Atomic-scale mechanistic features of oxide ion conduction in apatite-type germanates

Atomistic modelling studies of the apatite-type oxide ion conductor La9.33(GeO4)6O2 show that a key role of the O4 channel oxygen atoms appears to be as a reservoir for the creation of interstitial oxide ion defects, while the migration of these defects proceeds via the GeO4 tetrahedra.     

Effect of Sr content on the crystal structure and electrical properties of the system La2-xSrxNiO4+delta (0 <or=x<or= 1)

Materials formulated as La(2-x)Sr(x)NiO(4+delta) (0 相似文献   

Theoretical Studies on the Ground State and Excited State of 2,70-(Ethylene)-bis-8-hydroxyquinoline

Dielectric relaxation method was employed to study the properties of oxygen ion diffusion and phase transition in the oxide-ion conductors (La1-xLnx)2Mo2O9 (Ln=Nd, Gd, x=0.05-0.25). Two dielectric loss peaks were observed: peak Pd at about 600 K and peak Ph around 720 K. Peak Pd is a relaxational peak and associated with the short-range diffusion of oxygen ions, while peak Ph hardly changes its position and dramatically decreases in height with increasing frequency, exhibiting non-relaxational nature. With increasing Ln3+ concentration, the heights of peak Ph and Pd increase at first and then decrease after passing a maximum at 15% doping. It is suggested that peak Ph is related to the phase transition of a static disordered state to a dynamic disordered state in oxygen ions/vacancies distribution. It is found that the 15%Gd or 15%Nd doped La2Mo2O9 samples exhibit the highest conductivity in accordance with the highest height of peak Pd at this doping content.     

2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide as a new air-stable n-type dopant for vacuum-processed organic semiconductor thin films

2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) was synthesized and employed as a strong n-type dopant for fullerene C(60), a well-known n-channel semiconductor. The coevaporated thin films showed a maximum conductivity of 5.5 S/cm at a doping concentration of 8.0 wt% (14 mol%), which is the highest value reported to date for molecular n-type conductors. o-MeO-DMBI-I can be stored and handled in air for extended periods without degradation and is thus promising for various organic electronic devices.     

钙钛矿型稀土锰氧化物La0.7Sr0.3FexMn1—xO3的磁性和巨磁电阻效应

采用溶胶-凝胶工艺制备了La     

Doping strategies to optimise the oxide ion conductivity in apatite-type ionic conductors

The apatite-type phases, La(9.33+x)(Si/Ge)(6)O(26+3x/2), have recently been attracting considerable interest as potential electrolytes for solid oxide fuel cells. In this paper we report results from a range of doping studies in the Si based systems, aimed at determining the key features required for the optimisation of the conductivities. Systems examined have included alkaline earth doping on the rare earth site, and P, B, Ga, V doping on the Si site. By suitable doping strategies, factors such as the level of cation vacancies and oxygen excess have been investigated. The results show that the oxide ion conductivities of these apatite systems are maximised by the incorporation of either oxygen excess or cation vacancies, with the former producing the best oxide ion conductors. In terms of samples containing cation vacancies, conductivities are enhanced by doping lower valent ions, Ga, B, on the Si site. The presence of higher valent ions on these sites, e.g. P, appears to inhibit the incorporation of excess oxygen within the channels, and so limits the maximum conductivity that can be obtained. Overall the results suggest that the tetrahedral sites play a key role in the conduction properties of these materials, supporting recent modelling studies, which have suggested that these tetrahedra aid in the motion of the oxide ions down the conduction channels by co-operative displacements.     

Understanding the synergistic catalytic effect between La(2)O(3) and CaO for the CH(4) lean De-NO(x) reaction: kinetic and mechanistic studies

