Distribution by Scientific Domains

Kinds of Vacancies

  • anion vacancy
  • carbon vacancy
  • cation vacancy
  • oxygen vacancy

  • Terms modified by Vacancies

  • vacancy cluster
  • vacancy concentration
  • vacancy defect
  • vacancy diffusion
  • vacancy formation

  • Selected Abstracts

    Theoretical Defect Energetics in Calcium Phosphate Bioceramics

    Katsuyuki Matsunaga
    Vacancies, impurities, and foreign ions dissolving in calcium phosphate bioceramics play an important role in the biological properties of the materials. However, little is known about the thermodynamic stability of the defects. In this regard, point defects in hydroxyapatite (HAp) and octacalcium phosphate (OCP) were calculated in a first-principles manner, and the chemical-potential dependence of the defect formation energies was revealed. In particular, because calcium phosphates are usually subjected to an aqueous solution, a methodology to evaluate ionic chemical potentials under chemical equilibrium of the solid,aqueous solution was introduced. In the present article, recent results based on such a methodology (the solution pH dependence of Ca/P molar ratio of HAp and the ion-exchange ability with foreign cations in HAp and OCP) were reviewed. [source]

    First-Principles Calculations of Anion Vacancies in Oxides and Nitrides

    Isao Tanaka
    The formation energy, structural relaxation, and defect-induced states of neutral anion vacancies of five oxides (i.e., MgO, Al2O3, ZnO, In2O3, and SnO2) and four nitrides (i.e., AlN, Si3N4, Ge3N4, and InN) are systematically discussed, based on first-principles plane-wave pseudopotential calculations. Two types of polymorphs for each compound are compared. The number of atoms included in the supercells ranged from 54 to 96. When a localized vacancy-induced state appears within the band gap, as in a typical ionic crystal, the formation energy can be well scaled by the band gap of the perfect crystal. On the other hand, when an empty and localized vacancy-induced state is located above the highest occupied band or no localized state is formed, the formation energy has a tendency to be smaller. In compounds such as ZnO and SnO2, the formation energy is dependent largely on the crystal structure. This result can be explained by the transition of the vacancy-induced state from occupied to unoccupied, which is caused by the change in atomic arrangement, as represented by the cation coordination number. [source]

    Cerium iron sulfide, Ce3Fe1.94S7

    Allison M. Mills
    Tricerium(III) diiron(II,III) hepta­sulfide, Ce3Fe1.94S7, crystallizes in the polar hexagonal space group P63 and adopts the Ce6Al3.33S14 structure type. The Fe atoms occupy both tetrahedral and octahedral sites. Isolated FeS4 tetrahedra, all pointing in the same direction, are stacked along the threefold rotation axes. Chains of face-sharing FeS6 octahedra propagate along the 63 axis. Vacancies resulting from the partial oxidation of Fe2+ to Fe3+ occur exclusively in the octahedral Fe sites. The Ce atoms are coordinated by [7+1] S atoms, which form bicapped trigonal prisms. [source]

    ChemInform Abstract: Vacancies and Insertions in the RE10Ni9+xIn20 Series (RE: Ho,Tm, Lu).

    CHEMINFORM, Issue 52 2009
    Srinivasa Thimmaiah
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

