Oxygen Intercalation Research Articles

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa2Cu3−xCoxO6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10−5 and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa2Cu3−xCoxO6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa2Cu3O6+δ

Electrochemical Investigation of Oxygen Intercalation into La2CuO4+δPhases

In this work the electrochemical intercalation of oxygen in La2CuO4phases has been studied. Oxygen intercalation has been performed at different anodic potentials for fixed time in alkaline solution (1 M NaOH) at room temperature. The electrochemistry of the phenomena taking place at the oxide–solution interface has been investigated by cyclic voltammetry (CV), controlled-potential coulometry, and electrochemical impedance spectroscopy (EIS). The homogeneity of processed samples and the lattice parameters prior to and after oxygen intercalation have been verified by X-ray powder diffraction. SEM has been used to relate surface modification to the potential applied after electrochemical oxygen intercalation. The recent theories and knowledge of mechanisms of oxygen intercalation into the oxide lattice have been related to the experimental results. Oxygen intercalation seems to occur at potentials slightly lower than that of the oxygen evolution reaction (OER) and proceeds on a parallel pathway to O2evolution at more anodic potentials.

Crystal and Defect Structure of Nd1.9Ce0.1CuO4±y

The crystal and defect structure of neodymium cerium cu‐prate (Nd1.9 Ce0.1 CuO4±y) has been studied as a function of temperature (1200–1500 K) and oxygen pressure (100–103 atm) via X‐ray diffractometry, powder neutron diffraction, and thermogravimetry methods. The oxygen nonstoichiom‐etry changes from oxygen‐excess (positive y values) to oxygen‐ deficient oxides (negative y values). The absolute values of oxygen nonstoichiometry y are calculated from the neu‐tron diffraction and thermogravimetry data. The materials adopt the tetragonal structure (space group I4/mmm). A defect model is proposed for Nd2‐x Cex CuO4±y. It is con‐cluded that oxygen vacancies VÖ (which occupy O(1) crys‐tallographic positions (in planes)) and interstitial oxygen O(i (which occupy O(3) “apical” positions) are present simultaneously. Gaseous oxygen intercalates into the Nd1.9 Ce0.1 CuO4±y crystals and replaces oxygen vacancies in the O(1) position and interstitial oxygen atoms in the O(3) positions. The theoretical model and the experimental data are in good agreement with each other, and the equilibrium constants of defect formation have been calculated.

Pr-doping effect on the structure and superconductivity of Bi 2Sr 2Ca 1− xPr xCu 2O y single crystals

Effect of Pr-doping on the structure and superconductivity of Bi2212 system was studied for Bi 2Sr 2Ca 1− x Pr x Cu 2O y single crystals with x=0–0.78. The a- and b-axis lengths increase with increasing Pr concentration, while the c-axis length and the period of modulation decrease with Pr-doping. These can be explained by the higher valence of Pr ion substituting at Ca site, the extra oxygen in BiO bilayers and the lattice mismatch between perovskite layer and BiO bilayer. In addition, the changes in lattice parameters are more dramatic in the Pr-rich region than those in the Ca-rich region, which can be attributed to the difficulty in the intercalation of extra oxygen in the BiO bilayer at high-doping level. The superconducting transition temperature increases slightly at first and then drops gradually upon Pr doping and the complete suppression of superconductivity occurs when the Pr content reaches 0.60. The superconducting volume faction also decreases with Pr content and is proportional to 1− ax 2.

Room temperature topotactic oxidation of lanthanum cobalt oxide La 2CoO 4.0

Lanthanum cobalt oxide La 2CoO 4 exhibits unusually fast oxygen transport in the spontaneous intercalation of molecular oxygen at ambient temperature. The present work concerns investigations on the mechanism of the chemical and electrochemical intercalation process. It is shown that two nonstoichiometric phases and one line phase are formed as intermediates with different kinetics. Carbon dioxide was found to be an efficient inhibitor for the O 2 uptake reaction. Several models are discussed in order to explain the phenomenon of fast oxygen anion transport at low temperatures.

Effect of extra oxygen intercalation on the modulation structure and electronic structure of the Bi2201 system

The incommensurate modulation structure and electronic structure of the system were investigated by means of Raman scattering, transmission electron microscopy and x-ray photoemission. Analyses of Raman spectra and electron diffraction reveal that, even though the extra oxygen behaviours in layers for the three samples are very different, they possess a similar monoclinic modulation characteristic with modulation wavelength , which supports the crystal misfit model. Analysis of XPS results indicates that extra oxygen intercalation has a strong influence on the electronic structure; a possible Bi 6s contribution to the valence band is suggested.

