Disordered Oxygen Vacancies Research Articles

Anomalous Expansion of CeO2 Nanocrystalline Membranes

Nanocrystalline, nominally undoped films of CeO2–x undergo large (0.8–2 %) lateral expansion after local substrate removal and formation of freestanding membranes (see Figure). The structural, optical, and electrical properties are consistent with the hypothesis that substrate removal triggers disordering of oxygen vacancies leading to this expansion, which occurs irrespective of whether tensile or compressive stress existed in the films.

Oxide-Ion Conductivity of BaLaInMO5.5 (M=Sc, Ga and In) and BaLaIn2-xScxO5.5 (x=0.1, 0.2 and 0.3)

Solid solutions of BaLaIn2O5.5 and BaLaInMO5.5 that formed by partial replacement of In by Sc and Ga have a cubic perovskite structure. The Sc3+- and Ga3+-doped compounds, which had almost the same electrical conductivities, exhibited lower conductivities than the parent compound BaLaIn2O5.5 in the temperature range from 573 to 1273K. The Ga compound showed a transference number, ti, of unity at both high and low oxygen partial pressures in the temperature range from 973 to 1273K. As for the Sc compound kept at 973-1273K, a ti of nearly unity was observed at low oxygen partial pressure, while the ti was slightly lower than unity at high oxygen partial pressures at higher temperatures. The In compound had a ti of almost unity at high oxygen partial pressure, whereas the number decreased to a value less than 0.9 at low partial pressures of oxygen. The stepwise increase in the Sc content from 0.1 to 1.0 in BaLaIn2-xScxO5.5 caused a monotonic decrease in conductivity. The decrease in conductivity and the increase in activation energy caused by Sc3+ and Ga3+ doping were ascribed to the partial ordering of originally disordered oxygen vacancies and a decrease in the size of the triangle formed by two A-site cations and one B-site cation, through which the oxide ion proceeds, respectively.

On the relationships between structure, oxygen stoichiometry and ionic conductivity of CaTi 1− xFe xO 3− δ ( x=0.05, 0.20, 0.40, 0.60)

For the first time, Fe 4+ was observed and analysed in a systematic manner in CaTi 1− x Fe x O 3− δ ( x=0.05, 0.20, 0.40 and 0.60) perovskites and its presence related with the ionic conductivity of these materials. These oxides were prepared by solid state reaction under oxidizing conditions. Slowly cooled CaTi 1− x Fe x O 3− δ ( x=0.05, 0.20 and 0.40) oxides crystallise with the same orthorhombic symmetry of CaTiO 3. Some of the weaker peaks tend to disappear with increasing x and are no longer present for x=0.60, which can be indexed on a cubic unit cell. Mössbauer spectroscopy revealed the coexistence of Fe 4+ and Fe 3+ in all compositions, with prevalence of the Fe 3+ species. The relative amounts of Fe 3+ coordinated by 6, 5 and 4 oxygen ions were estimated. Oxygen stoichiometry changes, determined by solid electrolyte potentiometry–coulometry, were found to be in good agreement with the number of oxygen vacancies per unit formula based on estimates of the relative amounts of Fe 4+ and Fe 3+ obtained from the Mössbauer spectra. The ionic conductivity follows a typical Arrhenius behaviour with a sharp maximum at x=0.20. This behaviour is explained from combined Mössbauer spectroscopy and coulometric titration data, based on the existence of ordered and disordered oxygen vacancies associated with tetracoordinated and pentacoordinated Fe 3+ ions, respectively. Differences between the ionic conductivity of ceramics prepared under reducing and oxidising conditions suggest that Fe 4+ cations may stabilise disordered structures at low temperatures, thus enhancing the transport properties.