    ,-Ca3N2, a Metastable Nitride in the System Ca,N

    Peter Höhn Dr.
    Abstract A new page in the phase book: The metastable nitride ,-Ca3N2, the synthesis, crystal structure (see picture), and physical properties of which are reported, is an isotype of corundum (,-Al2O3). Vacancies in the crystal structure of ,-Ca3N2 are discussed by using full-potential local orbital methods and taking into account both the physical properties and the density of states. The structural chemistry of calcium nitrides is represented by just two different intermediate phases: cubic Ca3N2 (referred to as ,-Ca3N2, Mn2O3 type structure) and Ca2N (CdCl2 type structure). Other binary phases (",-Ca3N2", "Ca11N8") have been proven to represent cyanamide nitrides, Ca4N2[CN2] and Ca11N6[CN2]2, respectively. The metastable nitride ,-Ca3N2 is prepared either by reaction of freshly distilled calcium metal with nitrogen at 700,K, or by reaction of Ca2N with N2 at 500,K. ,-Ca3N2 transforms monotropically (Ttrans,810,K) to cubic ,-Ca3N2. The crystal structure of ,-Ca3N2 (rhombohedral Rc, no. 167, a=619.884(3),pm, c=1662.958(10),pm) is an isotype of the corundum structure (,-Al2O3) and comprises a network of NCa6/4 octahedra that share common faces and edges. Magnetic susceptibility and electrical resistivity measurements confirm that ,-Ca3N2 is a diamagnetic semiconductor. Die Strukturchemie der Calciumnitride wird von lediglich zwei intermediären Verbindungen bestimmt: kubisches Ca3N2 (nachfolgend als , -Ca3N2 bezeichnet; Mn2O3 -Strukturtyp) und das im CdCl2 -Strukturtyp kristallisierende Ca2N. Die binären Phasen ,, -Ca3N2" und ,Ca11N8" konnten inzwischen als die Cyanamid-Nitride Ca4N2[CN2] und Ca11N6[CN2]2 identifiziert werden. Metastabiles , -Ca3N2 kann entweder durch Reaktion von frisch destilliertem Calcium-Metall mit Stickstoff bei 700,K oder durch Umsetzung von Ca2N mit N2 bei 500,K erhalten werden. , -Ca3N2 transformiert monotrop (Ttrans,810,K) zur kubischen Phase , -Ca3N2. Die Kristallstruktur von , -Ca3N2 (rhomboedrisch Rc, No. 167, a=619.884(3),pm, c=1662.958(3),pm) ist isotyp zur Korundstruktur (, -Al2O3) und besteht aus einem Netzwerk von NCa6/4 -Oktaedern, die über gemeinsame Flächen und Kanten verknüpft sind. Messungen der magnetischen Suszeptibilität und des elektrischen Widerstands zeigen, daß , -Ca3N2 ein diamagnetischer Halbleiter ist. [source]

    Increasing the Number of Oxygen Vacancies on TiO2 by Doping with Iron Increases the Activity of Supported Gold for CO Oxidation

    Silvio Carrettin Dr.
    Abstract The addition of iron to high-area TiO2 (Degussa P25, a mixture of anatase and rutile) increases the number of oxygen defect sites that react with O2 to form peroxide and superoxide species. In the presence of gold nanoclusters on the TiO2 surface, the superoxide species become highly reactive, and the activity of the supported gold catalyst for CO oxidation is approximately twice that of the most active comparable catalysts described in the literature. Images of the catalyst obtained by scanning transmission electron microscopy combined with spectra of the catalyst measured in the working state (Raman, extended X-ray absorption fine structure, and X-ray absorption near-edge structure) indicate strong interactions of gold with the support and the presence of iron near the interfaces between the gold clusters and the TiO2 support. The high activity of the catalysts is attributed to the presence of defects in these sites that activate oxygen. [source]

    Departures, Arrivals and a Vacancy

    ANTIPODE, Issue 1 2008
    Noel Castree
    No abstract is available for this article. [source]

    Optical properties of ZnO nanotubes

    Hongju Zhai
    Abstract Tubular ZnO nanostructures have been obtained via a hydrothermal method at low temperature (90 °C) without any catalysts or templates. The XRD measurement reveals that the tubes are single crystals with hexagonal wurtzite structure. SEM shows that the diameters of ZnO nanotubes ranged from 400 to 550 nm. The Raman and PL spectra indicate that oxygen vacancies or Zn interstitials are responsible for the green emission in the ZnO nanotubes. A possible growth mechanism on the formation of crystalline ZnO nanotubes has been presented. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Work-hardening characteristics of Zn-Ti alloy single crystals

    G. Boczkal
    Abstract Shear stress , shear strain curves of 0.14 at.%Ti alloyed Zn single crystals were measured in compression at different temperatures and shear strain rates. The work-hardening coefficient for basal slip increases with decreasing temperature and increasing shear strain rate. The work-hardening characteristics are compared with those reported for Zn single crystals with different constituents and purities. It is discussed with respect to the interaction of dislocations with dislocations, vacancies, vacancy agglomerates and solute atoms. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Role of oxygen vacancies in the coloration of 0.65Pb(Mg1/3Nb2/3)O3 -0.35PbTiO3 single crystals

