ABO3 Perovskite Oxides Research Articles

Layer Resolved Cr Oxidation State Modulation in Epitaxial SrFe0.67Cr0.33O3-δ Thin Films.

Understanding how doping influences physicochemical properties of ABO3 perovskite oxides is critical for tailoring their functionalities. In this study, SrFe0.67Cr0.33O3-δ epitaxial thin films were used to examine the effects of Fe and Cr competition on structure and B-site cation oxidation states. The films exhibit a perovskite-like structure near the film/substrate interface, while a brownmillerite-like structure with horizontal oxygen vacancy channels predominates near the surface. Electron energy loss spectroscopy shows Fe remains Fe3+, while Cr varies from ∼Cr3+ (tetrahedral layers) to ∼Cr4+ (octahedral layers) within brownmillerite phases and becomes ∼Cr4.5+ in perovskite-like phases. Theoretical simulations indicate that Cr-O bond arrangements and the way oxygen vacancies interact with Cr and Fe drive Cr charge disproportionation. High-valent Cr cations introduce additional densities of states near the Fermi level, reducing the optical bandgap from ∼2.0 eV (SrFeO2.5) to ∼1.7 eV (SrFe0.67Cr0.33O3-δ). These findings offer insights into B-site cation doping in the perovskite oxide framework.

Perovskite-Like Carbodiimides AB(NCN)3: Synthesis and Characterization of MnHf(NCN)3 and FeHf(NCN)3.

Two novel ternary air-stable transition-metal carbodiimides, MnHf(NCN)3 and FeHf(NCN)3, were synthesized via solid-state metathesis using either ZnNCN or Na2NCN as the carbodiimide source and the corresponding binary metal chlorides. These two phases are the first examples of transition-metal carbodiimides with an AB(NCN)3 composition, akin to ubiquitous ABO3 perovskite oxides. The crystal structure of MnHf(NCN)3 was determined and refined from powder X-ray diffraction (XRD) data in the non-centrosymmetric space group P6322 allowing for chirality, the assignment of which is supported by second-harmonic generation (SHG) measurements. FeHf(NCN)3 was found to crystallize isotypically, and the presence of iron(II) in a high spin state was confirmed by 57Fe Mößbauer spectroscopy. The structures are revealed to be NiAs-derived and can be described as a hexagonal stack of NCN2- anions with metal cations occupying 2/3 of the octahedral voids. Both IR spectroscopic measurements and DFT calculations agree that the NCN2- unit is a bent carbodiimide with C2v symmetry, necessary to account for the size difference present in such a vacancy-ordered structure. Magnetic studies reveal predominantly strong antiferromagnetic interactions but no long-range order between the paramagnetic Mn2+ centers, likely due to the dilution of Mn2+ over the octahedral sites or perhaps even due to some degree of magnetic frustration. The optical and electrochemical properties of MnHf(NCN)3 were then studied, revealing a wide band gap of 3.04 eV and p-type behavior.

A Single Model for the Thermodynamics and Kinetics of Metal Exsolution from Perovskite Oxides.

Exsolution has emerged as an outstanding route for producing oxide-supported metal nanoparticles. For ABO3-perovskite oxides, various late transition-metal cations can be substituted into the lattice under oxidizing conditions and exsolved as metal nanoparticles after reduction. A consistent and comprehensive description of the point-defect thermodynamics and kinetics of this phenomenon is lacking, however. Herein, supported by hybrid density-functional-theory calculations, we propose a single model that explains diverse experimental observations, such as why substituent transition-metal cations (but not host cations) exsolve from perovskite oxides upon reduction; why different substituent transition-metal cations exsolve under different conditions; why the metal nanoparticles are embedded in the surface; why exsolution occurs surprisingly rapidly at relatively low temperatures; and why the reincorporation of exsolved species involves far longer times and much higher temperatures. Our model's foundation is that the substituent transition-metal cations are reduced to neutral species within the perovskite lattice as the Fermi level is shifted upward within the bandgap upon sample reduction. The calculations also indicate unconventional influences of oxygen vacancies and A-site vacancies. Our model thus provides a fundamental basis for improving existing, and creating new, exsolution-generated catalysts.