Doping of La(2)O(3) crystallites with Ca(2+) ions significantly enhances the intrinsic rate of NO reduction by CH(4) in the presence of 5% O(2) at 550 degrees C compared to pure La(2)O(3) and CaO solids, while the opposite is true after doping of CaO with La(3+) ions. It was found that the 5 wt % La(2)O(3)-95 wt % CaO system has one of the highest intrinsic site reactivities (TOF = 8.5 x 10(-3) s(-1)) reported at 550 degrees C for the NO/CH(4)/O(2) reaction among metal oxide surfaces. The doping process occurred after first dispersing La(2)O(3) and CaO crystallites in deionized water heated to 60 degrees C for 90 min, while the dried material was then ground and heated slowly in air to 800 degrees C and kept at this temperature for 5 h. The doping process had the effect of creating surface oxygen vacant sites (F-type defects) in the oxide lattices the concentration of which is a function of the wt % La(2)O(3) used in the mixed oxide system as revealed by photoluminescence and O(2) chemisorption studies. According to DRIFTS (15)NO transient isotopic experiments (SSITKA), oxygen vacant sites in Ca(2+)-doped La(2)O(3) promote the formation of a more active chemisorbed NO(x) species (NO(2)(-)) that contributes to the enhancement of reaction rate as compared to pure lanthana, calcium oxide, and La(3+)-doped CaO. These results were supported by the kinetic orders of the reaction with respect to NO and O(2) obtained as a function of wt % La(2)O(3) content in the mixed oxide system. Carbon dioxide (a reaction product) competes for the same oxygen vacant sites to form stable adsorbed carbonate-like species, thus lowering the reduction rate of NO. The dependence of the reaction TOF on the wt % La(2)O(3) loading at 550 degrees C was found to follow the trend of the dependence of photoluminescence intensity on the wt % La(2)O(3) content in the La(2)O(3)-CaO oxide system.     

Mixed conductivity, nonstoichiometric oxygen, and oxygen permeation properties in Co-Doped Sr3Ti2O(7-δ)

Electrical conductivity and oxygen permeation rates in Co-doped Sr(3)Ti(2)O(7-δ) with Ruddesden-Popper type structures were investigated. The effects of metal dopants (M) in the Ti site of Sr(3)Ti(2)(M)O(7-δ) on the mixed conductivity were also studied. Doping of Sr(3)Ti(2)O(7-δ) with Co was found to be effective for improving the electrical conductivity as well as the oxygen permeation rate, which could be assigned to the increased oxygen vacancy concentration by doping Co(3+) into Ti(4+) sites. The nonstoichiometric oxygen of these oxides was measured by using a thermal gravimetric method. The creation of oxygen vacancies, which is compensated with Co(3+) doping, leads to higher oxide ion conductivity. The oxygen permeation rate monotonously increased with increasing amounts of Co in the Ti site. Sr(3)Ti(0.8)Co(1.2)O(7-δ) exhibited high oxide ion conductivity and a large oxygen permeation rate. The highest oxygen permeation rate achieved a value of 2.02 cc min(-1) cm(-2) at 1273 K for Sr(3)Ti(0.8)Co(1.2)O(7-δ). Neutron diffraction analysis and redox titration suggests that the oxygen diffusion occurs through oxygen vacancies in the perovskite block, but not through excess oxygen in the rock salt block.     

La0.8Sr0.2Ga0.8Mg0.2O3与La0.8Sr0.2Ga0.8Mg0.15Co0.05O3电导的对比

采用固相合成法制备了La0.8Sr0.2Ga0.8Mg0.2O3(LSGM8282)和La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC5), 利用四电极交流阻抗法和Hebb-Wagner 极化法对比研究了两种材料的总电导率和电子电导率. 实验结果表明, LSGM8282 的总电导率与氧分压无明显依赖关系, 而LSGMC5 的总电导率在高氧分压区随氧分压降低而增加,在中等氧分压区域基本保持不变. 在973-1173 K的温度范围内, LSGM8282的自由电子电导率以及电子空穴电导率的氧分压级数分别为-1/4和1/4.在1073-1173 K的温度范围内, LSGMC5的自由电子电导率以及电子空穴电导率的氧分压级数分别为-1/4和约为1/8, 表明LSGMC5的空穴产生机制可能与LSGM8282不同. LSGM8282 的氧离子电导率与氧分压无关, 而LSGMC5 的氧离子电导率在高氧分压区随氧分压的减小而增加.     

