A-site Nonstoichiometry Research Articles

Thermoelectric properties of A-site deficient reduced Sr0.775La0.15-xEuxTiO3-δ (0 ≤ x ≤ 0.15)

There are numerous different schemes to improve the figure of merit (ZT) in oxide thermoelectrics, including creating additional electrons using oxygen vacancies, chemical doping, and A-site nonstoichiometry. A-site deficient (Sr,La)0.925TiO3- δ has shown promise as an n-type oxide thermoelectric; herein, additional A-site substitutional doping with Eu was used to improve ZT, through a combination of A-site size mismatch and non-stoichiometry. A solid solution series Sr0.775La0.15-xEuxTiO3-δ (0 ≤ x ≤ 0.15) was synthesised, showing a modest improvement in thermoelectric properties for x = 0.05, with a ZT of 0.18 at 600 °C, vs a ZT of 0.16 in the end member composition. Upon full characterisation, the additional electrons provided by the formation of oxygen vacancies were being used up by reduction of the added europium dopants, leading to a drop in electrical conductivity, and correspondingly, ZT. This gives a useful perspective in choosing dopants for schemes such as these.

Electrospun Nanofiber Electrodes with in situ Exsolved Nanocatalysts for Symmetrical SOCs

Solid oxide cells (SOCs) offer great promise for efficient energy generation and storage in a variety of applications. Symmetrical SOCs with identical anode and cathode materials configuration favoring an efficient reversible operation of the cells, allow both electricity production from traditional and renewable energy sources in fuel cell mode, and clean, efficient fuel production (in reverse/electrolysis mode). In addition, the symmetrical cell design is particularly advantageous, including: decreased number of cell components, simplification of fabrication process and alleviating the chemical stability issues related with materials, as well as the enhanced resistance to coking and sulphur poisoning, when cells are fuelled by alternative sources (non-H2) [1]. However, electrode material candidates with excellent redox stability and electrocatalytic activities in both oxidizing and reducing conditions for symmetrical SOCs are limited due to the strict criteria. The design of electrode materials with in situ exsolution of nanoparticles significantly benefits the performance of SOCs [2]. In the anode operation condition, metallic nanoparticles are in situ exsolved from the parent electrode materials decorating on the electrode surface, substantially improving the catalytic activity and electronic conductivity of anode, thus boosting the cell performance. The in situ exsolved nanoparticles can be socketed to the parent oxide, enhancing the stability and hydrocarbon coking tolerance, making the symmetrical SOCs fuelled by cheaper and readily available non-hydrogen alternatives. Moreover, the possible reversible exsolution/dissolution of nanoparticles can potentially resolve the particle agglomeration and coke formation during usage of non-hydrogen fuels [2, 3]. The electrospinning technique has been reported to obtain nanofiber-structured electrodes for SOCs, which efficiently contributes to the enhancement of the electrochemical performance of SOCs [4].In this work, the A-site deficient (Ln, Ba/Sr)2-α(Fe,M)2-x(Ni,Co)xO6-δ (Ln = selected lanthanides; M = Mn, Ti, Cr, Mo, W) perovskites were successfully obtained and evaluated as novel electrode materials for SOCs. It has been found that the A-site nonstoichiometry favours the in situ exsolution of metallic nanocatalysts (NixCo) from the parent materials in reducing condition. The selection of A-site cations (lanthanides) significantly affects the crystal structure (simple or double perovskite structure), while the B-site cations determine the stability of materials in air and reducing atmospheres. The reversibility of exsolved nanoparticles is systematically investigated under reducing and oxidizing conditions with different annealing temperatures. The possibility of in situ exsolution of nanooxides on the cathode (NixCoOy-like nanooxides) is also investigated. Nanofiber-structured electrodes with the ability of in situ exsolution of nanocatalysts are fabricated using the electrospinning technique, which significantly boost the electrochemical properties of constructed SOCs.