New synthetic routes for preparing perovskites: Electrochemical oxidation and oxidation by NO 2

Two new synthetic low temperature routes for preparing perovskite-related materials, especially metastable phases are depicted. The first one, the electrochemical oxidation, uses the electric field as the driving force for intercalating oxygen atoms within parent oxide networks. The reaction is achieved under anodic potential, in alkaline solution (1 M KOH or NaOH), at room temperature, in air. This process has been used for preparing various perovskite compounds such as AMO 3 (A=Sr, La; M=Fe 1− x Co x ) or A 2MO 4+ δ (A=La, Nd, Sr). The most relevant results are reported. Potentiostatic and galvanostatic experiments have shown that the amount of intercalated oxygen can be controlled and that the process is reversible. Structural as well as electronic aspects of the oxygen intercalation are discussed and a reaction mechanism is proposed. The second route is based on the exothermic reaction of nitrogen dioxide NO 2 with NH 4 + ions at moderate temperatures (typically T<300°C), which allowed the destruction of NH 4 + cations in situ. Topotactic reactions are described for preparing new hexagonal forms of WO 3 or MoO 3. The reaction process is discussed.

Structural Instability in the Bi-Based Superconductor Studied by Positron Lifetime

The positron lifetimes for Bi-based 2212 phase superconductors were measured as a function of time at room temperature. It is found that lifetime parameters, lifetimes (τ 1 , τ 2 ), mean lifetime (τ m ) and bulk lifetime (τ b ), decrease until 129 days, and then remain roughly unchanged as the time increases. This result confirms that the structure of oxide supercondutors is unstable even at room temperature. The decrease in the annihilation parameters is explained in terms of the intercalation of the excess oxygen in the region between the Bi-O layers. A possible correlation with the electron structure, oxygen content, hole density and transition temperature is discussed.

Temperature dependence of the conductivity and kinetics of oxygen intercalation ofC70films

Electrical conductivity values of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ to $1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ $(\ensuremath{\Omega}\mathrm{cm}{)}^{\ensuremath{-}1}$ at 30 \ifmmode^\circ\else\textdegree\fi{}C were found in undoped ${\mathrm{C}}_{70}$ films, and the conductivity is thermally activated with an activation energy of 0.51--0.81 eV and a pre-exponential factor of $0.5(\ensuremath{\Omega}\mathrm{cm}{)}^{\ensuremath{-}1}.$ We demonstrated that the conductivity decay due to oxygen intercalation follows $\ensuremath{\sigma}(t)=\ensuremath{\sigma}{(0)t}^{\ensuremath{-}\ensuremath{\alpha}},$ where \ensuremath{\alpha}\ensuremath{\ll}1 when $0&lt;t&lt;t1,$ \ensuremath{\alpha}\ensuremath{\geqslant}1 when $t1&lt;t&lt;t2,$ and 0.5\ensuremath{\leqslant}\ensuremath{\alpha}1.0 when $tt2,$ corresponding to three processes of oxygen reaction with the film.

Atmosphere-induced change of microhardness and plasticity of C 60 single crystals and polycrystalline films

The effect of air exposure and visible light illumination on the microhardness, plasticity and dislocation mobility in the C 60 single crystals was investigated. Microhardness values for defined structural states of the fullerene (pristine, oxygen-intercalated, photo-oxidized) are reported. It has been shown that oxygen intercalation in the fullerene lattice during air-aging in the dark results in the suppression of dislocation mobility. However, only a slight increase in the hardness of oxygenated samples was observed. A remarkable increase in the hardness and decrease in the plasticity of the C 60 crystals under illumination-assisted air exposure was noted. Photochemical transformation was found to be localized in the nearsurface layer of 1–5 μm. The data on the recovery of the micromechanical properties of the photo-oxidized surface layer under heating in air are presented.

