Low Oxygen Vacancy Concentration Research Articles

Enhanced Proton Conduction with Low Oxygen Vacancy Concentration and Favorable Hydration for Protonic Ceramic Fuel Cells Cathode

Protonic ceramic fuel cells (PCFCs), as an efficient energy storage and conversion device, have great potential to solve the serious problems of energy shortage and environmental pollution. Improving the proton conductivity of the promising cathode materials is an effective solution to promote the widespread application of PCFCs at low temperatures (450-650 °C). Herein, considering the high oxygen reduction reaction (ORR) activity of BaCoO3-based perovskite oxide and beneficial proton uptake capacity of Zn-doping, we construct BaCo0.4Fe0.4Zn0.1Y0.1O3-δ (BCFZnY) as the PCFCs cathode, and compare it with the classic triple-conducting cathode BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZrY). Different from the general strategy of increasing the initial oxygen vacancy concentration of cathode materials, this work unveils that enhancing the hydration of perovskite oxide with low oxygen vacancy concentration is a more effective strategy to accelerate the proton diffusion in the electrode. Therefore, the BCFZnY cathode achieved excellent proton conductivities of 8.05 × 10-3 and 6.38 × 10-3 S cm-1 as obtained by hydrogen permeation measurements and peak power densities of 982 and 320 mW cm-2 in a BaZr0.1Ce0.7Y0.1Yb0.1O3-δ-based anode-supported fuel cell at 600 and 450 °C, respectively.

Significantly enhanced hybrid improper ferroelectricity of Ca3Ti2O7 ceramics by the oxygen vacancy engineering

Ca3Ti2O7 materials with hybrid improper ferroelectricity call tremendous interest due to their great potential in designing novel room-temperature single-phase multiferroics with a strong magnetoelectric coupling effect. However, achieving Ca3Ti2O7-based ceramics with superior room-temperature ferroelectricity remains a crucial challenge. Herein, Ca3Ti2O7 ceramics were prepared by a tartaric acid sol–gel method and sintered in air and oxygen atmospheres, respectively. It was found that the long-time high-temperature sintering in air could lead to the formation of CaTiO3/Ca3Ti2O7/CaTiO3 sandwich structure, and oxygen-enriched sintering could effectively avoid the formation of CaTiO3 phase on the surface. The superior hybrid improper ferroelectricity is attained in Ca3Ti2O7 ceramics sintered in oxygen atmosphere with a higher remnant polarization (3.63 μC/cm2) and lower coercive electric field (72.6 kV/cm) obtained by positive-up and negative-down method, which is better than the ceramic sample sintered in air. The significantly better ferroelectricity is benefited from the synergistic effects of its larger distortion of oxygen octahedron, larger grain size, and lower oxygen vacancy concentration. Due to its low leakage current density, Ca3Ti2O7 ceramics sintered in oxygen atmosphere exhibit the remanent polarization as high as 7.40 μC/cm2 under the conditions of 1 Hz and 200 kV/cm obtained by dynamic hysteresis measurement mode. Furthermore, there are obvious stripe ferroelectric domains, which provide direct evidence for the excellent hybrid improper ferroelectricity of Ca3Ti2O7 ceramics. These findings demonstrate that the oxygen vacancy engineering is an effective strategy to improve hybrid improper ferroelectricity of Ca3Ti2O7 materials.

Domain Switching in BaTiO3 Films Induced by an Ultralow Mechanical Force.