Multicomponent Mixed Ionic and Electronic Conducting (MIEC) Air Electrodes and Their Heterointerface for Effective Application in Reversible Solid Oxide Cell (R-SOC): Efficacy of Stoichiometric and Off-Stoichiometric Compositions

Reversible solid oxide cell (R-SOC) has triggered research interest from the past few years for its reversible applicability for power-to-fuel (P2F) in electrolyser cell (SOEC) to generate hydrogen by splitting of high temperature steam and fuel-to-power (F2P) in fuel cell (SOFC) mode of operation to produce power by oxidation of fuel with >80% conversion efficiency [1]. Primarily, R-SOC consists of a dense electrolyte conventionally yttria stabilized zirconia (YSZ), bridged between two porous electrodes, e.g., strontium doped lanthanum manganite (LSM) as air electrode and Nickel (Ni)-YSZ cermet as fuel electrode. Significant research has been devoted to carry out the long-term stability (≥ 2000 h) degradation in endurance test primarily due to major challenges like highly humid environment in the fuel electrode, microstructural flaws and polarization losses under high operational temperature. In this context, mixed ionic and electronically conducting (MIEC) oxide-based La-Sr-Co-Fe-O (LSCF) and Ba-Sr-Co-Fe-O (BSCF) air electrodes has drawn significant attention for the effective oxygen reduction (ORR) and oxygen evolution (OER) reaction in the context of SOFC and SOEC operational mode [2,3].An attempt has been made in this present research work to establish the effect of off-stoichiometry in A-site lattice over the stoichiometric pristine compositions of La/Ba-Sr-Co-Fe-O (LSCF/BSCF)-based multicomponent MIEC-oxides and their associated heterostructure with optimised off-stoichiometric compositions that governs the electrochemical redox phenomena in conjunction with the functionally engineered Gd-doped ceria-based interlayer and buffer layer. Vacancy driven oxygen reduction (ORR) and oxide ion oxidation (OER) kinetics as a function of aliovalent charge compensation mechanism associated with the defect chemistry has also been investigated for these A-site non-stoichiometric compositions in the range of 1-0.92 for La/Ba(1-x)SrxCo1-yFeyO3-δ [x=0.4 and y=0.2]. A comparative study between the performance of the off-stoichiometric heterostructure synthesised using the optimised pristine compositions with that of the stoichiometric heterointerface has also been evaluated for their efficiency as catalyst for OER/ORR in reversible SOC mode of operation. The increasing trend of oxygen non-stoichiometry (δ) upon the transition from A-site stoichiometry to non-stoichiometric compositions reflected the formation of higher aliovalent cation of ‘Co’ to maintain the charge neutrality and thereby accelerates the associated oxygen redox kinetics. An optimum δ-value of 0.33 and 0.37 are found for LSCF and BSCF-compositions respectively having A-site non-stoichiometry of 0.94. Current density of 0.45 A.cm-2 at an applied voltage of 1.5 V @800℃ is obtained for stoichiometric composition of LSCF wherein 0.5 A.cm-2 is found for the off-stoichiometric batch corresponding to the hydrogen flux of 0.21Nl.h-1.cm-2. Such augmentation in performance may be related to the formation of higher lattice defect which eases the path for oxide ion oxidation. Formation of the interpenetrating heterointerface network at the air electrodes and electrode/electrolyte interfaces in the morphologically engineered cells having gradation in porosity has further been confirmed from Transmission Electron Microscopy (TEM) studies involving the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the STEM-EDS elemental spectrum mapping images of the cross-section of the sintered cells and clinically corelated with SOC performances. Orientation of the crystal lattice planes in the two types of grains across the heterojunction has also been determined from the High-Resolution Transmission Electron Microscopy (HRTEM) images. Further study to envisage the presumed hypothesis, based on the rate determining steps involving the defect lattice formation and probable mechanism for selective ORR/OER application, will also be described through hole and/or oxygen vacancy formalism using XPS and density functional theory (DFT) for such hetero-interpenetrating air electrodes.