Oxygen permeation in the system SrFeO 3− x–SrCoO 3− y

Oxygen flux through dense membranes with composition SrFeO 3− δ , SrFe 0.67Co 0.33O 3− δ and Sr 0.97Fe 0.33Co 0.67O 3− δ (0, 33% and 67% Co) have been measured primarily at 1000 °C as a function of membrane thickness with synthetic air on the primary side and He on the secondary side. Maximum flux rate was found to increase with cobalt content with an observed maximum for a membrane with composition 67% cobalt and thickness 0.5 mm corresponding to 3.8×10 −6 mol/cm 2 s. Based on experimental data, the critical thickness has been estimated to decrease from 2.0 mm for SrFeO 3− δ to 0.7 mm for a compound with 67% cobalt added. A severe oxygen flux reduction is observed below approx. 900 °C for membrane samples with composition Sr 0.97Fe 0.33Co 0.67O 3− δ , corresponding to a transition from a cubic perovskite with disordered oxygen vacancies to a rhombohedral brownmillerite structure with ordered oxygen vacancies.

Hard mode infrared spectroscopy of CaTiO3—CaFeO2.5 perovskites

Powder-absorption infrared (IR) spectra of perovskites CaFexTi1−x O3−x/2 (0≤x≤1) annealed at different temperatures were investigated at room temperature in the range 135–2000 cm−1. The spectra change as a function of composition, annealing temperature and structural state (order-disorder of oxygen vacancies). Autocorrelation analysis has been used to determine variations of average line widths of groups of peaks in the primary IR spectra. The band widths increase on increasing Fe content in the region of the structures with disordered oxygen vacancies and they decrease on going through the order-disorder boundary. High degrees of local structural heterogeneity are suggested by the effective line widths of the phases at intermediate compositions. The intensity of bands at ∼150 and ∼443 cm−1 decreases with increasing Fe content in the compositional range of the disordered structures. Finally, for every annealing temperature, the frequency of the band at ∼600cm−1 systematically shifts to higher values on increasing Fe content, these values decreasing again for the fully ordered structures.

The Structures of C–Ce2O3+δ, Ce7O12, and Ce11O20

The crystal structures of CeO1.68 (C–Ce2O3+δ), Ce7O12, and Ce11O20 with ordered oxygen vacancy distributions have been determined by refinement of single crystal neutron diffraction data collected at ambient temperature. CeO1.68 is cubic, of space group Ia3 with a=11.111(2) Å; one of the two oxygen positions is partially occupied with a probability of 37(3)%. For Ce7O12 the rhombohedral space group R3 with a=6.785(1) Å, α=99.42(1)° was confirmed. Ce11O20 is isomorphous to Tb11O20 with the triclinic space group P1. The lattice parameters are a=6.757(3) Å, b=10.260(5) Å, c=6.732(3) Å, α=90.04(4)°, β=99.80(4)°, and γ=96.22(4)°. For Ce11O20 there are no 12[1, 1, 1]F oxygen vacancy pairs, i.e., pairs corresponding to half of the space diagonal of the fluorite lattice along [1, 1, 1]F. The shortest vacancy pair type in that structure is 12[1, 1, 1]F, which is also present in the two other phases. High temperature neutron diffraction measurements showed that Ce7O12 is stable up to 1072 K. Then it transforms into a fluorite type structure with disordered oxygen vacancies.

Influence of cations vacancies on the K and LIII Zn edges of spinel-related compounds.

Compounds having oxygen vacancies exhibit particular physico-chemical properties. For example, in the ease of the perovskite type cubic system, disordered oxygen vacancies are intimately linked to ionic conductivity (Mori et al, 1994). Here we conduct a fundamental research study dedicated to spinel related compounds in order to discuss the information which can be extracted from the XANES part of the absorption spectra. In order to achieve this goal, we first recall the structural characteristics of the ZnAI204 reference compound and the structural parameters derived from the numerical simulations of the EXAFS modulations collected at the Zn K edge on a set of original materials coming from soft chemistry (Bazin et al, 1998). Then, we will discuss the XANES data obtained at the Zn K and Lm edges as well as the numerical simulations performed us ing the FeFF program and corresponding to several structural hypothesis (essentially influence of Zn or O vacancies).