Low-energy switching of ferroelectrics has been intensively studied for energy-efficient nanoelectronics. Mechanical force is considered as a low-energy consumption technique for switching the polarization of ferroelectric films due to the flexoelectric effect. Reduced threshold force is always desirable for the considerations of energy saving, easy domain manipulation, and sample surface protection. In this work, the mechanical switching behaviors of BaTiO3/SrRuO3 epitaxial heterostructure grown on Nb:SrTiO3 (001) substrate are reported. Domain switching is found to be induced by an extremely low tip force of 320 nN (estimated pressure ∼0.09 GPa), which is the lowest value ever reported. This low mechanical threshold is attributed to the small compressive strain, the low oxygen vacancy concentration in BaTiO3 film, and the high conductivity of the SrRuO3 electrode. The flexoelectricity under both perpendicular mechanical load (point measurement) and sliding load (scanning measurement) are investigated. The sliding mode shows a much stronger flexoelectric field for its strong trailing field. The mechanical written domains show several advantages in comparison with the electrically written ones: low charge injection, low energy consumption, high density, and improved stability. The ultralow-pressure switching in this work presents opportunities for next-generation low-energy and high-density memory electronics.

Increased Oxygen Vacancies in CeO2 for Improved Electrocatalytic Nitrogen Reduction Performance.

Electrochemical nitrogen fixation is a sustainable and economical strategy to produce ammonia. However, fabricating efficient electrocatalysts for nitrogen fixation is still challenging. Theoretical predictions prove that the oxygen vacancy is able to modulate the electronic state of CeO2 and enhance its electrical conductivity, thus promoting the electrochemical nitrogen reduction reaction (NRR) process. Herein, CeO2 with high oxygen vacancy concentration was prepared via a two-step pyrolysis strategy of Ce metal-organic frameworks (MOFs, denoted H-CeO2). Compared to CeO2 with low oxygen vacancy concentration synthesized via one-step pyrolysis of Ce-MOFs (denoted L-CeO2), H-CeO2 exhibits a large NH3 yield rate (25.64 μg h-1 mgcat-1 at -0.5 V vs reversible hydrogen electrode, RHE) and high faradaic efficiency (FE, 6.3% at -0.4 V vs RHE).

Lower oxygen vacancy concentration in BiPO4 with unexpected higher photocatalytic activity

Defect engineering has been demonstrated to be an appealing strategy to boost the photocatalytic activity of materials. However, can higher defect concentration bring about higher photocatalytic activity? This is an open question. In this work, BiPO4 photocatalysts with controllable oxygen vacancy concentrations were successfully synthesized. The photocatalytic activity of the obtained BiPO4 photocatalysts was determined by the removal of ciprofloxacin and 4-chlorophenol, as well as CO2 photoreduction. The BiPO4 materials with lower oxygen vacancy concentration could display unexpected higher photocatalytic efficiency. Through the investigation of different factors which may affect the photocatalytic performance, such as crystal structure, morphology, specific surface area, defect, and energy band structure, it can be found that the energy band structure difference was responsible for the enhanced photocatalytic activity.

Theoretical insights into the oxygen supply performance of α-Fe2O3 in the chemical-looping reforming of methane

The chemical looping reforming of methane to syngas is of great significance to the sustainable development of energy and environment, where the oxygen carrier (OC) is the core element of this technology. To study the oxygen migration properties of OCs and the matching problem of oxygen migration rate with the surface catalytic reaction rate, we present a theoretical study by taking the abundant and cheap α-Fe2O3 as the model OC. It is found that the migration rate of lattice oxygen and the dissociation of methane will be enhanced as the concentration of surface oxygen vacancy increases. By comparing the oxygen migration rate with the methane dissociation rate, we reveal that the two rates can match well at a low surface oxygen vacancy concentration, which is the key to the selective production of syngas. Lower or higher surface oxygen vacancy concentration will lead to the products towards carbon dioxide or coke.

Improving the Surface Oxygen Vacancy Concentration of Bi2O4 through the Pretreatment of the NaBiO3·2H2O Precursor as a High-Performance Visible Light Photocatalyst.