Impact of strontium non-stoichiometry of SrxTi0.3Fe0.7O3-δ on structural, electrical, and electrochemical properties for potential oxygen electrode of intermediate temperature solid oxide cells

This work presents the results of a comprehensive study on the impact of the A-site non-stoichiometry of SrxTi0.3Fe0.7O3-δ (x = 0.90, 0.95, 1.00, 1.05) ceramics on their physicochemical properties. The materials were fabricated by the conventional solid-state reaction method and their structure was determined by X-ray diffractometry, X-ray photoelectron spectroscopy and electron microscopy. Their sintering and thermal expansion properties were then evaluated. The electrical properties of the materials were determined by electrical conductivity and electrical relaxation measurements (on bulk materials) and by electrochemical impedance spectroscopy (EIS) studies of symmetrical, CGO-electrolyte based, porous electrodes. Finally, fuel cell tests with the non-stoichiometric electrodes were evaluated. To elucidate the electrochemical reaction pathways for oxygen reduction/evolution reaction, EIS measurements were carried out in different pO2 and were analysed via the distribution of relaxation times method. The results showed a dependence of materials’ properties on the A-site non-stoichiometry, which can be used to fine-tune their properties, e.g. increase the surface exchange rate or decrease the thermal expansion coefficient.

The positive effect of A-site nonstoichiometry on the electrical properties of Sr2Nb2O7 ceramics for high temperature piezoelectric sensor application

The structure and electrical properties of A-site nonstoichiometric perovskite layered structured (PLS) Sr2-xNb2O7-x piezoelectric ceramics prepared by the traditional solid-state reaction method are studied. The piezoelectric constant increases from 1.0 pC/N to 2.1 pC/N. Through XRD refinement and Raman analysis, the enhancement of the piezoelectric performance is mainly achieved by enhancing the distortion and/or rotation of the oxygen octahedron. The dielectric breakdown field strength at room temperature increases from 206 kV/cm to 435 kV/cm. The mechanism of dielectric breakdown enhancement has been explored by intrinsic factors such as dielectric constant and band gap. The resistivity at 700 °C increases from 6.7 × 105 Ω cm to 1.1 × 107 Ω cm which meets the requirements of high temperature vibration sensors at 650 °C. The improvement of high temperature resistance may be related to the inversion of oxygen vacancies. This work provides promising PLS piezoelectric ceramics for potential applications in high-temperature piezoelectric vibration sensors which uses a simple synthesis method without doping any elements.

Anomalous electrical performance of A-site double-bivalent-doped Bi0.49Na0.5TiO3- ceramics from nominal oxygen deficiency to excess

Perovskite structural Bi 0.5 Na 0.5 TiO 3 (BNT) ferroelectrics can exhibit considerable ionic conductivity, giving a new area for the application as oxygen ion conductors. Through acceptor doping and A-site nonstoichiometry, an ionic conductive mechanism associated with well-resolved arcs in the complex impedance spectra is proposed. Different dielectric responses in ceramics can be deduced to probe the electrical inhomogeneities in separative regions, such as bulk, grain boundary, and electrode, regions. Generally, ionic conductivity only arises at the nominal oxygen deficiency composition of BNT based ceramics. Although large current leakage can exist in the BNT ferroelectrics, they overall display an insulative nature and dominate the electronic conductive contribution, and only a large main arc can be identified in the Nyquist plots. In this work, A-site bivalent doped Bi 0.49- x (SrBa) x Na 0.5 TiO 3- δ and Bi 0.49- x (SrCa) x Na 0.5 TiO 3- δ ceramics ranging from oxygen deficiency to excess are investigated. However, anomalous ionic conductive characteristics are achieved at nominal oxygen excess composition. Herein, the polarization and conductive mechanisms are discussed to elucidate the effect of compositions and their phase and microstructure on the AC impedance, dielectric, and ferroelectric performances. The inhomogeneous distribution of oxygen vacancies, resulting from element segregation or distorted phases in the respective electroactive domains, is a crucial issue in the design of BNT based dielectrics or ionic conductors.