Comparative study of Nickel-based perovskite-like mixed oxide catalysts for direct decomposition of NO

The mixed oxides LaNiO 3, La 0.1Sr 0.9NiO 3, La 2NiO 4 and LaSrNiO 4 were prepared and used as catalysts for the direct decomposition of NO. The catalysts were characterized by means of XRD, XPS, O 2-TPD, NO-TPD and chemical analysis. By comparing the physico-chemical properties and catalytic activity for NO decomposition, a conclusion could be drawn as follows. The direct decomposition of NO over perovskite and related mixed oxide catalysts follows a redox mechanism. The lower valent metal ions Ni 2+ and disordered oxygen vacancies seem to be the active sites in the redox process. The oxygen vacancy plays an important role favorable for the adsorption and activation of NO molecules on one hand and on the other hand for increasing the mobility of lattice oxygen which is beneficial to the reproduction of active sites. The presence of oxygen vacancies is one of the indispensable factors to give the mixed oxides a steady activity for NO decomposition.

Oxygen and proton conduction in the yttrium tantalate pyrochlore phase

The pyrochlore solid solution Y3+xTa1–xO7–x in the Y2O3-Ta2O5 system exists from 17.5 to 25 mol% Ta2O5. The conductivity of Y3+xTa1–xO7–x increases with x to a maximum near x=0.2, as a result of the increased content of disordered oxygen vacancies. The activation energy and pre-exponential factor vary in a similar manner. The transference numbers measured in air indicate that these materials are predominantly proton conductors at temperatures below 400 °C, oxygen electrolytes at temperatures between 400 and 750 °C and predominantly hole conductors at higher temperatures.

Solid State Chemistry of New Perovskite and Ruddlesden-Popper Phases in the La2O3-CaO-CuO System at High Pressures

The crystal chemistry of the system LaO1.5-CaO-CuO at the 1:1:2 composition was studied at high pressures with the goal of stabilizing new perovskite cuprates with two-dimensional ordering of La, Ca cations and oxygen vacancies. Several phases, including the perovskites La4Ca4Cu8O20, La4Ca4Cu88O18 and the Ruddlesden-Popper (RP) phase La2Ca2Cu3O8, containing three copper oxide layers (n = 3), were revealed for the first time. The results are very sensitive to experimental conditions, and a variety of reaction channels are observed at the 1:1:2 composition depending on the choice of total pressure, Po2, temperature, and annealing conditions. The perovskite-related phases at this composition exhibited A-site cation disorder and three-dimensional ordering of oxygen vacancies. The Ruddlesden-Popper phase required the substitution of Sr on the A-sites to be metastably retained at room temperature. Due to its thermal instability, the RP phase could not be doped to a carrier concentration at which superconductivity might be observed.

Crystal structure of Ba 5In 2Al 2ZrO 13

The crystal structure of anion-deficient hexagonal perovskite-like oxide Ba 5In 2Al 2ZrO 3 ( a = 5.8707(7) A ̊ , a = 24.445(3) A ̊ , space group P6 3/ mmc, z = 2) was determined using single crystal data. A least-squares refinement gave conventional and weighted R factors of 0.037 and 0.040 respectively. This structure is a 10L ( cchcc) 2 sequence of close-packed BaO 3 and BaO□ 2 layers. The main feature of the structure is joint occupation of two 4f positions by In and Al atoms, which result in disordering of oxygen atoms and vacancies in the BaO□ 2 layers. It was shown that this structure can be considered formally as a result of the intergrowth of Ba 2InAlO 5-like blocks and BaZrO 3 ones along the c-axis of the hexagonal supercell.

Electrical transport and superconductivity in the (Y 0.8Ca 0.2)Ba 2Cu 3O y system with variable oxygen content

The effect of oxygen content and thermal treatment has been studied in the (Y 0.8Ca 0.2)Ba 2Cu 3O y system. The introduction of Ca altered the normal state and superconducting properties substantially. Particularly intriguing is that Ca-doped samples with more oxygen-vacancy disorder have higher superconducting transition temperatures ( T c) and lower resistivities, contrary to the non-Ca-doped system. The correlation between T c and carrier concentration cannot be described by the empirical relation observed in many high- T c oxides.