Mechanisms and Kinetics of Electrochemical Intercalation of Oxygen into Nd2NiO4+δ

ABSTRACTOverstoichiometric insertion of oxygen into thin films of Nd2NiO4+δwas performed electrochemically at room temperature (RT) in order to obtain an in-situ compositional analysis of the process. Thin Nd2NiO4+δlayers were deposited onto resonators of a Quartz Crystal Microbalance (QCM) by Pulsed Laser Deposition (PLD). Cyclic voltammograms showed oxidation and reduction peaks typical for these oxides. The change in oxygen stoichiometry was determined by in-situ QCM measurements during potential step experiments. The resulting maximum mass change was on the order of 50 ng and corresponded to a maximum excess oxygen content of δ = 0.12 in Nd2NiO4+δ. The time dependence of the mass change was described by a transport model which leads to oxygen diffusion coefficients in the range from 2.10−12 to 10−11 cm2/s depending on δ and the direction of the reaction.

Oxygen-Diffusion and Structural Modification in Air-Annealed Superconducting Bi2Sr2CaCu2Oy Single Crystals

Bi2Sr2CaCu2Oy single crystals were air-annealed at 300, 400, 600 and 750°C for 20 h in consequence. Measurements of ac susceptibility and x-ray diffraction showed that at annealing temperature below 400°C, the change of Tc was dominated by oxygen-diffusion involved in pervoskite unit and was strongly sample-dependent. This diffusion from pervoskite unit occurred earlier than that from Bi-O layers. At annealing temperature over 400°C, with the intercalation of excess oxygen into Bi-O layers, the hole concentration in the crystal was redistributed so that Bi-O reservoir fully determined superconductivity. From the changes of non-uniform strain in annealing process, it was found that structural modification was also closely correlated to superconductivity.

Novel two-dimensional perovskites

Using the example of rare earth titanium oxides, a new route for tailoring superlattices in bulk oxides will be discussed. Starting with well-known ABO 3 compounds, series of artificial structures can be created in a systematic way by oxygen intercalation, i.e. by an ordered incorporation of excess oxygen to form ABO 3+ y compositions. This intercalation process slices the three-dimensional ABO 3 structure into subunits of various thickness, each representing a defined oxygen composition. These basic units can be combined in multiple ways to form new superlattices whose physical character varies between semiconductor and ferroelectric insulator. These variations can be achieved without changing the chemical character of the compound, in contrast to the usual procedure of extrinsic doping, which creates disorder.

Electrochemical oxidation of La 2CuO 4 in fused nitrates

La 2CuO 4 has been electrochemically oxidized in fused alkali nitrates, a medium that stabilizes peroxo and superoxo groups. The oxidation takes place either in neutral (NaClO 4 electrolyte) or basic (KOH, Na 2O electrolytes) media, in the presence or absence of protons, and in the presence or absence of added oxo groups (e.g. NaClO 4). The oxidation potentials are lower as the basicity of the medium increases (NaClO 4 > KOH > Na 2O). The resulting La 2CuO 4 + δ materials have large superconductivity fractions when the oxidation is performed at 150°C, and very small fractions (along with phase decomposition into CuO) when it is performed at 235°C. The oxidized materials are totally unreactive versus iodide. That, along with the reported existence of peroxo and superoxo groups in this media, suggests that intercalation of oxygen in intermediate −1 and/or − 1 2 oxidation states takes place. However, the existence of such reaction in the absence of protons eliminates the possibility of a mechanism by which OH − groups are being intercalated. Potentiostatic experiments yield a phase-segregated material with phases of T c 45 and 35 K, while galvanostatic experiments allow to prepare the 35 K phase by itself.

Preparation of the Nd-123 phase in air with as high as 95 K

The possibility of preparing bulk Nd-123 by the partial-melting technique in air is reported for the first time. The effects of the starting composition and of processing conditions (temperature, annealing time) on the superconducting properties of the system were investigated. The electrical properties were found to depend critically on the starting composition as well as on the annealing times and the cooling rates. The best results (zero-resistance temperature with an onset ) were obtained starting with the stoichiometric composition (x = 0) by annealing at for 24 h and cooling to RT at . The results are discussed in terms of competition between the nucleation of Nd-123 and as secondary phase, variation of the Nd content in the 123 phase and kinetics of oxygen intercalation during the cooling process. .

Growth and electrochemical oxidation of MBE-grown c-axis La 2CuO 4 thin films on different substrates

The growth and subsequent electrochemical oxygen intercalation of c-axis La 2CuO 4 + δ thin films on substrates with different lattice mismatch [SrTiO 1 (001) (+3.03%), NdGaO 1 (001) (+1.90%). LaAlO 3 (001) (−0.05%) and SrLaAlO 4 (001) (−0.95%) substrates] are compared. The films grown on the different substrates can all be oxidized electrochemically, but their structural and transport properties differ vastly. The results can roughly be divided into two categories, i.e., those of films grown with a compressive or a tensile lattice mismatch where the former have a larger c-axis latice parameter and better transport properties.