    Zhiguo Xia
    Abstract The coloration and oxygen vacancies in 0.65Pb(Mg1/3Nb2/3)O3 -0.35PbTiO3 (PMN-PT(65/35)) (starting composition) single crystals grown by a so-called modified Bridgman technique were investigated in this paper. Light yellow and dark brown colored crystals were generally observed for the typical as-grown PMN-PT(65/35) single crystals. X-ray diffraction results demonstrated that they were both of pure perovskite structure, but good electric properties were only obtained for the light yellow crystal. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of its components. The O 1s photoelectron spectra of the dark brown colored crystals located at the higher binding energy side, which meant the existence of the more oxygen vacancies. It accordingly led to the formation of the low valence cations associated with the coloration of the crystals, which is also testified by the obtained X-ray photoelectron spectra of Ti and Nb. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Morphological, structural and optical study of quasi-1D SnO2 nanowires and nanobelts

    D. Calestani
    Abstract 0.1,0.3 mm thick entanglements of quasi-one-dimensional semiconducting Tin dioxide nanocrystals, in form of nanowires and nanobelts, are successfully grown by low cost Chemical Vapour Deposition directly on large area (100 mm2) Al2O3, SiO2 and Si substrates. Their lateral size ranges from 50 to 700 nm and their length can achieve several hundreds of micrometers. Transmission Electron Microscopy reveals either the nanowires and the nanobelts grow in the tetragonal Rutile structure. Diffraction contrast analyses and selected area diffraction investigations show the nanowires are single crystals without defects while the nanobelts sometimes present twins inside. An almost cylindrical shape and an average diameter of about 30,50 nm for the smallest nanowires is reported. X-ray diffraction investigations exclude the presence of spurious phases. A broad band structured in two emissions peaked at about 450 nm and 560 nm is revealed by large area Cathotoluminescence, while single nanocrystal spectroscopy shows that the reduction of the lateral dimension of the nanobelts from 1000 nm to 50 nm blue-shifts the main emission band at 560 nm of about 40 nm (at room temperature). These preliminary results suggest a possible role of oxygen vacancies and of the surface/volume ratio on the origin and the blue shift of Cathodoluminescence spectra. The near band edge emission, typical of bulk tin dioxide (,320 nm), is not found in nanobelts narrower than 1000 nm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    A TEM study on ZrO2 -rich phases in the quasibinary system ZrO2 -Zr3N4: Comparison between fast and slowly cooled samples

    A. T. Tham
    Abstract A thorough comparison between the slowly and fast cooled ,,-type oxynitrides of zirconium has been carried out by means of transmission electron microscopy. This work aims at detecting and analyzing the so-called "modulated" ,, phase, which has been involved in the introduction of ordered anion vacancies through nitridation of zirconia. The gained information from the occurrence of such a modulated phase and the manner, in what it appears, do support the suggested structure model, which had been developed in early works in order to get a better understanding of the typical phase transitions in zirconium oxynitrides. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Order-Disorder Phase Transition in Type-I Clathrate Cs8Sn44,2,

    Andreas Kaltzoglou
    Abstract The clathrate compound ,-Cs8Sn44,2 has been synthesized from its elements under inert gas conditions and has been characterized by single-crystal and powder X-ray diffraction. At room temperature, it crystallizes with cubic symmetry [a = 24.256(3) Å, space group Iad, Z = 8] and adopts a 2,×,2,×,2 superstructure of the type-I clathrate and a high ordering of the vacancies (,) in the Sn framework. Single crystals of ,-Cs8Sn44,2 reversibly transform at 90 °C to the high-temperature , form with primitive symmetry [a = 12.135(1) Å, space group Pmn, Z = 1] and a lower ordering of the defects. Differential thermal analysis corroborates the reversible character of the phase transition, which occurs with an enthalpy change of approximately 0.38 J,g,1. An atom-migration mechanism describing the order-disorder transition involving spiro-connected six-membered rings only (scsr mechanism) is proposed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Study of the Defects in Sintered SnO2 by High-Resolution Transmission Electron Microscopy and Cathodoluminescence

    David Maestre
    Abstract The defect structure of sintered SnO2 was investigated by high-resolution transmission electron microscopy (HRTEM), cathodoluminescence (CL), and electrical measurements. HRTEM shows the presence of the SnO phase in the sintered samples as well as twinning, stacking faults, and disordered intergrowths. The sintered samples annealed under an oxygen atmosphere show changes in the defect structure and in the CL spectra. In particular, the intensity of a CL band at 1.94 eV, related to oxygen vacancies, decreased as the electrical resistivity increased. The results are discussed by considering the presence of stoichiometric defects such as oxygen vacancies and Sn interstitials in the final structure and their evolution during the annealing process under an oxygen atmosphere. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Synthesis, Crystal Structure, and Magnetic Properties of Mn2(OH)2SO4: A Novel Layered Hydroxide

