Function Of Oxygen Stoichiometry Research Articles

Defect chemistry of lithium storage in TiO2 as a function of oxygen stoichiometry

We investigate the point defect chemistry of lithium storage in TiO2 and emphasize its great importance to lithium battery applications. Not only charge carrier concentration but also lithium diffusivity is successfully interpreted in terms of lithium content and activity as well as oxygen stoichiometry. A detailed defect chemical analysis reveals that ionic-electronic defect associations (Lii•+e′⇌LiiX and VO••+2e′⇌VO•+e′⇌VOX) play a significant role in determining the concentration of defects and lithium diffusivity, and hence can have large impact on overall lithium storage performance of oxide based electrode materials.

Synthesis and neutron powder diffraction structural analysis of oxidized delafossite YCuO 2.5

We report a study of the evolution of the structure of the delafossite-derived compounds YCuO 2+ δ as a function of oxygen stoichiometry. The structural details of the oxygenated material YCuO 2.5 were examined by means of high-resolution neutron powder diffraction. We confirmed that YCuO 2.5 adopts an orthorhombic superstructure ( a = 3 a H , b = c H , c = 2 a H ) in which the anions are located at the center of corner-sharing triangles to form undulating chains of Cu 2+ ( s = 1 / 2 ), running along a-axis direction.

Thermodynamic and nonstoichiometric behavior of promising Hi-Tc cuprate systems via electromotive force measurements: A short review

Electromotive force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made on the YBa2Cu3O x , GdBa2Cu3O x , NdBa2Cu3O x , and bismuth cuprate systems in the temperature range ∼400 °C to 750 °C by means of an oxygen titration technique with an yttriastabilized zirconia electrolyte. The shapes of the 400 °C isotherms as a function of oxygen stoichiometry for the Gd and Nd cuprate systems suggest the presence of miscibility gaps at values of x that are higher than those in the YBa2Cu3O x system. For a given oxygen stoichiometry, oxygen partial pressures above GdBa2Cu3Ox and NdBa2Cu3O x the higher (above x=6.5) than that for the promising YBa2-Cu3Ox system.

Crystal-field spectrum in RBa2Cu3Ox (R = Er, Ho) high-Tc superconductors: evidence for charge order in CuO2 planes

A model of the crystal field generated by a periodic array of charged tapes is developed to analyse the crystal-field interaction in RBa2Cu3Ox (R = Er, Ho) high-Tc copper oxides observed by the inelastic neutron scattering technique. The explicit calculation of the parameters of the Stevens Hamiltonian describing crystalline electric field effects in solids is performed for a specific charge density distribution uniformly extended in a certain direction of the crystal lattice. The model accounts for the x-dependence of the crystal-field parameters and allows us to determine the hole concentration in the CuO2 planes as a function of oxygen stoichiometry. The model of the periodic array of charged tapes suggests a charge order induced in the CuO2 planes by doping.

Dependence of the vortex-solid phase transition ofYBa2Cu3tO7−δtthin films on anisotropy: Evidence for a universal phase boundary

The vortex-solid phase boundary of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}\mathrm{}}$ (YBCO) thin films was studied as a function of oxygen stoichiometry, \ensuremath{\delta}, using magnetotransport measurements. Experimental evidence for a characteristic crossover field, ${\mathrm{H}}_{0}$, separating two distinct types of behavior, was observed for films with \ensuremath{\delta}g0.12 to occur at a temperature, ${\mathrm{T}}_{0}$, that is about half of the superconducting transition temperature (${\mathrm{T}}_{0}$\ensuremath{\approx}${\mathrm{T}}_{\mathrm{c}}$/2). When plotted as ${\mathrm{H}}_{\mathrm{g}}$(T)/${\mathrm{H}}_{0}$ versus (1-T/${\mathrm{T}}_{\mathrm{c}}$), all of the boundaries collapse onto a single universal curve. An implication of this collapse is that an empirical equation which depends only on ${\mathrm{T}}_{\mathrm{c}}$, the anisotropy \ensuremath{\gamma}, and ${\mathrm{T}}_{0}$, ${\mathrm{H}}_{\mathrm{g}}$(T)=${\mathrm{H}}_{0}$[(1-T/${\mathrm{T}}_{\mathrm{c}}$)/(1-${\mathrm{T}}_{0}$/${\mathrm{T}}_{\mathrm{c}}$)${]}^{\mathrm{n}}$, where ${\mathrm{H}}_{0}$=[1.2${\mathrm{\ensuremath{\varphi}}}_{0}$/(s\ensuremath{\gamma}${)}^{2}$], can be used to describe the ${\mathrm{H}}_{\mathrm{g}}$(T) boundary for any YBCO thin film and other disordered YBCO materials. Estimates of the vortex-glass (VG) correlation length, ${\ensuremath{\xi}}_{\mathrm{VG}}$, are made. The value of ${\ensuremath{\xi}}_{\mathrm{VG}}$ at the point where VG scaling finally breaks down is found to be independent of magnetic field for the fully oxygenated films, however, it is dependent on field for deoxygenated films. This result suggests that the pinning in the fully oxygenated films is dominated by correlated disorder while pinning in the deoxygenated films becomes dominated by quenched disorder.