Structural and magnetic properties of hexagonal perovskites La1.2Sr2.7MO7.33 (M = Ru, Ir) containing peroxide ions

The structures of the new compound La(1.2)Sr(2.7)IrO(7.33) and the recently discovered La(1.2)Sr(2.7)RuO(7.33) have been solved using a combination of X-ray and neutron diffraction. Both compounds crystallize in the trigonal space group Rm and consist of isolated MO6 (M = Ru, Ir) octahedra, which are arranged in well-defined hexagonal perovskite slabs. These slabs are separated by (Sr2O(1+delta)) layers containing both O2- and (O2)2- ions. The composition can therefore be written as La(1.2)Sr(2.7)MO(7-delta)(O2)delta with delta = 0.33. Results of the magnetic susceptibility and XANES measurements show that the transition metal cations are in a pentavalent state. While in La(1.2)Sr(2.7)RuO(7.33) an antiferromagnetic interaction between the Ru5+ ions is found, La(1.2)Sr(2.7)IrO(7.33) shows a very small temperature-independent paramagnetism down to 1.8 K due to the strong spin-orbit coupling characteristic for the 5d element iridium.     

Cation interdiffusion model for enhanced oxygen kinetics at oxide heterostructure interfaces

An interface between the perovskite La(0.8)Sr(0.2)CoO(3-δ) (LSC-113) and the K(2)NiF(4)-type (La(0.5)Sr(0.5))(2)CoO(4-δ) (LSC-214) heterostructure was recently shown to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest to develop better optimized materials for solid-state electrochemical devices, e.g. solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ～0.9 eV (～1.3 eV) energy gain for exchanging a Sr from LSC-113(25%Sr) (LSC-113(40%Sr)) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that the Sr concentrations (in the LSC-214 phase) in a typical experimental temperature range of 500-600 °C and in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr), may be about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase (Vashook et al., Solid State Ionics, 2000, 138, 99-104), an Sr enrichment from x = 0.5 to x = 0.75 in (La(1-x)Sr(x))(2)CoO(4-δ) is expected to enhance the oxygen vacancy concentration by 2-2.5 orders of magnitude under typical experimental conditions. An increased vacancy concentration in LSC-214 near the interface can explain most of the enhanced oxygen kinetics observed up until now in these heterostructures.     

Thermochemistry of Sr2Ce(1-x)Pr(x)O4 (x = 0, 0.2, 0.5, 0.8, and 1): variable-temperature and -atmosphere in-situ and ex-situ powder X-ray diffraction studies and their physical properties

A novel family of metal oxides with a chemical formula of Sr(2)Ce(1-x)Pr(x)O(4) (x = 0, 0.2, 0.5, 0.8, and 1) was developed as mixed oxide ion and electronic conductors for solid oxide fuel cells (SOFCs). All of the investigated samples were synthesized by the ceramic method at 1000 °C in air and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and electrochemical impedance spectroscopy (EIS). Ex-situ PXRD reveals that the Sr(2)PbO(4)-type Sr(2)CeO(4) decomposes readily into a mixture of perovskite-type SrCeO(3) and rock-salt-type SrO at 1400 °C in air. Surprisingly, the decomposed products are converted back to the original Sr(2)PbO(4)-type Sr(2)CeO(4) phase at 800 °C in air, as confirmed by in-situ PXRD. Thermal decomposition is highly suppressed in Sr(2)Ce(1-x)Pr(x)O(4) compounds for Pr > 0, suggesting that Pr improves the thermal stability of the compounds. Rietveld analysis of PXRD and SAED supported that both Pr and Ce ions are located on the 2a site in Pbam (space group no. 55). The electrical transport mechanism could be correlated to the reduction of Pr and/or Ce ions and subsequent loss of oxide ions at elevated temperatures, as shown by TGA and in-situ PXRD. Conductivity increases with Pr content in Sr(2)Ce(1-x)Pr(x)O(4). The highest total conductivity of 1.24 × 10(-1) S cm(-1) was observed for Sr(2)Ce(0.2)Pr(0.8)O(4) at 663 °C in air.     