    Mohsen Ben Salah
    Abstract Mn2(OH)2SO4, obtained as pink prismatic crystals by the hydrothermal reaction of MnSO4·H2O and NaOH at 240 °C for 24 h, consists of layers of Mn hydroxide connected to each other through µ6 -sulfate ions. Each layer exhibits vacancies, and each vacant space is capped at the top and bottom by the sulfate groups. The compound is paramagnetic above 50 K (C = 4.36 emu K mol,1, µeff = 5.91 µB, , = ,100 K). Below 45 K, the magnetization increases slightly, indicating canted-antiferromagnetic (TNéel = 42±1 K) behavior consistent with the linear dependence of the magnetization as a function of the field at 2 K, which reaches only 0.4 µB at 50 kOe, and the lack of any imaginary component of the ac-susceptibilities (ac = alternating current). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    Defect-Related Emissions and Magnetization Properties of ZnO Nanorods

    Bharati Panigrahy
    Abstract A clear correlation between defect-related emissions and the magnetization of ZnO nanorods synthesized by a one-step aqueous chemical method is demonstrated. The relative contribution of the emission bands arising from various types of defects is determined and found to be linked with the size of the nanorods and annealing conditions. When the size of the nanorods and the annealing temperature are increased, the magnetization of pure ZnO nanorods decreases with the reduction of a defect-related band originating from singly charged oxygen vacancies (). With a sufficient increase of annealing temperature (at 900,°C), the nanorods show diamagnetic behavior. Combining with the electron paramagnetic resonance results, a direct link between the magnetization and the relative occupancy of the singly charged oxygen vacancies present on the surface of ZnO nanorods is established. [source]

    Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls

    Haibo Zeng
    Abstract High concentrations of defects are introduced into nanoscale ZnO through non-equilibrium processes and resultant blue emissions are comprehensively analyzed, focusing on defect origins and broad controls. Some ZnO nanoparticles exhibit very strong blue emissions, the intensity of which first increase and then decrease with annealing. These visible emissions exhibit strong and interesting excitation dependences: 1) the optimal excitation energy for blue emissions is near the bandgap energy, but the effective excitation can obviously be lower, even 420,nm (2.95,eV,<,Eg,=,3.26,eV); in contrast, green emissions can be excited only by energies larger than the bandgap energy; and, 2) there are several fixed emitting wavelengths at 415, 440, 455 and 488,nm in the blue wave band, which exhibit considerable stability in different excitation and annealing conditions. Mechanisms for blue emissions from ZnO are proposed with interstitial-zinc-related defect levels as initial states. EPR spectra reveal the predominance of interstitial zinc in as-prepared samples, and the evolutions of coexisting interstitial zinc and oxygen vacancies with annealing. Furthermore, good controllability of visible emissions is achieved, including the co-emission of blue and green emissions and peak adjustment from blue to yellow. [source]

    Flame-Synthesized Copper Dimers: Flame-Synthesized Ceria-Supported Copper Dimers for Preferential Oxidation of CO (Adv. Funct.

    On page 369, R. Kydd et al. describe the creation of highly dispersed copper dimers (violet areas) on Ceria nanoparticles (green areas) via rapid flame-spray pyrolysis. These as-prepared copper dimers, in synergism with the oxygen vacancies in ceria, exhibit favourable electronic and catalytic interactions with incident CO molecules. Consequently, CO can be preferentially oxidised in H2 streams at low temperatures. [source]

    Nanostructured Titanium Oxynitride Porous Thin Films as Efficient Visible-Active Photocatalysts,

    E. Martínez-Ferrero
    Abstract Nanocrystalline mesoporous N-doped titania films have been prepared for the first time. The introduction of nitrogen into the anatase structure starts at 500,°C, with N bonding to titanium via oxygen substitution. Increasing the treatment temperature leads to the formation of TiN (TiN1,xOx) and N-doped rutile showing mixed-valence Ti states. Microstructural characterization shows that the ordered mesoporosity is maintained until 700,°C, where TiN (TiN1,xOx) begins to form. Optical characterization shows that the discrete introduction of N is able to shift the titania absorption edge. The photocatalytic tests give the best results under visible light excitation for the film nitrided at 500,°C. At this temperature the concentration of nitrogen in the structure is optimal since oxygen vacancies are still not important enough to promote the recombination of the photogenerated electrons and holes. [source]

    Cover Picture: Colloidal Synthesis of Hollow Cobalt Sulfide Nanocrystals (Adv. Funct.