Efficient and Stable Symmetrical Solid Oxide Fuel Cell Via A-Site Non-Stoichiometry

Efficient and Stable Symmetrical Solid Oxide Fuel Cell Via A-Site Non-Stoichiometry

Significantly improved dielectric and piezoelectric properties by defects in PbNb2O6-based piezoceramics

PbNb2O6 (PN)-based piezoceramics are considered important materials used in high temperatures. Whereas, the main problem of PN-based ceramic is its low electrical performance such as piezoelectric constant (d33), severely restricting its practical applications. Here, based on the principle of regulating structure and performance via defect engineering, a new way of artificially tailoring point defects by A-site nonstoichiometry is implemented in this study. The results show that the PN-based ceramics with A-site deficiency exhibit significantly enhanced dielectric and piezoelectric properties. Particularly, the relative dielectric permittivity (εr) increased by 165% (from 197 to 523) and the d33 increased by 177% (from 30 to 83 pC/N) while maintaining a high Curie temperature (TC ~ 543 °C), and the resultant superior comprehensive performance is nearly highest in PN-based ceramic system. The X-ray photoelectron spectrometer and Raman spectra reveal that A-site deficiency leads to a decreased level of oxygen vacancies and NbO6 octahedral distortion as well as disorderly motions of A-site cations, which further causes the markedly decreased domain size exhibited in piezoresponse force microscopy (PFM) images. Consequently, the improved domain structure gives rise to improved dielectric and piezoelectric properties. The results of this work provide a simple and effective strategy to improve the dielectric and piezoelectric properties while keeping a high TC in the PN-based ceramics system, meeting the urgent demands of high-temperature applications.

Non-stoichiometry, structure and properties of proton-conducting perovskite oxides

The demand for clean and sustainable energy has garnered great interest in new energy materials. Among them, high temperature proton-conducting perovskite oxides are, or can be widely used in clean energy applications (including fuel cells, electrochemical reactors, solid-state separators and supports of catalytic components via various reduction and oxidation reactions) in the intermediate temperature range. The control of defect chemistry is the main strategy to fine tune properties for these applications. This review provides a critical discussion about non-stoichiometry-structure-property relation in terms of structure distortions by intrinsic octahedral titling and extrinsic acceptor doping, chemical stability, hydration behavior, transport properties, and catalytic effects in some typical classes of proton conducting perovskites and perovskite-related derivatives. A good understanding of A-site and B-site non-stoichiometry is also given to allow the perovskite structure with desired properties.

Structure and electrical properties of [(Na0.825K0.175)0.5Bi0.5]1+xTiO3 piezoceramics with A-site nonstoichiometry

ABSTRACT A type of nonstoichiometric piezoceramics, [(Na0.825K0.175)0.5Bi0.5]1+x TiO3 (BKNT-x, x = 0–0.009), was synthesized by the conventional sintering method. The effects of Bi3+, Na+ and K+ content on the structure and electrical properties of 0.825Bi0.5Na0.5TiO3-0.175Bi0.5K0.5TiO3 piezoceramics were investigated.The XRD patterns demonstrated that BKNT-x piezoceramics form a solid solution without a secondary phase. Meanwhile, SEM micrographs indicated that excess [(Na0.825K0.175)0.5Bi0.5]2+ suppresses grain growth and that BKNT-x ceramics become considerably denser. BKNT-0.002 ceramics offer the following enhanced properties: d 33 = 154 pC/N, k p = 33%, ε r = 1087, tanδ = 0.036, and Q m = 116. This improvement of properties is most likely related to the dense structure and the maintenance of morphotropic phase boundaries through the introduction of a proper A-site cation excess. For ceramics containing volatile elements, stoichiometric control is an efficacious method of enhancing performance.