A new high temperature modification of face-centered cubic Y2O3

A new high temperature form of Y2O3 has been synthesized at 2220 °C by a laser beam heating technique. The new form has a face-centered cubic structure with a = 5.2644(3)Å, Z = 2, Dx = 5.14 g/cm3, and is considered to have a fluorite-type structure containing disordered oxygen vacancies.

Investigation of defect ordering in the perovskite system SrCr0.1Fe0.9O3 –yby Mössbauer spectroscopy

The oxygen-deficient perovskite system SrCr0.1Fe0.9O3 –y has been studied by Mössbauer spectroscopy and X-ray powder diffraction techniques. The material obtained is strongly dependent upon the conditions of preparation. Partially oxidized materials annealed in air and quenched are simple cubic perovskites with considerable cation and oxygen vacancy disorder. Annealing at 1200°C in vacuo and then cooling step-wise produces an ordered brownmillerite superlattice with the chromium ordered onto octahedral sites. However, rapid cooling gives a microdomain-textured material containing a random distribution of cations; the introduction of chromium stabilizes the high-temperature state of Sr2Fe2O5 which has never been quenched to room temperature. Annealing in an argon stream at 1200 °C gives a partially oxidized crystalline brownmillerite in which the chromium (and extra oxygen) are incorporated in the tetrahedral layers. The magnetic ordering temperatures are correlated with the observed site distribution of the chromium.

New Oxide-Ion Electrolytes

AbstractStrategic considerations for the design of new O2--ion solid electrolytes are presented, and preliminary measurements of the oxide-ion conductivity σo = (B/T) exp (-EA/kT) and transport number tO = σO/σ are reported for several oxides with perovsldte-related structures. Ba2In2O5 exhibits a first-order transition at Tt ∼ 930°C from the brownmillerite structure to a cubic perovskite with disordered oxygen vacancies and fast O2-ion conduction. Ba3In2MO8 (M = Ce, Zr, or Hf) contains oxygen vacancies ordered into every second BO2 (001) plane. An octahedralsite preference of the disordered M(IV) cations makes incomplete the oxygen-vacancy ordering, so a high extrinsic σo with EA ∼ 0.6 eV is found for T ≤ 400 °C. The transport properties of Ba3In2ZrO8 at 400 °C are equivalent to those of the stabilized zirconias at 800 °C. BagIn6O17 consists of an intergrowth of (BaO)2 rocksalt and Ba3In3O7.5 oxygen-deficient perovskite layers. Although there is no evidence of long-range ordering of the oxygen vacancies, nevertheless an EA = 1.06 eV and transport properties equivalent to stabilized zirconia indicate retention of short-range correlations to at least 950 °C. Attempts to prepare Bag Y6O17 yielded a mixture of hexagonal Ba3Y4O9 and unidentified phases. An EA = 0.45 eV above a Tt=370 °C gives a remarkable O2--ion conductivity at temperatures T > Tt in both the mixed-phase and pure Ba3Y4O9 at temperatures T > 370 °C, but the samples are extremely hygroscopic.

Separate paramagnetism and superconductivity in YBa2Cu3Oz

The magnetic properties of YBa2Cu3Oz with different values of z in the temperature range 1.5–300 K were measured and compared with those of DyBa2Cu3Oz and EuBa2Cu3Oz. The susceptibility data of YBa2Cu3Oz and DyBa2Cu3Oz followed the Curie–Weiss law above Tc, while those of EuBa2Cu3Oz exhibited the Van Vleck and Frank behavior [Phys. Rev. 34, 1494 (1929)] with σ=33. The paramagnetism persisted even below Tc. The effective moment per copper ion increased as the oxygen content decreased. The results suggest that the paramagnetism in YBa2Cu3Oz originates mainly from the nonsuperconducting domains of the material which have disordered oxygen vacancies or have lower oxygen content than the superconducting ones.

Experimental features of the antiferromagnetism in Ba2GdCu3O7-y.