C60: A host lattice for the intercalation of oxygen?

The investigation of structural and electronic properties of the novel family of fullerenes depends on the existence of pure reference materials. Sublimation of the van-der Waals solids is a suitable purification method. Little attention has been paid to the question about the air stability of such sublimed samples in form of crystals or thin films. A combination of thermal desorption spectroscopy, thermal analysis and diffuse reflectance FT-IR spectroscopy is used to show the extent to which oxygen from dry air is intercalated into fullerenes and which detrimental reactivity occurs from attempts to thermally remove („nneal”) air-exposed samples. The conclusion is that any fullerene sample exposed to air will be transformed in part into a polymeric non-fullerene carbon upon thermal treatment to above 400 K irrespective of its initial purity.

A New Class of Pyrochlore Solid Solution Formed by Chemical Intercalation of Oxygen

Oxygen intercalation into a pyrochlore at room temperature is reported. A simple chemical route was employed. Previously only perovskite or a few closely related phase have demonstrated an ability to act as hosts for such intercalation. The specific system studied was the interstitial solid solution Ce2Zr2O7+x (0 ≤ x ≤ 0.36). Neutron diffraction reveals that interstitial oxygen enters the tetrahedral 8b sites (space group Fd3̄m), which are empty in stoichiometric pyrochlore, but displacement of existing oxygen from the tetrahedral 8a sites also occurs. The lattice contracts on intercalation due to oxidation of Ce3+. The changes in structure and the diffusion pathways for oxygen are discussed.

Growth, microstructure, and electrochemical oxidation of MBE-grown c-axis La2CuO4 thin films.

The growth, microstructure, and electrochemical oxygen intercalation of c-axis ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ thin films on substrates with different lattice mismatch [${\mathrm{SrTiO}}_{3}$ (001) and ${\mathrm{SrLaAlO}}_{4}$ (001) substrates] are compared. Except for the absence of planar defects in the latter case, the microstructural properties of both film types are very similar. For films on ${\mathrm{SrTiO}}_{3}$, oxygen can be intercalated electrochemically into the grown c-axis thin film with a high diffusion coefficient (${\mathit{D}}_{\mathrm{ox}}$=${10}^{\mathrm{\ensuremath{-}}13}$--${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$/s), and subsequently additional (00l) reflections, l=2n+1, are observed by x-ray diffraction measurements. A double transition with a resistivity drop at \ensuremath{\simeq}55-58 K, suggesting a more strongly oxidized phase, and a zero-resistance state at \ensuremath{\simeq}42 K are found. For films on ${\mathrm{SrLaAlO}}_{4}$, the value of ${\mathit{T}}_{\mathit{c}}$ could not be raised further, as the films decompose during the anodic polarization. This comparison reveals the role of planar defects, and we propose an electrochemical oxidation mechanism that occurs in two steps: First oxygen is transported into the film by intercalation into the planar defects, and then a slower oxygen diffusion into the interstitial sites occurs along the ab planes. \textcopyright{} 1996 The American Physical Society.

Electrochemical oxygen intercalation in La 2NiO 4 + δ crystals

The electrochemical intercalation of oxygen from an aqueous KOH electrolyte into La 2NiO 4 + δ has been investigated by a potential step method at potentials below oxygen evolution. The first reduction cycle occurs at nearly constant potential for the whole composition range and is characteristic of nucleation and growth behavior. On the first oxidation and subsequent reduction-oxidation cycles, the results are similar to those obtained previously for polycrystalline electrodes and show the presence of a two-phase region for 0 ≤ δ ≤ 0.06 and a single phase with the composition 0.06 ≤ δ ≤ 0.13. The P4 2 ncm phase with δ ∼ 0.02 and a narrow range of stoichiometry, is apparent in dδ dV versus δ data and in the behavior of the current decay curves. Examination of the crystals at various stages of cycling show the formation of extensive cracks that apparently increase in density with cycling. Measurements of the transient currents after potential steps have been used to estimate the diffusion coefficient of oxygen in La 2NiO 4 + δ in the single phase region. Data were obtained for both polycrystalline and single crystal electrodes.