复合阴极材料La1-xSrxCuO3-δ-SDC的制备及电学性能

合成具有单相正交钙钛矿结构的La1-xSrxCuO3-δ(x=0.15, 0.2, 0.3, 0.4)系列样品, 碘量滴定法实验结果表明, 随着Sr掺入量的增加, Cu3+离子的含量逐渐增加. 电学性能研究结果表明, La0.7Sr0.3CuO3-δ电导率最高, 与La0.6Sr0.4CoO3-δ相比, La0.7Sr0.3CuO3-δ具有更好的电化学性能, 可作为一种新的中温固体氧化物燃料电池(IT-SOFC)阴极材料. 将La0.7Sr0.3CuO3-δ与不同质量比的中温电解质Ce0.85Sm0.15O2-δ(SDC) 固相混合, 制备复合阴极材料, 电化学性能测试结果表明, 掺入适量的SDC有利于降低La0.7Sr0.3CuO3-δ电极的极化, 获得性能更优越的IT-SOFC阴极材料, 提高在中温区单电池的输出功率.     

Doped CeO2-LaFeO3 composite oxide as an active anode for direct hydrocarbon-type solid oxide fuel cells

Direct utilization of hydrocarbon and other renewable fuels is one of the most important issues concerning solid oxide fuel cells (SOFCs). Mixed ionic and electronic conductors (MIECs) have been explored as anode materials for direct hydrocarbon-type SOFCs. However, electrical conductivity of the most often reported MIEC oxide electrodes is still not satisfactory. As a result, mixed-conducting oxides with high electrical conductivity and catalytic activity are attracting considerable interest as an alternative anode material for noncoke depositing anodes. In this study, we examine the oxide composite Ce(Mn,Fe)O(2)-La(Sr)Fe(Mn)O(3) for use as an oxide anode in direct hydrocarbon-type SOFCs. High performance was demonstrated for this composite oxide anode in direct hydrocarbon-type SOFCs, showing high maximum power density of approximately 1 W cm(-2) at 1073 K when propane and butane were used as fuel. The high power density of the cell results from the high electrical conductivity of the composite oxide in hydrocarbon and the high surface activity in relation to direct hydrocarbon oxidation.     

Epitaxial thin films of ATiO(3-x)H(x) (A = Ba, Sr, Ca) with metallic conductivity

Epitaxial thin films of titanium perovskite oxyhydride ATiO(3-x)H(x) (A = Ba, Sr, Ca) were prepared by CaH(2) reduction of epitaxial ATiO(3) thin films deposited on a (LaAlO(3))(0.3)(SrAl(0.5)Ta(0.5)O(3))(0.7) substrate. Secondary ion mass spectroscopy detected a substantial amount and uniform distribution of hydride within the film. SrTiO(3)/LSAT thin film hydridized at 530 °C for 1 day had hydride concentration of 4.0 × 10(21) atoms/cm(3) (i.e., SrTiO(2.75)H(0.25)). The electric resistivity of all the ATiO(3-x)H(x) films exhibited metallic (positive) temperature dependence, as opposed to negative as in BaTiO(3-x)H(x) powder, revealing that ATiO(3-x)H(x) are intrinsically metallic, with high conductivity of 10(2)-10(4) S/cm. Treatment with D(2) gas results in hydride/deuteride exchange of the films; these films should be valuable in further studies on hydride diffusion kinetics. Combined with the materials' inherent high electronic conductivity, new mixed electron/hydride ion conductors may also be possible.     

Effect of oxygen content on the 29Si NMR, Raman spectra and oxide ion conductivity of the apatite series, La8+xSr2-x(SiO4)6O2+x/2

29Si NMR data have been recorded for the apatite series La8+xSr2-x(SiO4)6O2+x/2 (0 < or = x < or = 1.0). For x = 0, a single NMR peak is observed at a chemical shift of approximately -77 ppm, while as the La : Sr ratio and hence interstitial oxygen content is increased, a second peak at a chemical shift of approximately -80 ppm is observed, which has been attributed to silicate groups neighbouring interstitial oxide ions. An increase in the intensity of this second peak is observed with increasing x, consistent with an increase in interstitial oxide ion content, and the data are used to estimate the level of interstitial oxide ions, and hence Frenkel-type disorder in these materials. The increase in second 29Si NMR peak intensity/interstitial oxide ion content is also shown to correlate with an increase in conductivity. The effect of interstitial oxygen content can also be studied by means of Raman spectroscopy, with a new mode at 360 cm(-1) appearing for samples with x > 0 in the symmetric bending mode energy region of the SiO4 group. The intensity of this mode increases with increasing oxygen content, yielding results comparable to those from the NMR studies, showing the complementarities of the two techniques.     

