectra for different temperatures and compositions indicates that dynamics of oxygen ions is independent of temperatures and compositions.

Local structure and oxygen ion dynamics in La doped ceria: 17O NMR study

The local order and oxygen dynamics in La doped ceria (LDC) has been studied by 17O NMR techniques. High resolution 17O MAS NMR spectra show several oxygen resonances which are assigned to the lattice sites with 0, 1 and 2 nearest neighbor (nn) La 3+ ions. The site with 1 nn La contributes two different lines depending on the presence or absence of oxygen vacancy. The sites with 2 nn La ions are represented by one single line, which indicates that two neighboring La 3+ ions always bind one vacancy. Spin-lattice relaxation ( T 1) measurements on static samples were carried out between 293 K and 1073 K. The temperature dependence of T 1 has been interpreted assuming two independent relaxation paths. The relaxation below 600 K is assigned to the oxygen dynamics in the lattice without involving vacancy diffusion. The relaxation at a higher temperature is interpreted to be caused by the oxygen hopping between vacant and occupied sites.

Cation size effects in oxygen ion dynamics of highly disordered pyrochlore-type ionic conductors

In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, E_(dc), in highly disordered pyrochlore-type ionic conductors A_(2)B_(2)O_(7). Twenty six compositions with the general formula, Ln_(2)Zr_(2−y)Ti_(y)O_(7), Ln_(1.7)Mg_(0.3)Zr_(2)O_(7) (Ln=Y, Dy, and Gd), and Gd_(2−y)La_(y)Zr_(2)O_(7), were prepared by mechanical milling, and their electrical properties were measured by using impedance spectroscopy as a function of frequency and temperature. By using the coupling model we also examine the effect of cation radii R_(A) and R_(B) on the microscopic potential-energy barrier, E_(a), which oxygen ions encounter when hopping into neighboring vacant sites. We find that, for a fixed B-site-cation radius R_(B), both activation energies decrease with increasing A-site-cation size, R_(A), as a consequence of the increase in the unit-cell volume. In contrast, for a given R_(A) size, the E_(dc) of the Ln_(2)Zr_(2−y)Ti_(y)O_(7) series increases when the average R_(B) size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with R_(B).

Network-Forming Ions Diffusion in Metasilicate and Pyrophosphate MeIts: Molecular Dynamics Simulation

The specific features of the dynamics of oxygen ions in Ме2O · SiO2 (Ме = Li, Na, K, Cs) and Na2O·ZnO·P2O5 melts at а temperature of 2000 K were investigated bу the molecular dynamics method. It is demonstrated that, as in the systems studied earlier, the formation of defect complexes is а necessary condition fог an oxygen diffusion event to bе successful. The scenarios of generating defect соmplexes аrе described, and the lifetimes of these complexes are calculated. The structure of the defect complexes is determined. It is shown that two-membered rings, free and threefold-coordinated oxygen ions сап also bе involved in the formation of defect complexes.

Influence of thermally induced oxygen order on mobile ion dynamics inGd2(Ti0.65Zr0.35)2O7

We report on the influence of oxygen order in the oxygen-ion dynamics in the ionic conductor Gd_(2)(Ti_(0.65)Zr_(0.35))_(2)O_(7). The metastable Gd_(2)(Ti_(0.65)Zr_(0.35))_(2)O_(7) powders prepared by mechanical milling present an anion-deficient fluorite type of structure, stable up to about 800 °C. Thermal treatments at higher temperatures facilitate the gradual rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects. Interestingly, metastable pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process. We have found that the ionic conductivity due to mobile oxygen ions increases significantly with increasing sintering temperature from 800 to 1500 °C as a result of a systematic decrease in the activation energy for the dc conductivity from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by stretched exponentials of the form φ(t)=exp[-(t/τ)^(1−n)], and the fractional exponent n decreases systematically from n=0.51 to 0.18 with increasing sintering temperature. These results are explained in terms of weaker ion-ion interactions in the increasingly ordered structure of the samples sintered at higher temperatures, and point to the importance of structural disorder in determining the dynamics of mobile oxygen ions.

Oxygen ion dynamics in yttria-stabilized zirconia as evaluated by solid-state 17O NMR spectroscopy

Solid-state 17O NMR measurements between room temperature and 973 K were performed for the first time on 17O-enriched yttria-stabilized zirconia samples. Spin–lattice relaxation is found to exhibit a strong temperature dependence which can be traced back to motional displacements of the oxygen ions and which is almost unaffected by the actual sample constitution. Analysis of the spin–lattice relaxation data provides the motional correlation times. The derived activation energies are relatively low with values of about 30 kJ/mol. In addition, large temperature effects are observed for the 17O NMR line widths, and thus for spin–spin relaxation, which are again attributed to the oxygen ion mobility. In this case, the underlying oxygen motions, however, occur on a length-scale which is different from that probed by spin–lattice relaxation.

