Ab-initio study of Sr-doping effects on nitric oxide adsorption on the LaO (001) surface of LaFeO3

Density-functional calculations of molecular nitric oxide (NO) on defective (La,Sr)O (001) surfaces of (La,Sr)FeO_3 ? δ using slab models are performed to elucidate the oxygen vacancy formation problem on the LaO (001) surface of LaFeO_3 ? δ.From the estimation of the NO adsorption energy, NO adsorption is found on (La,Sr)O surfaces of (La_0.83,Sr_0.17)FeO_3 ? δ with δ = 0 or 0.25.The absolute value of the NO adsorption energy shows a remarkable increase at oxygen vacancies in the top surface layer, where the nitrogen atoms of the adsorbed molecules are embedded in the first (La,Sr)O layer, because a bond with Fe in the second FeO₂ layer is formed.Our data shows that Sr doping promotes formation of oxygen vacancies, which keep the NO adsorption ability high.Thus, we conclude that if Sr doping increases the number of oxygen vacancy sites by a charge compensation effect, NO adsorption on LaFeO₃ is enhanced, which provides an explanation for several experimental observations.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.     

Ti/CeOx(111) interfaces studied by XPS and STM

Low coverage of Ti was deposited on the well-ordered CeO_x(111) (1.5 < x < 2) thin films grown on Ru(0001) by physical vapor deposition at room temperature. The structure and interaction of Ti/ceria interfaces were investigated with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques under ultrahigh vacuum conditions. XPS data indicate that the deposition of Ti on both oxidized and reduced ceria surfaces causes the partial reduction of Ce from + 4 to + 3 state. Ti is formally in the + 4 state. STM data show the formation of small atomic-like titania features at 300 K, which coalesce to form chain structures upon heating. It is demonstrated in the study that the deposition of Ti can form mixed metal oxides at the interface and modify both electronic and structural properties of the ceria support. The structural study of Ti/ceria interfaces can be a key for understanding the higher catalytic activity of the Ti–CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

Oxygen transport in La2NiO4 + δ: Assessment of surface limitations and multilayer membrane architectures

The steady-state oxygen permeation through dense La₂NiO_4 + δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La₂NiO_4 + δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in the case when one dense layer, ~ 60 μm in thickness, is sandwiched between relatively thin (< 150 μm) porous layers. Whilst the permeability of such membranes is still affected by surface-exchange kinetics, increasing thickness of the porous supporting components leads to gas diffusion limitations.     

Color center of YAlO3 with cation vacancies

High quality pure YAlO₃ crystal with dimension of Φ 30 × 50 mm² was grown by Czochralski technique. UV irradiation and air annealing bring additional absorption in the region 200–800 nm. The absorption spectra of perfect YAlO₃ and YAlO₃ containing cation vacancy (aluminium vacancy and yttrium vacancy) were calculated using density functional theory code CASTEP. Comparison of the simulated absorption spectra with the experimental absorption spectra of YAlO₃ after UV irradiation and air annealing treatments shows that cation vacancies are responsible for part of the coloration on YAlO₃ crystal.     

Oxygen permeation characteristics of zirconia with surface modification

Yttria-stabilized zirconia (YSZ) can be used as an oxygen-permeating membrane at elevated temperature (> 1400 °C) due to its chemical and mechanical stability. It was previously shown that the oxygen transport through YSZ membrane in reducing oxygen partial pressure (P_O2) was highly influenced by the surface-exchange kinetics that can be improved by porous surface coating layers such as YSZ, GDC (Gd-doped ceria) or YSZ–GDC mixture [H.J. Park, G.M. Choi, J. Eur. Ceram. Soc. 25 (2005) 2577]. However, the increased oxygen flux was still lower than that estimated assuming bulk-diffusion limit and rapidly decreased with time due to the sintering of coating layers and the reaction between bulk YSZ and coating layers. In this study, the oxygen fluxes through YSZ with LaCrO₃, GDC + LaCrO₃ (bilayer), LaCrO₃ + 5 wt.% GDC (mixture), or LaCr_0.7Co_0.3O₃ coatings were measured under controlled P_O2 gradient (permeate-side P_O2: ∼ 3 × 10^− 12 atm, feed-side P_O2: 2 × 10^− 10–2 × 10^− 8 atm) at 1600 °C. The oxygen flux drastically increased with these coatings. The highest increase in oxygen flux was shown with GDC + LaCrO₃ (bilayer) coating and was maintained for a long time. The presence of highly catalytic Ce ions while maintaining porous structure in the coating layer may explain the observation. The prevention of formation of resistive layer due to ceria coating may also be partly responsible for the observation.     

Spin-polarized metastable deexcitation spectroscopy study of iron films

A spin-polarized metastable deexcitation spectroscopy apparatus using a nozzle-skimmer pulsed discharge metastable atom source was developed. The oxygen adsorption dependence of the surface magnetism of thin iron films deposited on MgO(100) was investigated using this apparatus. The surface local density of states spilling towards the vacuum (SDOS) at around the Fermi energy, E_F, for the clean surface shows a negative polarization to the bulk. The oxygen derived SDOS for the lightly oxygen adsorbed surfaces (2–10 L) also shows a negative polarization while the SDOS at E_F changes its polarity to positive. The polarization of SDOS is not detected for the heavily oxidized surfaces (20–100 L).     

Oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an oxygen-permeable membrane at high pressures

Oxygen permeation fluxes across the dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membrane disks were measured under an air/helium oxygen partial pressure gradient at high pressures (up to 10 atm) and various temperatures (973–1123 K). The fabricated BSCFO membrane exhibited good oxygen permeability with a high oxygen permeation flux of 2.01 ml min^− 1cm^− 2 (thickness: 1.37 mm) at 1123 K and 10 atm. Oxygen permeation results were analyzed theoretically using the surface exchange current model. The dependences of the oxygen permeation fluxes on the oxygen partial pressure gradient, suggested that the bulk oxygen ionic diffusion was the rate-limiting step for the overall oxygen permeation process across the BSCFO membrane. The ambipolar diffusion coefficients (D_a), the oxygen vacancy diffusion coefficients (D_v) and the oxygen ionic conductivities (σ_i) of the BSCFO material at different temperatures (973–1123 K) were calculated. It was found that BSCFO possessed high oxygen diffusion coefficients and ionic conductivities, which resulted in the good oxygen permeability of BSCFO. In addition, the BSCFO membrane exhibited good stability of oxygen permeation at 1123 K, while the deterioration of oxygen permeation stability was observed at 1098 K due to structural changes occurring at the surface of the BSCFO membrane disk as demonstrated by XRD.     

DFT-GGA study of NO adsorption on the LaO (001) surface of LaFeO3

In order to demonstrate the adsorption of the nitrogen monoxide molecule (NO) on the LaO (001) surface of LaFeO₃, we perform simulations based on density functional theory. The generalized gradient approximation (GGA) for the exchange-correlation energy functional indicates that the electronic state of the LaFeO₃ bulk is an anti-ferromagnetic insulator with a local magnetic moment of 4.1 μ_B at each Fe atom. Using the ultrasoft pseudo-potential method with spin-polarized GGA, fully optimized internal parameters as well as charge and spin density are determined for the NO-adsorbed structure prepared in a slab model. The calculated adsorption energy of NO is around ? 1.4 eV on the LaO (001) surface of LaFeO₃. This value decreases down to ? 4.46 eV at an oxygen vacancy site, where the nitrogen atom of NO is embedded in the 1st LaO layer forming a bond with Fe in the 2nd FeO layer.     

Adsorption of Xe atoms on the TiO2(1 1 0) surface: A density functional study

The interaction of xenon atoms with the TiO₂(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO₂(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO₂(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.     

The interaction of O2 with the surface of polycrystalline gadolinium at the temperature range 300–670 K

Auger-Electron-Spectroscopy (AES) and Direct-Recoils-Spectrometry (DRS) were applied to study the interaction of O₂ with a polycrystalline gadolinium surface, in the temperature range 300–670 K and oxygen pressure up to 2 × 10^? 6 Torr. It has been found that initial uptake of oxygen, at coverage measurable by the techniques used here, results in rapid oxide island formation. The subsurface is believed to be a mixture of oxide particles and oxygen dissolved in the Gd metal, the latter being the mobile species, even at relatively low temperatures.Enhanced inward diffusion of oxygen starts as early as 420 K and dictates the surface oxygen concentration and effective thickness of the forming oxide. The oxygen accumulation rate at the near-surface region, as measured by the O(KLL) AES signal intensity, goes through a maximum as a function of temperature at 420 K. This is a result of the combination of still efficient oxygen chemisorption that increases surface occupation and slow inward diffusion. The thickest oxide, ~ 1.7 nm, is formed at 300 K and its effective thickness was found to decrease with increasing temperature (due to oxygen dissolution into the metal bulk).Diffusion coefficients of the oxygen dissolution into the bulk were evaluated for various temperatures utilizing models for infinitely thin oxide layer and thick oxide layer, respectively. The best fit under our experimental procedure was obtained by the thick layer model, and the coefficients that were calculated are D₀ = 2.2 × 10^? 16m²s^? 1 and E_a = 46kJ/mol.     

Correlation of defect structure and ionic conductivity in δ-phase solid solutions in the Bi3NbO7–Bi3YO6 system

Defect structure and conductivity behaviour are discussed in the solid solution Bi₃Nb_1−xY_xO_7−x(0.0 ≤ x ≤ 1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800 °C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions.     