Structure, resistivity, Meissner effect, magnetization and specific-heat measurements were conducted on Ba/sub 2/GdCu/sub 3/O/sub 7-//sub y/ with different oxygen contents. The results indicate that the local symmetry breaking due to twinning or disorder of oxygen vacancies may give rise to a frustration of the collinear spin structure. The critical behavior of the specific heat is logarithmic but with different amplitudes above and below the transition temperature, implying that the magnetic transition is driven by quasi-three-dimensional magnetic interactions.

Experimental study of La1Ba2Cu3Ox.

We have prepared samples in the ${\mathrm{La}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$ series with superconducting transition temperatures ${T}_{c}$ ranging from 23 to 72 K (onset temperatures from 43 to 88 K). Results from scanning-electron-microscopy characterization, x-ray diffraction analysis, thermogravimetric analysis, the temperature dependence of the resistivity, and temperature dependence of the magnetic susceptibility are presented. The samples with ${T}_{c}$ greater than 50 K are orthorhombic while samples with ${T}_{c}$ less than 50 K show tetragonal structure. The transition temperature is found to depend on the oxygen content x with an apparent peak near x=7. Our sample with the highest ${T}_{c}$ (72 K) had x=6.75. As x decreases, so does ${T}_{c}$, reaching 44 K when x=6.47. For oxygen content greater than 7, ${T}_{c}$ decreases rapidly to a value of 23 K when x=7.27. The room-temperature resistivity increases systematically from 0.8 to 7.9 m\ensuremath{\Omega} cm as ${T}_{c}$ decreases from 72 to 27 K. The increase is resistivity in the normal state and the decrease in the resistivity ratio (${\mathrm{\ensuremath{\rho}}}_{300\phantom{\rule{0ex}{0ex}}\mathrm{K}}$/${\mathrm{\ensuremath{\rho}}}_{\mathrm{residual}}$ are interpreted in terms of disorder in the microscopic structure which suppresses ${T}_{c}$. The magnetic susceptibility of these samples separates into a Pauli susceptibility ${\ensuremath{\chi}}_{0}$ and a Curie-Weiss susceptibility C/(T-FTHETA), after the core contribution is subtracted. A systematic increase in ${\ensuremath{\chi}}_{0}$ and a decrease in the Curie constant C has been observed as ${T}_{c}$ varies from 23 to 72 K. This is interpreted as an increase in the density of states at the Fermi surface coupled with a disappearance of local magnetic moments. We interpret our experimental results in terms of two types of disorder: disordered oxygen vacancies and an additional disorder of the lanthanum and barium ions.

Local structural excitations in the new superconducting oxides

The local structural excitations (LSEs) in the new superconducting oxides, based on the migration of the oxygen atoms in the Cu-O planes, are analysed with a simple microscopic model which involves two different local configurations and the transitions between them. A new electronic pairing mechanism via exchange of LSEs is suggested. By taking into account both the electron-phonon and the electron-LSE interactions, a formula for Tc in the strong-coupling case is derived. The results give a new insight into the relation between the oxygen vacancy structure and the superconducting transition temperature in these materials. It is shown that the orthorhombic structure with an ordered distribution of oxygen vacancies is more favourable for high Tc than is the tetragonal structure with disordered oxygen vacancies; for superconductors with a dominant LSE mechanism the isotopic effect is not easily observed under ordinary conditions. These arguments are in qualitative agreement with the current experimental results. Methods for obtaining further experimental confirmation of this are suggested.

Mössbauer and magnetic investigations of YBa 2Cu 3O 7 superconductors

The d.c. electric resistivity, static and a.c. magnetic susceptibility as well as Mössbauer effect methods have been employed to study properties of YBa 2Cu 3O 7 superconductors. Substitution of 0.1 at .% of Cu atoms by 57Fe atoms was found to have no noticeable influence on electric and magnetic properties of the investigated sample. The a.c. magnetic susceptibility method has indicated an energy dissipation of the a.c. magnetic field inside the investigated samples while the Mössbauer effect method has indicated the distribution of an electric field gradient at the Cu-II positions caused by a disorder of oxygen vacancies. The transition to the superconducting state does not change an electronic configuration of the 57Fe atoms.