Interpretable machine learning model of effective mass in perovskite oxides with cross-scale features

The interpretability of machine learning reveals associations between input features and predicted physical properties in models, which are essential for discovering new materials. However, previous works were mainly devoted to algorithm improvement, while the essential multi-scale characteristics are not well addressed. This paper introduces distortion modes of oxygen octahedrons as cross-scale structural features to bridge chemical compositions and material properties. Combining model-agnostic interpretation methods, we are able to achieve interpretability even using simple machine learning schemes and develop a predictive model of effective mass for a widely used material type, namely perovskite oxides. With this framework, we reach the interpretability of the model, understanding the trend of the effective mass without any prior background information. Moreover, we obtained the knowledge only available to experts, i.e., the interpretation of effective mass from the s–p orbitals hybridization of B-site cations and O2− in ABO3 perovskite oxides.

(Invited) Chemical Intricacies of Mixed Ionic-Electronic Conducting Perovskite Oxides

Mixed ionic-electronic conductors (MIEC) play a critical role in electrochemical devices, such as batteries and fuel cells. Ideally, both the electrodes (anode and cathode) should support a fast transport of electrons and the working ions to realize acceptable reaction kinetics and current density/rates. Crystal chemistry and the resulting chemical bonding control the MIEC properties and the device performances ensuing from them. This presentation will illustrate the richness and complexities displayed by perovskite-based MIECs, which are employed as electrodes in solid oxide fuel cells, oxygen separation membranes, etc., by taking a few basic fundamental materials parameters/factors: By taking the ionic sizes of the A and B cations in ABO3 perovskite oxides, the presentation will show the ability to tune the electronic and ionic conductivities in MIECs. Specifically, for example, how the approach of B-O-B bond angle towards 180o by replacing the A cations with progressively increasing size can cause an increase in bandwidth, electron delocalization, and electronic conductivity, and eventually a semiconductor-to-metal transition.By taking the size difference between the A cations in the double perovskite oxides, such as ALnCo2O6- d (A = alkaline earth and Ln = lanthanide), how oxygen content (6-d) and hole concentration can be altered drastically under even identical synthesis conditions due to the preference of different coordination numbers for A and Ln ions, e.g., the preference of larger cations like Ba2+ for 12-fold coordination while smaller cations like Y3+ for 8-fold coordination.By taking the coordination number of trivalent cobalt ions as an example, the presentation will illustrate how the low-spin to high-spin transition that has plagued the cobalt-containing perovskite oxides with huge thermal expansion coefficient can be eliminated by designing materials with tetrahedral-site cobalt ions instead of octahedral-site cobalt ions.By taking the differences in the magnitude of the charge-transfer gap between the B3+/4+:3d band and the top of the O2-:2p band in ABO3 perovskite oxides, the presentation will explain how the oxygen flux in oxygen gas separation membranes can be altered due to the differences in the degree of covalence in the B-O bond.

Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films.

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Exploring heterogeneous phases in highly A-site-deficient titanate with Ni exsolution

In-situ exsolution technique, as an efficient and controllable strategy of surface modification, has been extensively applied in reversible solid oxide cells (RSOCs). Here, we demonstrate LaxSrxTi0.9Ni0.1O3-δ fiber decorated by exsolved Ni nanoparticles and highlight the impacts of A-site deficiency on Ni exsolution and electrochemical performance of RSOCs. The La0.4Sr0.4Ti0.9Ni0.1O3 (A/B = 0.8) fibrous fuel electrode decorated by Ni nanoparticles with moderate Ni exsolution displays optimum electrochemical performance at 800 °C, achieving ∼540 mW cm−2 in SOFC mode and −0.742/0.385 A cm−2 under 1.3/0.7 V in RSOC mode. In contrast, high A-site deficiency (A/B < 0.8) allows heterogeneous phases (NiTiO3) and excessive Ni exsolution along with alterations of fibrous morphology, leading to slow gas diffusion, sluggish catalysis, and Ni agglomeration. The distribution of relaxation time (DRT) results show that the rate-limiting steps in symmetrical cells are gas diffusion, hydrogen adsorption, and dissociation, accounting for over 89% of the total RP. Ni-equilibrium model based on X-ray Rietveld refinement calculates the degree of exsolution with various A-site deficiency, revealing NiTiO3 is the key factor of excessive exsolution. Our work of high A-site deficiency exemplified here may serve in the design and nanoengineering of fibrous ABO3 perovskite oxides for RSOCs.