Electrode reaction of Sr1−xLaxCo0.8Fe0.2O3−δ with x = 0.1 and 0.6 on Ce0.9Gd0.1O1.95 at 600 ≤ T ≤ 800 °C

The electrode reaction of the perovskite phases Sr_1−xLa_xCo_0.8Fe_0.2O_3−δ (x = 0.1 and 0.6) on Ce_0.9Gd_0.1O_1.95 has been investigated by impedance spectroscopy in the temperature range 600 ≤ T ≤ 800 °C. Thick porous electrodes (t ∼ 20 μm) were sprayed on Ce_0.9Gd_0.1O_1.95 and ac impedance spectra were recorded on symmetrical cells at the equilibrium. The analysis of the complex impedance diagrams clearly indicates the presence of two contributions. The low frequency one was assigned to the gas phase oxygen diffusion through the porous electrode and a finite length diffusion (Warburg) impedance was used to describe the high frequency (HF) data. The polarization resistance of the HF impedance contribution (R_w) is higher for x = 0.1 while the activation energy of R_w is higher for x = 0.6. The variations of R_w versus the La content, temperature and thickness indicate that the Warburg-type impedance contains information of both bulk oxygen diffusion and surface processes.     

Mixed oxygen ion and electron conducting hollow fiber membranes for oxygen separation

Phase inversion spinning technique was employed to prepare dense perovskite hollow fiber membranes made from composition BaCo_xFe_yZr_zO_3−δ (BCFZ, x + y + z = 1.0). Scanning electron microscope (SEM) shows that such hollow fibers have an asymmetric structure, which is favored to the oxygen permeation. An oxygen permeation flux of 7.6 cm³/min cm² at 900 °C under an oxygen gradient of 0.209 × 10⁵ Pa/0.065 × 10⁵ Pa was achieved. From the Wagner Theory, the oxygen permeation through the hollow fiber membrane is controlled by both bulk diffusion and surface exchange. The elements composition of fresh fiber and the fiber after long-term experiments were analyzed by energy-dispersive X-ray spectra (EDXS). Compared to the fresh fiber, sulphur was found on the tested hollow fiber membrane surface exposed to the air side and in the bulk, and Ba segregations occur on the tested hollow fiber membrane surface exposed to the air side. A decrease of the oxygen permeation flux was observed, which was probably due to the sulphur poisoning.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions

The thermogravimetric and Mössbauer spectroscopy studies showed that, at atmospheric oxygen pressure, the oxygen content in Ca₂Fe₂O₅ brownmillerite is very close to stoichiometric at 300–1270 K. The orthorhombic lattice of calcium ferrite undergoes a transition from primitive (space group Pnma) to body-centered (I2mb) at 950–1000 K, which is accompanied with decreasing thermal expansion coefficient (TEC) and increasing activation energy for the total conductivity, predominantly p-type electronic. The steady-state oxygen permeation through dense Ca₂Fe₂O₅ ceramics is limited by the bulk ionic conduction. The ion transference numbers in air vary in the range 0.002–0.007 at 1123–1273 K, increasing with temperature. Analysis of stereological factors, which may affect oxygen diffusivity, suggests a dominant role of the ion jumps along octahedral and, possibly, tetrahedral layers of the brownmillerite structure. The ionic conductivity of calcium ferrite is higher than that of Ca₂FeAlO_5+δ, but lower compared to the oxygen-deficient perovskite phases based on SrFeO_3−δ where the diffusion pathways form a three-dimensional network. The average TECs of Ca₂Fe₂O₅ ceramics, calculated from dilatometric data in air, are 13.1 × 10^− 6 K^− 1 at 370–950 K and 11.3 × 10^− 6 K^− 1 at 970–1270 K.     

Diffusion of a Ga adatom on the GaAs(001)‐c(4 × 4)‐heterodimer surface: A first principles study

The adsorption and diffusion behavior of a Ga adatom on the GaAs (001)‐c(4 × 4)-heterodimer surface were studied by employing ab initio density functional theory (DFT) computations in the local density approximation. Structural and bonding features of the c(4 × 4)-heterodimer reconstruction surface were examined. A comparison with the c(4 × 4)-ss reconstruction was performed. Minimum energy sites (MES) on the c(4 × 4)-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top hopping- and exchange-diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top hopping- and exchange‐diffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping.     

Oxygen adsorption properties of YBaCo4O7-type compounds

The oxygen adsorption and desorption of newly found compounds RBaCo₄O₇ (R = Y, Dy–Lu, In) were investigated by thermogravimetry (TG) in the temperature range from room temperature to 1100 °C. The influence of Co replaced by Zn and Fe on the oxygen diffusion properties of YBaCo₄O₇ was also studied. TG results showed clearly that all RBaCo₄O₇ compounds basically experience two oxygen adsorption and desorption processes in the temperature range 20∼1100 °C in oxygen flow. One happens at about 200∼450 °C and the another happens at about 660∼1050 °C. The differences between the resulting states by adsorbing oxygen at lower and high temperature were discussed based on the X-ray diffraction (XRD) patterns and TG data. We showed evidence that the oxygen adsorption at the lower temperature has a small activation energy, while the oxygen adsorption at the higher temperature has a large activation energy. The oxygen adsorbed at high temperature will destroy the RBaCo₄O₇ structure. Zn substituting in the YBaCo_4 − xZn_xO₇ influences the oxygen diffusion behavior prominently, the amount of oxygen adsorbed becomes increasingly weak with the increase of Zn content and disappears completely for the samples with x ≥ 2.0. However, replacement of Co by Fe has little effect on the oxygen absorption process.