The oxygen-deficient bismuth oxide, Bi2O4, synthesized by a typical hydrothermal method using commercial NaBiO3·2H2O as a raw material only has a relatively low concentration of surface oxygen vacancies (OVs). How to improve the visible light photocatalytic performance of Bi2O4 via tuning its surface OV concentration is still a huge challenge. In this study, improving the surface OVs of Bi2O4 was successfully realized through the pretreatment of commercial NaBiO3·2H2O, including thermal treatment in air and hydrothermal treatment in 10 M NaOH solution, forming NaBiO3·xH2O intermediate products first, and then hydrothermal preparation of Bi2O4 target products using NaBiO3·xH2O instead of commercial NaBiO3·2H2O as the precursor. The enhanced surface OV content not only narrows the band gap of Bi2O4 and thus extends its optical response range but also captures more photoexcited electrons and thus increases the charge carriers' separation efficiency and prolongs the charge carriers' lifetime of Bi2O4. Among the above-mentioned two pretreatment methods, the effects of the hydrothermal pretreatment are superior to those of the thermal treatment, involving the increase of surface OVs, the optical harvesting capacity, and the charge carriers' separation efficiency. Accordingly, Bi2O4 prepared by the hydrothermal pretreatment route exhibits the optimal visible light catalytic performance toward the removal of methyl orange (MO) and phenol due to its most abundant surface OV concentration, which is 2.59 times and 4.26 times higher than that of Bi2O4 synthesized directly by the commercial NaBiO3·2H2O route, respectively. Holes (h+) and superoxide radicals (•O2-) are identified as the main active species, while singlet oxygen (1O2) and hydroxyl radicals (•OH) are verified as the second and third important active species for organic pollutant removal, respectively. This work has developed a novel strategy to promote the catalytic performance of single Bi2O4 induced by the enhanced surface OV concentration through the pretreatment of the precursor, commercial NaBiO3·2H2O.

Modified Dynamic Physical Model of Valence Change Mechanism Memristors.

Valence change-type resistance switching behaviors in oxides can be understood by well-established physical models describing the field-driven oxygen vacancy distribution change. In those models, electroformed residual oxygen vacancy filaments are crucial as they work as an electric field concentrator and limit the oxygen vacancy movement along the vertical direction. Therefore, their movement outward by diffusion is negligible. However, this situation may not be applicable in the electroforming-free system, where the field-driven movement is less prominent, and the isotropic oxygen vacancy diffusion by concentration gradient is more significant, which has not been given much consideration in the conventional model. Here, we propose a modified physical model that considers the change in the oxygen vacancies' charged state depending on their concentrations and the resulting change in diffusivity during switching to interpret the electroforming-free device behaviors. The model suggests formation of an hourglass-shaped filament constituting a lower concentration of oxygen vacancies due to the fluid oxygen diffusion in the thin oxide. Consequently, the proposed model can explain the electroforming-free device behaviors, including the retention failure mechanism, and suggest an optimized filament configuration for improved retention characteristics. The proposed model can plausibly explain both the electroformed and the electroforming-free devices. Therefore, it can be a standard model for valence change memristors.

Lead-free (Sr0.7Ca0.3)1−1.5xBixTiO3 ceramics with temperature stable energy storage density and discharge efficiency for pulsed power technology

• Homogeneous phase transition restricts the abrupt variation of polarization. • Reduction of oxygen vacancy concentration improves the breakdown strength. • The activation and ordered arrangement of PNRs enhance the polarization. Lead-free energy storage ceramics have attracted a large concentration for their significant role in pulsed power technology. Here, environmentally friendly (Sr 0.7 Ca 0.3 ) 1−1.5x Bi x TiO 3 relaxor ferroelectric ceramics are systematically studied. The introduction of Bi 2 O 3 can enhance polarization. As the Bi 2 O 3 content increases, the dielectric constant rises from 300 to 774, while the dielectric loss is less than 0.5% under a measurement frequency of 1 kHz. For the Sr 0.49 Ca 0.21 Bi 0.2 TiO 3 ceramic, an energy storage density ( W ) of 2.035 J cm −3 and a high energy storage efficiency ( η ) of 91.84% achieved under an electric field of 260 kV cm −1 . The variation rate of the dielectric constant is less than 15% (compared with the capacitance at 25 °C) in the temperature range of − 55–125 °C, meeting the requirement of the EIA X7R specification, and the W and discharge energy density ( W d ) also exhibit desirable temperature stability in the temperature range of 30 °C to 150 °C. These results are related to the homogeneous phase transition and low concentration of oxygen vacancies. The thermal activation of polar nanoregions ( PNRs ) partially compensates for the reduction of polarization caused by lattice expansion, which also contributes to the temperature stability. Taken together, the results of this study indicate the Sr 0.49 Ca 0.21 Bi 0.2 TiO 3 ceramics have potential as a target for developing high energy storage lead-free ceramic capacitors.