Accurate prediction of oxygen vacancy concentration with disordered A-site cations in high-entropy perovskite oxides

Entropic stabilized ABO3 perovskite oxides promise many applications, including the two-step solar thermochemical hydrogen (STCH) production. Using binary and quaternary A-site mixed {A}FeO3 as a model system, we reveal that as more cation types, especially above four, are mixed on the A-site, the cell lattice becomes more cubic-like but the local Fe–O octahedrons are more distorted. By comparing four different Density Functional Theory-informed statistical models with experiments, we show that the oxygen vacancy formation energies ({E}_{V}^{f}) distribution and the vacancy interactions must be considered to predict the oxygen non-stoichiometry (δ) accurately. For STCH applications, the {E}_{V}^{f} distribution, including both the average and the spread, can be optimized jointly to improve Δδ (difference of δ between the two-step conditions) in some hydrogen production levels. This model can be used to predict the range of water splitting that can be thermodynamically improved by mixing cations in {A}FeO3 perovskites.

Enhanced strain of (Ba, Ca)(Ti, Zr, Sn)O3 ferroelectrics with multiphase polar nanoregions coexistence and BO6 octahedral rotation

The cation displacement closely related to the local symmetry and corner-connected oxygen octahedral rotation are the structural origin of macroscopic properties in ABO3 perovskite oxides. This work describes in detail the relationship between microstructure and macroscopic properties in 0.78BT-0.12CT-0.1BZS ceramic, which achieves the enhanced strain of 0.19 % with a hysteresis of 6.56 %. The atomic level structure is revealed by aberration corrected scanning transmission electron microscope (AC-STEM). The AC-STEM HAADF images reveal the coexisting of polar nanoregions with O and T symmetries. Such a microstructure has a lower energy barrier, facilitating dynamic polarization rotation flexibly under external stimuli. The AC-STEM ABF image reveals the existence of the BO6 octahedral rotation, with rotation patterns of the a0b0c0 and a-b+c-, and rotation angles are quantitatively measured. The BO6 octahedral rotation and cation displacement are found to be consistent, the greater octahedral rotation with greater cation displacements. These results have important implications for understanding the structural origin of the excellent properties in ferroelectrics.

A dual-site doping strategy for developing efficient perovskite oxide electrocatalysts towards oxygen evolution reaction

Perovskite-type ABO3 transition metal oxides are promising electrocatalysts for oxygen evolution reaction (OER), but still suffer from insufficient activities. Traditional efforts are mainly based on a single-site doping of heteroatom ions into either B-sites or O-sites of the ABO3 structures. Here we propose a dual-site doping strategy by simultaneous incorporation of iodine cations and anions into the B-sites and O-sites of ABO3 perovskite oxides, respectively. Consequently, both activated B-site transition metals with optimal eg orbital occupancy and large amount of active oxygen species were achieved for remarkably improved OER activities. Using this approach, significantly improved OER activities are achieved in some representative perovskite oxides, including Ba0.5Sr0.5(Co0.8Fe0.2)0.9O3–δ (BSCF), SrCo0.9O3−δ (SCO) and SrNi0.9O3−δ (SNO). Specifically, the iodine dual-site doped BSCF boosts a small overpotential of 290 mV at 10 mA cm−2 and a small Tafel slope of 53 mV dec−1, which is about 130 mV and almost half times lower than that of pristine BSCF, respectively. Besides, this general dual-site doping strategy can also realize scale-up synthesis and achieve a gram-scale production. Our findings show a unique doping strategy to design efficient perovskite oxide electrocatalysts beyond the conventional approaches.

Bond-Valence Parameterization for the Accurate Description of DFT Energetics.

We report a bond-valence method (BVM) parameterization framework that captures density functional theory (DFT)-computed relative stabilities using the BVM global instability index (GII). We benchmarked our framework against a dataset of 188 experimentally observed ABO3 perovskite oxides, each of which was generated in 11 unique Glazer octahedral tilt systems and optimized using DFT. Our constrained minimization procedure minimizes the GIIs of the 188 perovskite ground state structures predicted by DFT while enforcing a linear correlation between the GIIs and DFT energies of all 2068 competing structures. GIIs based on BVM parameters determined using our framework correctly identified the DFT ground state perovskite structure in 135 of 188 compositions or one of the two lowest energy structures in 152 of 188 compositions. Using the most common approach to parameterize BVM, which minimizes the root-mean-square deviation of the BVM site discrepancy factors, GIIs correctly identified the DFT ground state perovskite structure in only 41 of 188 compositions. Our new parameterization framework is therefore a marked improvement over the existing procedure and an important first step toward BVM-based structure generation protocols that reproduce DFT.