Structural Characterization and Ionic Conductivity of Metastable Gd2(Ti0.65Zr0.35)2O7 Powders Prepared by Mechanical Milling

AbstractWe analyze in this work the influence of ordering on the oxygen ion dynamics in the ionic conductor Gd2(Ti0.65Zr0.35)2O7, prepared by mechanical milling. As-prepared powder phase presents a metastable anion deficient fluorite-type of structure below 800°C becoming a disordered pyrochlore above this temperature. Such phase transformation implies a significant increase in the ionic conductivity of this material as a result of a systematic decrease in the activation energy for the dc conductivity, from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by the Kohlrausch-Williams-Watts (KWW) stretched exponential function with the fractional exponent n decreasing systematically with increasing sintering temperature (increasing ordering) as a result of decreasing ion-ion interactions in better ordered samples.

Mechanochemical synthesis and ionic conductivity in the Gd 2(Sn 1–yZr y) 2O 7 ( [formula omitted]) solid solution

Several compositions within the Gd 2(Sn 1− y Zr y ) 2O 7 solid solution with a pyrochlore-type of structure, have been prepared at room temperature in a planetary ball mill starting from stoichiometric mixtures of the constituent oxides, SnO 2, ZrO 2 and Gd 2O 3. Phase evolution in the powder mixtures as a function of composition and milling time was followed by X-ray diffraction (XRD) finding out that after 18 h, every mixture consist of a single phase. Mechanically activated chemical reaction takes place after an intial step of particle size refinement and polymorphic transformation of C-Gd 2O 3. We have performed Impedance Spectroscopy measurements to investigate oxygen ion dynamics in the series and conclude that Zr substitution for Sn hardly modifies the importance of ion–ion correlations in the oxygen diffusion process. We also estimate from conductivity measurements the value of the microscopic activation energy needed for independent ion hopping, which is found to be 0.56±0.06 eV, independent of Zr content.

Effects of cooperativity on ion dynamics in oxygen conducting Gd 2Ti 2− yZr yO 7

We analyze ion dynamics in the oxygen ion conductor Gd 2Ti 2− y Zr y O 7 by impedance spectroscopy measurements. We find that electrical conductivity relaxation is well described by stretched exponentials of the form Φ( t) = exp(−( t/ τ) 1− n ), with the fractional exponent n increasing systematically with Zr content. Increasing Zr content also leads to an increase in the activation energies for long-range ion diffusion from 0.7 to 1.0 eV. These results are explained in terms of the enhanced cooperativity in oxygen ion dynamics when increasing the concentration of mobile oxygen vacancies with Zr content.

Cooperative oxygen ion dynamics inGd2Ti2−yZryO7

We report on dispersive conductivity measurements in the oxygen ion conductor Gd_(2)(Ti_(2−y)Zr_(y))O_(7). Increasing Zr content leads to higher concentration of oxygen vacancies and results in higher activation energies for long-range ion transport, whilst the microscopic energy barrier for single ion hopping remains constant. We find evidence that, besides oxygen binding energy, enhanced cooperativity in oxygen ion dynamics determines the activation energy for long-range diffusion.

Defects and oxygen diffusion in metasilicate melts: Molecular dynamics simulation

The specific features of the dynamics of oxygen ions in Me 2O ⋅ SiO2 (Me = Li, Na, K, Cs) melts at a temperature of 2000 K are investigated by the molecular dynamics method in the partially ionic potential approximation. It is demonstrated that, as in the systems studied earlier, the formation of defect complexes is a necessary condition for an oxygen diffusion event to be successful. The methods for generating defect complexes are described, and the lifetimes of these complexes are calculated. The structure of the defect complexes is determined. It is found that, in the Li2O ⋅ SiO2 melt, successful oxygen diffusion events occur predominantly with the participation of fivefold-coordinated silicon ions. The threefold-coordinated defects begin to predominate in the Na2O ⋅ SiO2 melt, and their role becomes crucial in the diffusion in the potassium and cesium metasilicate melts. It is shown for the first time that free and threefold-coordinated oxygen ions can also be involved in the formation of defect complexes.

Conjectured chaotic nature of the ionosphere-magnetosphere coupling in a reconfigurating magnetosphere

Abstract. During substorms the magnetic field configuration changes in time; stretching of the magnetosphere during growth phase is followed by its collapse after the onset of the expansion phase. In this study the ionospheric origin oxygen ion dynamics in a time-dependent magnetosphere is analyzed. An induction electric field of several mV/m due to the reconfigurating magnetic field determines the details of the ion extraction from the auroral topside ionosphere. Two regimes of motion are discernible; a regime in regions far from the equatorial plane where the magnetic moment is conserved and a regime near the equatorial plane, in which the motion produces magnetic moment jumps and oscillations. The time spent by the ion in the regions is determined by the initial characteristics of the ion and by the field transition features. Lyapunov characteristic exponents are calculated to estimate the sensitivity of the system to initial conditions. Their values are higher for orbits with chaotic segments compared with the orbits in the static magnetic field and depend on the amplitude of the induced electric field. It follows from the study that the region of chaos usually localized far beyond 10 RE in the plasma sheet is expected to approach closer to the Earth ( r = 6 - 7 RE) during substorm associated reconfigurations of the magnetosphere, due to the auroral ionospheric ions.