Structural features of defect cascades in YBa2Cu3Oxas a function of oxygen stoichiometry

Single crystals of YBa2Cu30,, of various oxygen stoichiometries, were irradiated with 50 keV Kr' to create defect cascades similar to those resulting from fast neutron irradiation. Two-beam dark-field transmission electron microscopy in weak dynamic conditions was used to measure the defect cascade size distribution in the principal axes for four different oxygen stoichiometries, and high-resolution electron microscopy verified the mean size of these cascades. The mean size of the defect cascades generated was independent of oxygen stoichiometry from x = 6.35 to x = 6.9, which indicates that magnetic flux pinning does not vary because of any association between the oxygen concentration and the cascade size. For Kr' incident near the c axis the defect cascades have on average a circular cross-section of 3.5 nm diameter perpendicular to the ion beam, while an elliptical cross-section was found for ions incident near the x axis. The minor axis (3.5nm) was in the [OOl] direction and the major axis (5.5nm) was in the [OlO] direction. This aspect ratio of 1.6 matches that found for high-energy heavy-ion track damage in YBa2Cu3Ox, which implies that the shape is determined not by the recoil damage mechanism but in the cooling or relaxation stage where the anisotropy in thermal conductivity determines the defect shape.

Magnetic order and disorder in YBa 2(Cu 1- xFe x) 3O 6+ y

Neutron-diffraction and neutron-polarisation analysis have been used to map the magnetic phase diagram of YBa 2(Cu 1- x Fe x ) 3O 6+ y , with x=0.06 and 0.10, as a function of oxygen stoichiometry, y. For x=0.06 ( y=0.35) and x=0.10 ( y<0.76) magnetic order is observed in which the Cu(1) and Cu(2) planes each form 2D antiferromagnetic layers coupled antiferromagnetically along the c-axis. The Cu(1) and Cu(2) moments lie in the ab plane taking maximum values of 0.21 μ B and 0.52 μ B, respectively, for the x=0.10, y=0.42 sample. The ratio μ Cu(1)/μ Cu(2)=0.4 remains constant with varying x and y. A paramagnetic-like diffuse scattering persists undiminished from above T N/ T c to low temperatures. It is suggested that this scattering arises from the Fe moments which constitute an almost independent magnetic subsystem to the Cu spins.

Electronic structure studies of YBa2Cu3Ox(6.2≤×≤6.9) using angle-resolved photoemission

Using high-resolution angle-resolved photoemission, the electronic structure of YBa 2Cu 3O x is examined when oxygen stoichiometries are varied in the range 6.2≤×≤6.9. Detailed measurements of the Fermi surface for YBa 2Cu 3O 6.9 are presented and are compared with predictions of band theory. In the metallic region of the phase diagram, changes in the Fermi surfaces are measured as a function of oxygen stoichiometry. The electronic structure is monitored as the oxide changes from a metal to a semiconductor with additional oxygen depletion. For intermediate stoichiometries, effects of oxygen-vacancy ordering are considered. Unusual resonant effects observed at several photon energies are examined as the oxygen content is varied.

Correlation Between Nonlinear Charge Transfer and Behaviour of Tc in YBa2Cu3O7-y System

ABSTRACTThe charge transfer in YBa2CU3O7-y system have been analysed by bond valence sum calculation. It is found that there exists an additional effective charge-transfer between the CuO2 planes and the CuO chain which varies nonlinearly with the oxygen stoichiometry and can be expressed as q = 0.151 exp(∼0.007/y4) + 0.35 exp(0.5 – 0.5/y4). This nonlinear charge-transfer is associated with the structural transitions of orthorhombic (I)-to-orthorhombic(II) and orthorhombic(II)-to-tetragonal symmetry, and thus related to the oxygen ordering in the CuO chains. A new relationship between the mobile hole number P(2) the localised hole number P(l) and the total hole number P has been established as P(2) = P - P(1), P(1) = 0.5 - y - q. The results also show that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry correspond to the plateaus in the curve of q versus y, indicating that both the nonlinear variations of Tc and the charge-transfer with the oxygen stoichiometry are intrinsically correlated.