Use of Interplay between A‐Site Non‐Stoichiometry and Hydroxide Doping to Deliver Novel Proton‐Conducting Perovskite Oxides

AbstractThe magnitude of ionic conductivity is known to depend upon both mobility and number of available carriers. For proton conductors, hydration is a key factor in determining the charge–carrier concentration in ABO3 perovskite oxides. Despite the high reported proton mobility of calcium titanate (CaTiO3), this titanate perovskite has thus far been regarded as a poor proton conductor due to the low hydration capability. Here, the enhanced proton conductivity of the defective calcium titanate Ca0.92TiO2.84(OH)0.16 prepared by replacing lattice oxygens with hydroxyl groups via a solvothermal route is shown. Conductivity measurements in a humidified Ar atmosphere reveal that, remarkably, this material exhibits one order of magnitude higher bulk conductivity (10−4 Scm−1 at 200 °C) than hydrated stoichiometric CaTiO3 prepared by traditional solid‐state synthesis due to the higher concentration of protonic defects and variation in the crystal structure. The replacement of Ca2+ by Ni2+ in the Ca1−xTi1O3−2x(OH)2x, which mostly exsolve metallic Ni nanoparticles along orthorhombic (100) planes upon reduction, is also demonstrated. These results suggest a new strategy by tailoring the defect chemistry via hydration or cation doping followed by exsolution for targeted energy applications.

Anisotropic chemical expansion due to oxygen vacancies in perovskite films

In scientifically intriguing and technologically important multifunctional ABO3 perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.

Enhanced redox activity and oxygen vacancies of perovskite triggered by copper incorporation for the improvement of electro-Fenton activity

ABO3 perovskite oxides doped with cation substitutions have been proved to change the inherent electronic and structural properties of perovskite. However, the role of perovskites substituted by B sites element in tailoring their electronic structure, crystal structure and surface chemical reaction on the degradation of organic pollutants by heterogeneous electro-Fenton (EF) process still remained obscure. Herein, the incorporation of Cu in LaCoO3 perovskite (LaCoxCu1−xO3−δ) was designed as heterogeneous EF catalyst for almost completely removal of target pollutant ciprofloxacin (CIP) within 120 min. The results showed that incorporation of Cu could effectively improve EF activity due to the enhancement of redox activity and oxygen vacancies (OVs) of LaCoxCu1−xO3−δ, which synergistically promoted the activation of hydrogen peroxide to hydroxyl radical (·OH). Furthermore, the superoxide free radical (O2−) was remarkably increased due to the electron-rich OVs and the redox pairs of Co2+/Co3+ and Cu+/Cu2+. Meanwhile, the pseudo-first order kinetic constant of the degradation process was investigated to evaluate the degradation effect of LaCoxCu1−xO3−δ. Finally, the degradation mechanism was investigated by EPR and scavenger experiments, which unraveled the important role of cation substitutions in perovskite oxides for the improvement of heterogeneous EF activity. The results provided insights into the incorporation strategy in perovskite oxides for improving heterogeneous EF activity in a rational approach.

Manipulating Surface Termination of Perovskite Manganate for Oxygen Activation

AbstractFor ABO3 perovskite oxides, one of the key issues limiting their utilization in heterogeneous catalysis is the dominant presence of catalytically inactive A‐site cations at the surface. The engineering of B‐site terminated perovskites is considered as an effective method to address this issue, especially when dealing with Mn/Co‐based perovskite catalysts. However, to date, such a strategy has not been fully successful and remains a major challenge in the field. Herein, a Mn‐terminated La0.45Sr0.45MnO3 (B‐LSM) is successfully synthesized via a one‐pot hydrothermal method, in which low‐valence Mn ions partially occupy the A site to form the active Mn‐excess phase. Experimental results and theoretical calculations reveal that the presence of the surface Mn termination in B‐LSM optimizes the hybrid orbitals of Mn 3d‐O 2p and promotes the activation of surface lattice oxygen, where the pristine inert lattice O2− is evolved into active and stable lattice O2−x. Such structural optimization significantly reduces the activation energy barriers on going from O2− species to important intermediate O− species during O2 activation. Moreover, this results in good stability and Pt‐like activity for the B‐LSM during CO oxidation. This work offers a new chemical route for the design of advanced perovskite‐type oxides possessing novel functions.

A Resol-Assisted Cationic Coordinative Co-assembly Approach to Mesoporous ABO3 Perovskite Oxides with Rich Oxygen Vacancy for Enhanced Hydrogenation of Furfural to Furfuryl Alcohol.