Periodic density functional theory study of methane activation over La(2)O(3): activity of O(2-), O(-), O(2)(2-), oxygen point defect,and Sr(2+)-doped surface sites

Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at O(s)(2-), O(s)(-), O(2)(s)(2-) point defect, and Sr(2+)-doped surface sites on La(2)O(3)(001). The results show that the anionic O(s)(-) species is the most active surface oxygen site. The overall reaction energy to activate methane at an O(s)(-) site to form a surface hydroxyl group and gas-phase (*)CH(3) radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site O(s)(-) is -102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr(2+)/La(3+) exchange at the surface. The O(-)-like nature of the surface site is reflected in a calculated hydrogen binding energy of -109.7 kcal/mol. Calculations indicate that surface peroxide (O(2(s))(2-)) sites can be generated by adsorption of O(2) at surface oxygen vacancies, as well as by dissociative adsorption of O(2) across the closed-shell oxide surface of La(2)O(3)(001). The overall reaction energy and apparent activation barrier for the latter pathway are calculated to be only 12.1 and 33.0 kcal/mol, respectively. Irrespective of the route to peroxide formation, the O(2)(s)(2-) intermediate is characterized by a bent orientation with respect to the surface and an O-O bond length of 1.47 A; both attributes are consistent with structural features characteristic of classical peroxides. We found surface peroxide sites to be slightly less favorable for H-abstraction from methane than the O(s)(-) species, with DeltaE(rxn)(CH(4)) = 39.3 kcal/mol, E(act) = 47.3 kcal/mol, and DeltaE(ads)(H) = -71.5 kcal/mol. A possible mechanism for oxidative coupling of methane over La(2)O(3)(001) involving surface peroxides as the active oxygen source is suggested.     

La0.5>RE0.3Sr0.2FeO3－δ (RE = Nd、Ce、Sm)体系双稀土阴极材料的制备与电性能

采用甘氨酸-硝酸盐法(GNP)合成了La0.5RE0.3Sr0.2FeO3－δ(RE=Nd、Ce、Sm)系列复合氧化物粉体. 用X射线衍射(XRD)和TG-DSC分析了样品钙钛矿物相的形成过程, 用Archimedes排水法测量体积密度并计算烧结样品的相对密度, 用四端子技术测量电导率. 结果显示, 掺Nd的样品1200 ℃烧结2 h成为单一立方钙钛矿结构, 掺Ce样品有明显的CeO2立方相析出, 掺Sm样品主相为钙钛矿结构伴有微弱的杂峰. 1250 ℃烧结2 h的La0.5Nd0.3Sr0.2FeO3－δ在600 ℃时电导率高达100 S•cm-1以上, 明显高于La0.5Ce0.3Sr0.2FeO3－δ及La0.5Sm0.3Sr0.2FeO3－δ样品的电导率, 预示着La0.5Nd0.3Sr0.2FeO3－δ可能是一种良好的中温固体氧化物燃料电池(SOFC)阴极材料.     

Ca2+xLa8-x(SiO4)6O2-0.5x的合成及其导电机理

Ca2+xLa8-x(SiO4)6O2-0.5x的合成及其导电机理;溶胶凝胶法;硅酸盐氧基磷灰石;空位导电机理;电化学阻抗谱     

Doping and defect association in AZrO(3) (A = Ca, Ba) and LaMO(3) (M = Sc, Ga) perovskite-type ionic conductors

Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO(3)(A = Ca, Ba) and LaMO(3)(M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant-defect association at the atomic level. The energetics of dopant incorporation in AZrO(3) show strong correlation with ion size. We predict Y(3+) to be one of the most favourable dopants for BaZrO(3) on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy-dopant pairs in BaZrO(3) are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO(3) is via a curved pathway around the edge of the ScO(6) octahedron. Dopant-vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO(3), but very low energies in LaGaO(3). This points to greater trapping of the oxygen vacancies in doped LaScO(3), perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data.