Optical and magnetic properties of fluoride-doped CeO2 obtained by an oxidative insertion method

• CeO 2 : F sample is synthesized using molten flux promoted oxidative insertion method. • PXRD and Raman spectral measurements confirmed the fluorite structure of CeO 2 :F. • Besides Ce and O, EDX analysis also revealed the presence of 18 at. % of fluoride. • PL and magnetic studies revealed a lower concentration of O vacancies in CeO 2 :F. • The F-doped sample had a higher concentration of Ce 3+ in CeO 2 :F. In the present study, we generated a fluoride-doped CeO 2 sample through the molten hydroxide flux promoted oxidative insertion approach using CeF 3 as the precursor. We analyzed the results and compared them with pure CeO 2 prepared by the same approach starting from Ce(NO 3 ) 3 . High-resolution powder X-ray diffraction, FTIR, and Raman spectral measurements confirmed both samples’ fluorite structure. The CeO 2 resulted from the oxidative flux treatment of CeF 3 had 18 at.% of fluoride ion-doped in it. The optical band gap narrowed marginally from 2.98 to 2.88 eV upon fluoride doping in CeO 2 . The blue emission intensity decreased with fluoride doping, indicating a reduction in oxygen vacancies compared to undoped CeO 2 . F-doped CeO 2 showed ferromagnetic behavior with magnetization, H c, and M r values higher than the undoped sample. A higher concentration of Ce 3+ and a lower concentration of oxygen vacancies were the consequences of fluoride doping in CeO 2 by this synthetic method.

Robust Ni-doped Ruddlesden-popper (La,Sr)Fe1-xNixO4+δ oxide as solid oxide cells fuel electrode for CO2 electrolysis

Ruddlesden-popper (La, Sr)FeO 4+δ perovskite oxide with excellent redox stability shows insufficient electrochemical catalytic activity for CO 2 reduction because of low conductivity and oxygen vacancy concentration. In this work, Ni modified (La, Sr)Fe 1-x Ni x O 4+δ cathode was developed to improve the conductivity and catalytic activity for CO 2 electrolysis. The introduction of the Ni element significantly increases the conductivity of (La, Sr)FeO 4+δ perovskite oxide in both air and 50%CO 2 /CO due to the increasing charge carrier's concentration. Furthermore, the symmetric cell with (La, Sr)Fe 0·9 Ni 0·1 O 4+δ (RPLSFNi0.1) electrode exhibits the lowest polarization resistance in 50%CO 2 /CO, suggesting that the RPLSFNi0.1 electrode has the best catalytic activity for CO 2 electrolysis. Moreover, the addition of Sm 0.2 Ce 0·8 O 2-δ (SDC) in RPLSFNi0.1 electrode further enhances the electrochemical performance, and the current density of −1170 mA cm −2 is obtained at 850 °C and 1.5 V. In addition, the electrolysis cell exhibits excellent reversible cycling operating stability between 0.6 V at fuel cell mode and 1.2 V at electrolysis mode, indicating that RPLSFNi0.1 is a robust cathode material for solid oxide cells (SOCs) fuel electrode. • Ni doping increases the conductivity of (La, Sr)FeO 4+δ perovskite oxide in both air and 50%CO 2 /CO. • The substitution of Fe with Ni favors the formation of oxygen vacancy. • The current density of electrolysis cell with RPLSFNi0.1-SDC electrode attains −1170 mA cm −2 @ 1.5 V.