It is a challenge to obtain ABO3 perovskite oxides with favorable crystal phase and well-defined porous structure via existing approaches. Here, we design an effective and versatile strategy to construct mesoporous ABO3 perovskite oxides with functionalized nanocrystal frameworks and abundant oxygen vacancy sites via a resol-assisted cationic coordinative co-assembly approach. The as-prepared oxygen vacancy-rich mesoporous LaMnO3 as heterogeneous catalyst exhibits remarkable catalytic activity and stability for hydrogenation of furfural to furfuryl alcohol, including over 99 % conversion and 96 % selectivity. Combined with density functional theory calculation, the catalytic mechanism is elucidated, revealing that porous LaMnO3 nanocrystal framework is conducive to expose oxygen deficiency sites, which can facilitate the interaction between catalyst surface and catalytic substrate, leading to lower barrier in hydrogenation process.

A Resol‐Assisted Cationic Coordinative Co‐assembly Approach to Mesoporous ABO3 Perovskite Oxides with Rich Oxygen Vacancy for Enhanced Hydrogenation of Furfural to Furfuryl Alcohol

AbstractIt is a challenge to obtain ABO3 perovskite oxides with favorable crystal phase and well‐defined porous structure via existing approaches. Here, we design an effective and versatile strategy to construct mesoporous ABO3 perovskite oxides with functionalized nanocrystal frameworks and abundant oxygen vacancy sites via a resol‐assisted cationic coordinative co‐assembly approach. The as‐prepared oxygen vacancy‐rich mesoporous LaMnO3 as heterogeneous catalyst exhibits remarkable catalytic activity and stability for hydrogenation of furfural to furfuryl alcohol, including over 99 % conversion and 96 % selectivity. Combined with density functional theory calculation, the catalytic mechanism is elucidated, revealing that porous LaMnO3 nanocrystal framework is conducive to expose oxygen deficiency sites, which can facilitate the interaction between catalyst surface and catalytic substrate, leading to lower barrier in hydrogenation process.

Effect of elemental doping on the catalytic activity of ABO3 perovskite oxides during low-pressure gas nitriding

In order to optimize the catalytic activity of the LaFeO3 oxide during low-pressures gas nitriding, the elemental doping by chromium, nickel and cerium was applied in the present work. The effects of doped LaFeO3 oxides on catalytic nitriding efficiency were systematically investigated. A new catalytic feature was proposed to explain the differences in catalytic activity among ABO3 oxides. The microstructure, phase composition and thickness of nitrided layers were characterized by the optical microscope, scanning electron microscope, X-ray diffraction and micro-hardness tester, respectively. The chemical bonds on nitrided surfaces were carefully investigated by X-ray photoelectron spectroscopy. The first-principles calculation was applied to reveal the inner diffusion behavior of nitrogen atoms. The results show that all ABO3 oxides exhibit an excellent catalytic activity in low-pressure gas nitriding. The LaFeO3 and (La0.5Ce0.5)FeO3 oxide show the enhanced catalytic activity as nitriding time prolonging while the LaCrO3 and LaNiO3 oxide present an opposite phenomenon. It depended on the state of active oxygen on nitrided surfaces, which is affected by the valence of doped elements. A critical ratio of adsorbed oxygen to hydroxyl on nitrided surfaces was proved to be 1.33. Beyond this value, ABO3 oxides have a remarkable improvement on catalytic nitriding.

The emergence of magnetic ordering at complex oxide interfaces tuned by defects

Complex oxides show extreme sensitivity to structural distortions and defects, and the intricate balance of competing interactions which emerge at atomically defined interfaces may give rise to unexpected physics. In the interfaces of non-magnetic complex oxides, one of the most intriguing properties is the emergence of magnetism which is sensitive to chemical defects. Particularly, it is unclear which defects are responsible for the emergent magnetic interfaces. Here, we show direct and clear experimental evidence, supported by theoretical explanation, that the B-site cation stoichiometry is crucial for the creation and control of magnetism at the interface between non-magnetic ABO3-perovskite oxides, LaAlO3 and SrTiO3. We find that consecutive defect formation, driven by atomic charge compensation, establishes the formation of robust perpendicular magnetic moments at the interface. Our observations propose a route to tune these emerging magnetoelectric structures, which are strongly coupled at the polar-nonpolar complex oxide interfaces.

Emerging Perovskite Electromagnetic Wave Absorbers from Bi-Metal–Organic Frameworks

Highly efficient electromagnetic wave absorbers are urgently demanded for the removal of electromagnetic pollution and interference in consumers’ electronics. ABO3 perovskite oxides with fascinatin...