Dislocations Promoted A-Site Nonstoichiometry and Their Impacts On the Proton Transport Properties of Epitaxial Barium Zirconate Thin Films

In the epitaxial barium zirconate (BaZrO3) thin films deposited on NdGaO3 substrates through pulsed laser deposition, the large lattice mismatch between film and substrate can be accommodated by ge...

The effect of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of NaNbO3 polycrystalline ceramics

The impact of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of sodium niobate ceramics (Na1+xNbO3, x = −2 to 1 mol %) was investigated. All the components maintained an orthorhombic antiferroelectric (AFE) structure. The grain size increased from 3.9 to 14.3 μm with the variation in x from −2 to 1. The AFE–FE phase transition electric field dramatically increased from 100 kV cm−1 at x = 0 to 170 kV cm−1 at x = −2, confirming the enlarged energy barrier between AFE Pbma and FE Pmc21 phase under external field in A-site deficient components. This is attributed to the lattice compressive stress generated by introducing proper A-site vacancies. Combined results of transmission electron microscopy and Raman spectroscopy indicated that the AFE distortion of Pbma phase was significantly enhanced in A-site deficient components, which jointly contributed to the stability of AFE phase in A-site deficient NaNbO3 material.

Effect of A-Site Nonstoichiometry on Defect Chemistry and Electrical Conductivity of Undoped and Y-Doped SrZrO3.

The effect of Sr-nonstoichiometry on phase composition, microstructure, defect chemistry and electrical conductivity of SrxZrO3−δ and SrxZr0.95Y0.05O3−δ ceramics (SZx and SZYx, respectively; x = 0.94–1.02) was investigated via X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy followed by distribution of relaxation times analysis of impedance data. It was shown that at low Sr deficiency (x > 0.96 and 0.98 for SZx and SZYx, respectively) a solid solution of strontium vacancies in strontium zirconate crystal structure forms, whereas at higher Sr deficiency the secondary phase, zirconium oxide or yttrium zirconium oxide, is precipitated. Yttrium solubility limit in strontium zirconate was found to be close to 2 mol%. Y-doped strontium zirconates possess up to two orders of magnitude higher total conductivity than SZx samples. A-site nonstoichiometry was shown to have a significant effect on the electrical conductivity of SZx and SZYx. The highest total and bulk conductivity were observed at x = 0.98 for both systems. Increasing the conductivity with a rise in humidity indicates that proton conduction appears in the oxides in wet conditions. A defect model based on consideration of different types of point defects, such as strontium vacancies, substitutional defects and oxygen vacancies, and assumption of Y ions partitioning over Zr and Sr sites was elaborated. The proposed model consistently describes the obtained data on conductivity.

Enhanced catalytic activity of nanostructured, A-site deficient (La0.7Sr0.3)0.95(Co0.2Fe0.8)O3−δ for SOFC cathodes

Significant enhancement in the catalytic activity of the benchmark (LaSr)(CoFe)O3−δ (LSCF) electrode is obtained by modifying the A-site non-stoichiometry.

Structural characterization of A-site nonstoichiometric (1 − x)Bi0.5Na0.5TiO3–xBaTiO3 ceramics