Emission Spectroscopy Investigation of the Enhancement of Carrier Collection Efficiency in AgBiS2-Nanocrystal/ZnO-Nanowire Heterojunction Solar Cells.

Eco-friendly solar cells were fabricated using interdigitated layers comprising ZnO nanowires (NWs) and infrared absorbing AgBiS2 nanocrystals (ITO/ZnO NWs/AgBiS2/P3HT/Au). The quality of ZnO NWs was studied using photoluminescence and Raman spectroscopy to identify the defects in ZnO NWs influencing solar cell performance. Oxygen vacancies and Zn interstitial sites, among various recombination sites, were observed to be the main sites for carrier recombination, which hinders the carrier collection in the solar cells. Accordingly, the power conversion efficiency of AgBiS2 solar cells exhibited a good correlation with the number of oxygen vacancies. The structural order and electron-phonon interaction in ZnO NWs were also investigated via Raman scattering spectroscopy. A lower concentration of oxygen vacancies and zinc interstitials (Zni) resulted in a higher structural order as well as a weaker electron-phonon interaction in ZnO NWs. When ZnO NWs were treated at 500 °C in oxygen with the lowest oxygen vacancy concentration, the solar cells (500-O2 solar cell (SC)) demonstrated an external quantum efficiency of approximately 70% in the visible region and a corresponding internal quantum efficiency of more than 80%. The 500-O2 SC exhibited a power conversion efficiency of 5.41% (JSC = 22.21 mA/cm2, VOC = 0.41 V, and FF = 60%) under quasi one-sun illumination. New methods that can efficiently reduce oxygen vacancies and Zni without affecting the structural order of ZnO NWs would further enhance the carrier collection efficiency. Moreover, since ZnO is a key electron transport material for constructing not only colloidal quantum dot solar cells but also other emerging solar cells, such as organic thin-film solar cells, the present findings provide significant information for improving their performance.

MOCVD growth of MgGa2O4 thin films for high-performance solar-blind UV photodetectors

Epitaxial MgGa2O4 thin films with a bandgap of 5.18 eV were grown on the c-plane sapphire substrate by using metal organic chemical vapor deposition. The structure, optical, electronic, and optoelectronic properties of MgGa2O4 thin films have been investigated before and after high-temperature annealing in an oxygen or nitrogen atmosphere. In particular, the O2-annealed MgGa2O4 thin film reveals a high crystalline quality and a low concentration of oxygen vacancies. Moreover, a quick response speed (tr = 20 ns, td = 400 ns), a low dark current (∼17 pA at 10 V), and a high UV/Visible rejection ratio (>105) can be demonstrated in the O2-annealed MgGa2O4 photodetector, indicating the excellent solar-blind ultraviolet (SBUV) photodetection characteristics. The effect of annealing atmosphere on the photoelectric properties of the film and its physical mechanism were studied. This research provides an effective way to realize high-performance SBUV photodetectors and opens up the application of MgGa2O4 spinel in the field of semiconductor optoelectronic and microelectronic devices.

Effects of crystallographic orientation on the corrosion behavior of stainless steel 316L manufactured by laser powder bed fusion

The corrosion behavior of the {100}, {110}, {111} planes of a single-crystalline-like stainless steel 316L (SS316L) produced by laser powder bed fusion (LPBF) was investigated in 3.5 wt% aqueous NaCl and compared with their LPBF-polycrystalline and wrought counterparts. The general corrosion resistance followed: wrought ≈ {110} < polycrystalline < {100} < {111} trend, which was rationalized by the surface atom density and corrosion activation energy. All LPBF samples exhibited a significantly higher pitting corrosion potential than the wrought SS316L due to the lower concentration of oxygen vacancies in their passive oxide films. These findings provide insights into improving the corrosion performance by tailoring the crystallographic texture of LPBF-SS316L.