    Abstract Hollow nanocrystals have been synthesized through a mechanism analogous to the Kirkendall Effect. When a cobalt nanocrystal reacts with sulfur in solution, the outward diffusion of cobalt atoms is faster than the inward diffusion of sulfur atoms through the sulfide shell. The dominating outward diffusion of cobalt cations produces vacancies that can condense into a single void in the center of the nanocrystal at high temperatures. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds and is described in the Full Paper by A.,P. Alivisatos and co-workers on p.,1389. Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at >,120,°C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature-dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell-size statistics and transmission electron microscopy images of as-grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system. [source]

    Graphene Monolayers: Chemical Vapor Deposition Repair of Graphene Oxide: A Route to Highly-Conductive Graphene Monolayers (Adv. Mater.

    ADVANCED MATERIALS, Issue 46 2009
    Graphene oxide (GO) is a promising precursor for the mass production of graphene. As an important step in this direction, the electrical conductivity of GO has been enhanced by six orders of magnitude, thus closely approaching that of exfoliated graphene. The novel two-step process reported by Cristina Gómez-Navarro and co-workers on p. 4683 involves hydrogen reduction and healing by a gaseous carbon feedstock. The inside cover shows a snapshot of the crucial second step. The oxidized regions in GO are represented in red, and the blue patches correspond to vacancies. [source]

    ,McJobs', ,good jobs' and skills: job-seekers' attitudes to low-skilled service work

    Colin Lindsay
    This article focuses on unemployed job-seekers' attitudes towards entry-level jobs in three areas of the service sector , retail, hospitality and call-centre work. The article examines whether job-seekers are reluctant to pursue these opportunities, and provides an analysis of the motives of those ruling out service work. A range of potential barriers is discussed, including the extent to which job-seekers perceive the service economy as offering only so-called ,McJobs', low-skilled, low-paid jobs with few opportunities for development. However, the article also focuses on perceived skills mismatches, with some job-seekers arguably over-qualified for entry-level service jobs, while others consider themselves to lack the necessary ,soft' skills. The analysis is based on interviews with 220 unemployed people in Glasgow. The article concludes that policy action may be required to encourage job seekers to consider a broader range of vacancies and to provide tailored training in partnership with service employers. On the demand side, service employers must address the need for entry-level positions that offer realistic salaries, decent work conditions and opportunities for progression and development. [source]

    A Family of Electronically Reconfigurable Nanodevices

    ADVANCED MATERIALS, Issue 37 2009
    J. Joshua Yang
    AFM image of 17 nanodevices with a zoom-in cartoon schematically shows an individual crosspoint device consisting of two Pt metal electrodes separated by a TiO2 bi-layer memristive material. By applying an electric field across the memristive material, oxygen vacancies can drift up and down, leading to four current-transport end-states. The switching between these end-states results in a family of nanodevices. [source]

    A One-Step Method for the Growth of Ga2O3 -Nanorod-Based White-Light-Emitting Phosphors

    ADVANCED MATERIALS, Issue 35 2009
    Sampathkumar Chrisolite Vanithakumari
    A one-step synthesis of Ga2O3 nanorods by heating molten gallium in ambient air at high temperatures is presented. The high-temperature synthesis creates oxygen vacancies and incorporates nitrogen from the environment. The oxygen vacancy in Ga2O3 is responsible for the emission in the blue,green region, while nitrogen in Ga2O3 is responsible for red emission. [source]

    The operation and management of agency workers in conditions of vulnerability

    Sonia McKay
    ABSTRACT This article focuses on the operation and management of agency labour by employers and observes that there are strong contradictions between the employers' stated reasons for using agency labour and the employment agencies' perceptions of why such labour is utilised. While discussing agency labour generally, the article also takes account of the position of migrant workers within the agency sector, since agencies have represented a significant route into employment for migrant labour. It draws primarily on 22 in-depth interviews with employers and employment agencies mainly in the food processing, cleaning and care sectors. The research revealed that while some employers were using agency staff to cover for specific peaks in production, others had used it as an alternative method of dealing with vacancies that otherwise might have resulted in permanent employment. [source]