Lead-free 1 − x(Bi0.5Na0.5TiO3)–xBaTiO3, x = 0.055, 0.06 and 0.07, ceramics [i.e., near the morphotropic phase boundary (MPB)] were modified by varying the A-site cation stoichiometry to induce acceptor and donor doping through the addition of excess Na and Bi, respectively. The role of A-site nonstoichiometry on the crystal structure was investigated by using Raman spectroscopy and high-energy X-ray diffraction (XRD) techniques. The Raman spectra were analyzed via spectral deconvolution, and the resultant fitted data demonstrated deviations in the dominant bands (at 200–400 cm−1 and 400–600 cm−1) which indicated a shift toward tetragonal distortions in all Na-excess samples. High-energy XRD data showed that all the characteristic peaks corresponding to tetragonal P4bm structure became more prominent in Na-excess sample. Rietveld refinement data confirmed the coexistence of rhombohedral (R3c) and tetragonal (P4bm) phases with a significant increase in P4bm phase fraction in Na-excess composition. The clear agreements between Raman spectroscopy and high-energy XRD data suggest that Bi-excess samples (i.e., donor doping) had little or no effect on crystal structure, whereas Na-excess samples (i.e., acceptor doping) had a significant influence on the structure near the MPB such that tetragonal distortions were induced in both the local structure and the long-range average structure.

Tailoring structural and electrical properties of A-site nonstoichiometric Na0.5Bi0.5(Ti0.97Ni0.03)O3 ferroelectric films deposited on LaNiO3(100)/Si substrate

Highly (l00)-oriented Ni-doped Na0.5Bi0.5TiO3 (NBTNi) thin films with different A-site cation nonstoichiometry were deposited on the LaNiO3 (100)/Si substrates. We find that low levels of Na/Bi nonstoichiometry in the original composition of NBTNi films have obvious influence on the crystal structure and ferro-/dielectric properties. Na deficiency or Bi excess can lower the leakage current compared to the stoichiometric sample due to the decreased oxide-site vacancies. However, the mechanisms for the two types of films are different. That is, the mobile oxygen vacancies are tied by the Na vacancies in Na deficiency film whereas the formation of oxygen vacancies is suppressed for Bi-rich film. A good combination of ferroelectric property (Pr = 22.7 μC/cm2) and dielectric property (εr = 360 and tan δ = 0.11) can be achieved in Bi-rich NBTNi (Na0.5Bi0.54TNi) film. Besides, the effect of voltage and frequency on the capacitance and dielectric tunability for the Na0.5Bi0.54TNi film is investigated solely. These results show that NBT-based thin film is quite flexible in A-site nonstoichiometry, which provides a broad space for performance improvement.

The influence of excess K2O on the electrical properties of (K,Na)1/2Bi1/2TiO3 ceramics

The solid solution (KxNa0.50-x)Bi0.50TiO3 (KNBT) between Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 (KBT) has been extensively researched as a candidate lead-free piezoelectric material because of its relatively high Curie temperature and good piezoelectric properties, especially near the morphotropic phase boundary (MPB) at x ∼ 0.10 (20 mol. % KBT). Here, we show that low levels of excess K2O in the starting compositions, i.e., (Ky+0.03Na0.50-y)Bi0.50TiO3.015 (y-series), can significantly change the conduction mechanism and electrical properties compared to a nominally stoichiometric KNBT series (KxNa0.50-x)Bi0.50TiO3 (x-series). Impedance spectroscopy measurements reveal significantly higher bulk conductivity (σb) values for y ≥ 0.10 samples [activation energy (Ea) ≤ 0.95 eV] compared to the corresponding x-series samples which possess bandgap type electronic conduction (Ea ∼ 1.26–1.85 eV). The largest difference in electrical properties occurs close to the MPB composition (20 mol. % KBT) where y = 0.10 ceramics possess σb (at 300 °C) that is 4 orders of magnitude higher than that of x = 0.10 and the oxide-ion transport number in the former is ∼0.70–0.75 compared to <0.05 in the latter (between 600 and 800 °C). The effect of excess K2O can be rationalised on the basis of the (K + Na):Bi ratio in the starting composition prior to ceramic processing. This demonstrates the electrical properties of KNBT to be sensitive to low levels of A-site nonstoichiometry and indicates that excess K2O in KNBT starting compositions to compensate for volatilisation can lead to undesirable high dielectric loss and leakage currents at elevated temperatures.