Dependence of hole concentration on oxygen vacancy order in YBa 2Cu 3O 7−δ: A chemical valence model

Using a simple chemical model, the hole concentration is calculated for YBa 2Cu 3O 7−δ as a function of oxygen stoichiometry δ. For the calculations, it is assumed that 2-coordinated Cu is monovalent while 3- and 4-coordinated Cu's are nominally divalent. Because monovalent Cu's bind more electrons than do divalent Cu's, the degree of hole doping of the conduction band depends on the oxygen vacancy configuration. We examine extreme ordering conditions and apply the chemical valence model to an extended phase field of the double-cell (Ortho II) structure. For δ > 0.2, the dependence of the calculated hole concentration on δ, obtained for the (defect) double-cell structure, is similar to the observed dependence of T c on δ suggesting a simple relationship between T c and hole concentration. If T c is assumed to depend quadratically on hole concentration (as observed for other oxide superconductors), then the calculated T c versus δ corresponds closely with experiment. Low temperature aging behavior is also simulated in the model.

Determination of oxygen content and carbonate impurity in YBa2Cu3O7−x by diffuse reflectance infrared spectroscopy

Samples of YBa2Cu3O7−x with x ranging from ∼0 to 0.65 have been analyzed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in the midinfrared region (400–6000 cm−1). Spectral line shapes vary gradually as a function of oxygen stoichiometry, and the reflectance at 400 and 1000 cm−1 decreases linearly with decreasing oxygen content. Spectra of samples that were incompletely synthesized or exposed to a 4% CO2 atmosphere at 650 °C clearly indicated the presence of carbonate. DRIFTS is therefore a quick, nondestructive method for determining oxygen content in YBa2Cu3O7−x powders, and for detecting carbonate species due to synthesis error or reaction with CO2-bearing atmosphere.

Influence of the oxygen content on the critical temperature T c and the thermopower of Y 0.7Ca 0.3Ba 2Cu 3O z

Superconducting properties of Ca-substituted YBa 2Cu 3O z have been investigated. Electrical and magnetic measurements as a function of oxygen stoichiometry have been carried out. The results show that Y 0.7Ca 0.3Ba 2Cu 3O z samples remain superconducting in the 6.02≤ z≤6.845 range as first predicted by theoretical study. We also correlate this superconducting behaviour with the thermopower measurements performed.

High temperature X-ray diffraction studies of the compound YBa2Cu3Ox

Ambient and high temperature X-ray diffraction methods were used to determine the unit cell parameters of the orthorhombic and tetragonal phases of the YBa2Cu3Ox compound, as a function of oxygen stoichiometry and temperature. The total oxygen content was obtained by the thermogravimetric method (TGA) by determining the weight loss in hydrogen up to 1000 degrees C. Reduction reactions in hydrogen were also performed in situ using a high temperature X-ray unit. Using the TGA method the oxygen content in the structure was obtained by weight loss in 100% oxygen. Thermal expansion coefficients of the lattice parameters were determined for both orthorhombic and tetragonal materials with different, constant x values.

Structure and valence from complementary anomalous X-ray and neutron powder diffraction

We introduce a new type of crystallography whose major aim is to study the crystal chemistry and the electronic structure of constituent atoms or ions in crystals, rather than the crystal structure itself. These results are obtained from recent experiments that take advantage of the complementary nature of anomalous X-ray and neutron diffraction data, and of the fact that anomalous scattering can provide substantial changes in the scattering factors for atoms or ions with similar atomic number Z or with the same Z but different valence. Examples presented include: the determination of constituent ordering among several sites in Fe/Co/V alloys and in certain high temperature superconductors; the study of ordering that arises via valence fluctuation in metallic cerium; the determination of anomalous X-ray scattering factor variation for copper with oxidation state; and finally, the determination of site-specific valence of the Cu ions in the chain and plane sites in the high temperature superconductor YBa 2Cu 3O 6+ x as a function of oxygen stoichiometry.

R2Ba4Cu7O15- delta : A 92-K bulk superconductor.