Revealing the solid‐state processing mechanisms of antiferroelectric AgNbO3 for energy storage

AbstractAgNbO3 is one of the prominent lead‐free antiferroelectric (AFE) oxides, which readily exhibits a field‐induced AFE to ferroelectric phase transition and thus a high energy storage density. The solid‐state synthesis of AgNbO3 is considered difficult and an oxidizing atmosphere is typically employed during AgNbO3 processing, on the premise that oxygen can prevent possible decomposition of the silver oxide at high temperatures. However, details about the influence of processing parameters on the functional properties of AFE AgNbO3 are insufficiently understood. In this work, the solid‐state reaction of a stoichiometric AgO and Nb2O5 mixture was investigated. We found that ball milling can convert AgO into metallic Ag, which is beneficial for lowering the reaction temperature for the formation of the perovskite phase to 500‒600℃. Moreover, the influence of the processing atmosphere (air, O2, and N2) was investigated by thermal analysis and in situ X‐ray diffraction. Since the reaction between Ag and Nb2O5 to form AgNbO3 requires oxygen uptake, AgNbO3 was only found to form in air and O2, whereby the kinetics were faster in the latter case. All the sintered AgNbO3 samples exhibited a similar crystallographic structure, although the samples processed in O2 had a lower oxygen vacancy concentration. Despite this, well‐defined AFE double polarization loops were obtained in all cases. Our results indicate that decomposition of sliver oxide during ball milling is beneficial for the solid‐state reaction, while a pure O2 atmosphere is not essential for the synthesis of high‐quality AgNbO3. These findings may simplify the processing and facilitate further research of AgNbO3‐based antiferroelectrics.

Quenching effects on depolarization temperature and &lt;sup&gt;18&lt;/sup&gt;O tracer diffusion in (Bi&lt;sub&gt;0.5&lt;/sub&gt;Na&lt;sub&gt;0.5&lt;/sub&gt;)TiO&lt;sub&gt;3&lt;/sub&gt; ceramics with acceptor and donor additives

Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 (BNT), with Mg and Nb additives were prepared and the concentration of oxygen vacancies as the 18O tracer diffusion of these ceramics were clarified by secondary ion mass spectroscopy (SIMS). Moreover, the samples were quenched after sintering to examine the depolarization temperature Td, and then their electrical properties were investigated. As a result, the obtained values of volume diffusion coefficient D of BNT doped with 0.4 wt % Mg and 0.4 wt % Nb were determined to be 9.2 × 10−11 and 1.1 × 10−13 cm2/s, respectively. The oxygen vacancy concentration increased with the amount of Mg added and decreased with that of Nb added. Moreover, quenching increased Td by ∼50 °C in BNT with the Mg additive (Mg x) and ΔTd also increased. Thus, the effect of quenching was promoted by Mg x. On the other hand, the Td of BNT with the Nb additive (Nb y) also increased despite the low concentration of oxygen vacancies. This is because the D of pure BNT (no additives) was revealed found to be 2.5 × 10−11 cm2/s, that is, pure BNT originally contained a high concentration of oxygen vacancies. The concentration was maintained even after quenching, then D showed 1.8 × 10−11 cm2/s. In addition, the electromechanical coupling factor k33 of the quenched ceramics remained similar to that of the ordinarily fired samples of BNT with the Mg and Nb additives.