Perovskite Oxide Nanoparticles As High Performance Bifunctional Catalyst

Developing efficient electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are vital to the new generation of electrochemical storage and conversion devices such as electrolyzer, metal-air batteries, and fuel cells. For the demand of reversible fuel cells and rechargeable metal-air batteries, the bifunctional catalyst that can be used for both ORR and OER is gathering attention. In order to address the longstanding problems with high cost and poor stability of noble metal electrocatalysts, perovskite (ABO3 formula, where A is rare-earth or alkaline metal and B is transition metal) with low cost, high versatility in structure and potential as bifunctional catalyst is of special interest. Applying our successful and novel polymer-assisted chemical solution (PACS) method, we will present the controlled synthesis of (La0.8Sr0.2)x(Mn1-yBy)O3 (B: Ir and Co, x=0-0.1, y=0-0.2) nanoparticles with different particle sizes, A-site nonstoichiometry, B-site doping, and oxygen vacancy, and epitaxial thin films with different crystal orientations. Stuctural characterizations and electrochemical analysis are utilized to investigate the correlation between their compositions, structures, and electrochemical properties.

A-site nonstoichiometry and B-site doping with selected M3 + cations in La2-xCu1-y-zNiyMzO4-δ layered oxides

In this work studies on a modification of Cu-based, layered oxides with K2NiF4-type structure are presented concerning formation of cationic nonstoichiometry in the A-sublattice of the parent La2CuO4, as well as an introduction of selected M3+ cations (Sc, Ga, In) at the B-site into La1.9Cu1-xMxO4 and Ni-containing La1.9Cu0.45Ni0.45M0.1O4. It is shown that synthesis of phase-pure La2-xCuO4 can be done with values of x≤0.1, and in the A-site nonstoichiometric materials further doping with M3+ is also achievable. Formation of solid solutions for M3+-doped samples follows the expected trend, with the widest range observed for the smallest Sc3+ cation, z≤0.1. Apart from a different structural behavior of the doped oxides at high temperatures, it is documented that the discussed substitution affects strongly the oxygen content (e.g. high concentration of the oxygen vacancies is present in La1.9CuO4-δ), as well as electrical conductivity. This is documented by the high-temperature X-ray diffraction, thermogravimetric, iodometic titration and electrical measurements. Since the compounds possess good transport properties, are stable at elevated temperatures, exhibit moderate thermal expansion and are Co-, Sr- and Ba-free, the proposed doping strategy seems interesting concerning development of the oxygen-transporting membranes based on Cu-containing oxides. For example, 1mm thick La1.9Cu0.9Sc0.1O4-δ membrane was found to deliver 0.37mLcm−2min−1 oxygen flux at 950°C, a very high value for the K2NiF4-type materials.

The microstructure, ferroelectric and dielectric properties of Ni-doped Na0.5Bi0.5TiO3 nanocrystalline films: A-site nonstoichiometry study

Effects of A-site nonstoichiometry on 3at% Ni-doped Na0.5Bi0.5TiO3 thin film were investigated. The influence of Na deficiency and Bi excess on the microstructure and electrical properties for the films were determined in comparison to the stoichiometric sample. The polycrystalline perovskite structure can be maintained at all compositions without secondary phase. Compared with the stoichiometric film, either Na deficiency or Bi excess can result in a decreased grain size. The enhanced insulating characteristic, ferroelectric and dielectric properties can be obtained in the Na or Bi nonstoichiometric thin films. Especially, the Na0.5Bi0.5(1+0.08)Ti0.97Ni0.03O3 thin film exhibits larger remanent polarization (Pr) of 22.3 μC/cm2 and dielectric tunability of 24.4%. These results show that Na deficiency or Bi excess lead to improved electrical performances for Na0.5Bi0.5TiO3-based thin films to some extent, rendering it suitable for ferroelectric and dielectric applications.