Superconductivity is investigated in {ital R}{sub 2}Ba{sub 4}Cu{sub 7}O{sub 15{minus}{delta}} ({ital R}=Y or Er) as a function of oxygen stoichiometry. High-quality single-phase ceramic samples prepared at 869 {degree}C in oxygen at one atmosphere were annealed under various oxygen partial pressures and temperatures then rapidly quenched into liquid nitrogen to fix the value of {delta}. {ital T}{sub {ital c}} was found to decrease monotonically with {delta} from a maximum of 92 K when {delta}=0, to a minimum of 30 K when {delta}=10. The normal-state carrier concentration has a one-to-one correspondence with {delta}. There is no significant difference between the oxygen unloading curves for {delta} as a function of temperature and pressure for Y{sub 2}Ba{sub 4}Cu{sub 7}O{sub 15{minus}{delta}} and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}; i.e., the unloading in the latter is twice that of the former. The volume change per oxygen vacancy is 3.1 A per formula unit compared to 3.6 A for YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. The material remains orthorhombic for all values of {delta}.

Structural anomalies, oxygen ordering and superconductivity in oxygen deficient Ba 2YCu 3O x

We report the characterization of series of oxygen deficient Ba 2YCu 3O x samples for 7 ≥ x ≥ 6 prepared by Zr gettered annealing at 440°C. Measurements include complete crystal structure analysis at 5 K by powder neutron diffraction, electron microscopy study of the oxygen ordering, and magnetic measurements of the superconducting transitions, with particular attention to the transition widths. The results show for the first time that the 90 K and 60 K plateaus in T c as a function of oxygen stoichiometry are associated with plateaus in the effective valence of the plane coppers. We also correlate the disappearance of superconductivity for x < 6.5 with an abrupt transfer of negative charge into the CuO 2 planes. We propose that different ordering schemes of oxygen have different characteristic T c's between 90 and 60 K.

Physical and structural properties of the oxygen vacancy phases of ErBa 2Cu 3O x

Samples of ErBa 2Cu 3O x with defined oxygen contents 6.0 ⩽ x ⩽ 7.0 were prepared by an oxygen desorption-absorption procedure. Superconducting transition temperatures T c , room-temperature lattice parameters and the high-temperature orthorhombic to tetragonal (O-T) phase boundary were determined as a function of oxygen stoichiometry. T c vs. x shows a step-like behaviour at about 92 K (6.9 ⩽ x ⩽ 7.0) and around 65 K (6.6 ⩽ x ⩽ 6.7). At room temperature, the ErBa 2Cu 3O x structure is tetragonal for x < 6.25, and orthorhombic for x > 6.25. The high-temperature O-T phase boundary determined by resistance measurements extends from x ∼- 6.5 at 750 K to x ∼- 6.59 at 920 K. A partial, pseudobinary phase diagram for the O-T phase transition is proposed based on these results.

Effect of iron doping and oxygen stoichiometry on infrared absorption in Y1Ba2Cu3O7−δ

We report infrared absorption of Y1Ba2Cu3O7−δ as a function of oxygen stoichiometry (0<δ<1) and copper substitution by iron in the spectral range of 450–700 cm−1. The strong bands associated with Cu-O vibrations undergo significant changes in their frequencies and intensities asδ is varied across the orthorhombic to tetragonal phase. These changes coupled with those arising as a result of doping with iron has helped in identifying the nature of the vibrational modes.

Far-infrared spectroscopy of RBa2Cu3Ox with variations in R and x.

We report far-infrared reflectance measurements of R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$, where R=Y, Er, Nd, and 6&lt;x&lt;7 for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$. A phonon mode at 150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ appears to be a sensitive probe of disorder in the Cu-O linear chains, while the mode at 191 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is shown to be primarily an R vibration. The reflectance spectra vary smoothly as a function of oxygen stoichiometry with quite distinct features characteristic of low- and high-oxygen-content samples. The gap which has been reported previously in fully oxygenated samples does not behave as expected for a Bardeen-Cooper-Schrieffer superconducting energy gap when the oxygen content is varied.

Oxygen Diffusion in Nickel Ferrous Ferrite

The diffusion coefficients of oxygen in single crystals of nickel ferrite with Fe/Ni=3.4 were determined as a function of oxygen stoichiometry between 1140° and 1340°C. The diffusion kinetics were followed by measuring the rate of exchange of 18O between the gas and solid slabs of ferrite, and the oxygen stoichiometry was established by controlling total pressure of oxygen in the gas phase. At higher oxygen contents, the diffusion coefficient is independent of stoichiometry and is given by D=10‐2.3exp cm2/s. At low oxygen contents the diffusion rate depends strongly on oxygen stoichiometry and is smaller. In polycrystalline ferrite the diffusion at 1197°C is 40 times faster than in single crystals but shows the same dependence on oxygen stoichiometry. Probable diffusion mechanisms are discussed.