    Electronic Manifestation of Cation-Vacancy-Induced Magnetic Moments in a Transparent Oxide Semiconductor: Anatase Nb:TiO2

    ADVANCED MATERIALS, Issue 22 2009
    Shixiong Zhang
    Nb-doped anatase TiO2 thin films grown by pulsed-laser deposition show Kondo scattering in elctronic-transport measurements, providing evidence for the formation of magnetic moments. The origin of magnetism is attributed to cation (Ti) vacancies, confirmed by X-ray absorption spectroscopy and first-principle calculations. The Ti vacancies are controlled by oxygen partial pressure during growth. [source]

    Structure,Property Relation of SrTiO3/LaAlO3 Interfaces

    ADVANCED MATERIALS, Issue 17 2009
    Mark Huijben
    Abstract A large variety of transport properties have been observed at the interface between the insulating oxides SrTiO3 and LaAlO3 such as insulation, 2D interface metallicity, 3D bulk metallicity, magnetic scattering, and superconductivity. The relation between the structure and the properties of the SrTiO3/LaAlO3 interface can be explained in a meaningful way by taking into account the relative contribution of three structural aspects: oxygen vacancies, structural deformations (including cation disorder), and electronic interface reconstruction. The emerging phase diagram is much richer than for related bulk oxides due to the occurrence of interface electronic reconstruction. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques, and provides an extension to the range of exceptional electronic properties of complex oxides. [source]

    Lanthanum Chromite-Based Interconnects as Key Materials for SOFC Stack Development

    Natsuko Sakai
    The historical investigations on the physicochemical and transport properties of lanthanum chromite-based perovskite oxides are reviewed to evaluate the compatibility as interconnects in solid oxide fuel cells. These materials improve sinterability in air. This has led to investigations on other physicochemical properties of these materials, such as thermal expansion, mechanical strength, and chemical stability. Lanthanum and chromium ion can be substituted by alkaline earths or transition metals, and this translates into a large flexibility in physicochemical properties. However, the formation of oxygen vacancies in a reducing atmosphere can result in an undesirable isothermal expansion or oxygen permeation. The chemical stability of these materials is governed by the fast cation diffusion at the grain boundary of the polycrystals. [source]

    Periodic models in quantum chemical simulations of F centers in crystalline metal oxides

    Yuri F. Zhukovskii
    Abstract We present a survey of recent first principles simulations of the neutral oxygen vacancies (F centers) existing as native or radiation-induced point defects in various crystalline metal oxides in different forms (bulk, bare substrate surface, and on the interface with metal adsorbates). We mainly consider periodic models in calculations of point defects using the metal oxide supercell or cyclic clusters. We compare different formalisms of first principles calculations, mostly the Density Functional Theory (DFT) as implemented in the framework of either localized basis set of atomic orbitals or delocalized basis sets of plane waves. We analyze in detail the structural and electronic properties of F centers in binary oxides of light metals (MgO and Al2O3), and ternary metal oxides (SrTiO3, BaTiO3, PbTiO3, KNbO3, and PbZrO3 perovskites). When available, we compare results of ab initio periodic defect calculations with experimental data, results of the first principles cluster calculations (both embedded and molecular) as well as with semi-empirical calculations. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

    X, -DVM modelling of oxygen transport in YBa2Cu3O7-, and SrFeO3

    Elmira I. YuryevaArticle first published online: 4 APR 200
    Abstract The reasons for oxygen transport in YBa2Cu3O7-, and SrFeO3 were investigated, using the X, -discrete variation method. Reduced binding energy and other parameters of the electronic energy structure were calculated for a number of crystal fragments within the T and O phases of YBa2Cu3O7-, and the cubic phase of SrFeO3. The following results were obtained: (i) test calculations of reduced binding energy confirmed that high temperature should be a condition of existence of the T phase, as compared with the O phase of YBa2Cu3O7-,; (ii) an increase in temperature up to ,400°C points to the beginning of the Jahn,Teller dynamic effect in SrFeO3; and (iii) transport of neutral and charged oxygen atoms results in distributions of stable and unstable oxygen vacancies, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]