Corrosion Behavior of Laser Powder Bed Fusion Fabricated Stainless Steel 316L

Metal additive manufacturing techniques have been recognized for their capability of controlling the crystallographic orientations of stainless steels. However, the inherent anisotropic corrosion behavior has not been extensively studied. In this study, the corrosion properties of 316L stainless steels prepared by Laser Powder Bed Fusion (LPBF) additive manufacturing were investigated. The effects of different crystallographic textures, namely {100}, {110} and {111} on both general and pitting corrosion were characterized by several electrochemical measurements, including Electrochemical Impedance Spectroscopy (EIS), potentiodynamic polarization and Mott-Schottky analysis. The results were also compared to the polycrystalline and wrought 316L counterparts. It was found that the LPBF-{111} sample offered the highest general corrosion resistance, followed by the LPBF-{100}, LPBF-polycrystalline and LPBF-{110} samples (Figure 1). The origin of this trend was related to the atomic surface density. The LPBF-{111} surface exhibited a stronger atomic bonding than that of LPBF-{100} and LPBF-{110} samples, resulting in a higher corrosion activation energy and thus a higher general corrosion resistance. All the LPBF samples also offered a significantly higher pitting corrosion resistance (Figure 2), which was attributed to the lower concentration of oxygen vacancies (donor levels) in the passive film that serve as pits nucleation sites, as observed by the Mott-Schottky analysis (Figure 3). Figure 1

High-entropy A2B2O7-type oxide ceramics: A potential immobilising matrix for high-level radioactive waste

The sustainable development of civil nuclear energy requires the fabrication of the durable nuclear wasteforms, in particular for high-level radioactive waste, which involves the design of the composition and microstructure. Herein, we demonstrated that high-entropy ceramics (Eu1-xGdx)2(Ti0.2Zr0.2Hf0.2Nb0.2Ce0.2)2O7 are the potential candidate as immobilizing hosts for high-level radioactive waste. The static aqueous leaching test indicates that the normalized leaching rates for the simulated radionuclides Ce (LRCe) and Gd (LRGd) in as-prepared high-entropy ceramics are approximately 10−6~10−8 g·m−2·d−1 after 42 days testing, much lower than those reported values in doped-Gd2Zr2O7 (10−6~10−3 g·m−2·d−1). The excellent chemical durability is mainly due to the synergistic effects of the compositional complexity and severe lattice distortion. Compared to their ternary oxides, the low oxygen vacancy concentration slows down the migration and diffusion of cations. Moreover, the lattice distortion increases the lattice potential energy, also inhibiting the migration of cations. This study provides a strategy for the development and application of high-entropy ceramics as the wasteforms.

Lead-Free BiFeO3-BaTiO3 Ceramics with High Curie Temperature: Fine Compositional Tuning across the Phase Boundary for High Piezoelectric Charge and Strain Coefficients

BiFeO3-BaTiO3 is a promising high-temperature piezoelectric ceramic that possesses both good electromechanical properties and a Curie temperature (TC). Here, the piezoelectric charge constants (d33) and strain coefficients (d*33) of (1 - x)BiFeO3-xBaTiO3 (BF-xBT; 0.20 ≤ x ≤ 0.50) lead-free piezoelectrics were investigated at room temperature. The results showed a maximum d33 of 225 pC/N in the BF-0.30BT ceramic and a maximum d*33 of 405 pm/V in the BF-0.35BT ceramic, with TCs of 503 and 415 °C, respectively. To better understand the performance enhancement mechanisms, a phase diagram was established using the results of XRD, piezoresponse force microscopy, TEM, and electrical property measurements. The superb d33 of the BF-0.30BT ceramic arose because of its location in the optimum point in the morphotropic phase boundary, low oxygen vacancy (VO··) concentration, and domain heterogeneity. The superior d*33 of the BF-0.35BT ceramic was attributed to a weak relaxor behavior between coexisting macrodomains and polar nanoregions. The presented strategy provides guidelines for designing high-temperature BF-BT ceramics for different applications.

Relation among Oxygen Stoichiometry, Structure, and Co Valence and Spin State in Single-Layer La2-xAxCoO4±δ (A = Ca, Sr) Perovskites.

We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2-xAxCoO4±δ (A = Sr, Ca; 0 ≤ x ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+ independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+ as in La2CoO4.2 and Co2.9+ as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+ and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the ab plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the c axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the c lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.