Oxygen-deficient Perovskite Research Articles

Stabilization of oxygen vacancy ordering and electrochemical-proton-insertion-and-extraction-induced large resistance modulation in strontium iron cobalt oxides Sr(Fe,Co)Oy

Electrochemically inserting and extracting hydrogen into and from solids are promising ways to explore materials’ phases and properties. However, it is still challenging to identify the structural factors that promote hydrogen insertion and extraction and to develop materials whose functional properties can be largely modulated by inserting and extracting hydrogen through solid-state reactions at room temperature. In this study, guided by theoretical calculations on the energies of oxygen reduction and hydrogen insertion reactions with oxygen-deficient perovskite oxides, we demonstrated that the oxygen vacancy ordering in Sr(Fe1−xCox)Oy (SFCO) epitaxial films can be stabilized by increasing the Co content (x ≥ 0.3) and revealed that it plays a key role in promoting proton accommodation into the SFCO lattice. We also show that the electrical resistance of SFCO films can be reversibly modulated by electrochemical proton insertion and extraction, and the modulation exceeds three orders of magnitude for Sr(Fe0.5Co0.5)O2.5 epitaxial films. Our results provide guidelines for controlling material properties through the insertion and extraction of hydrogen and for designing and exploring hydrogen-insertion materials.

Regulating Oxygen Vacancy Distribution in Perovskites via A-Site Cation Engineering for Water Oxidation

Oxygen-deficient perovskite oxides hold great potential as highly efficient catalysts for oxygen evolution reaction (OER), but regulating and in-depth understanding oxygen vacancies (Vo) effectively remains challenging. This study reports the...

Irreversible oxygen uptake and thermal expansion in the solid electrolyte oxygen-deficient perovskite Ca(Ti0.8Fe0.2)O3−δ

The efficient operation of solid oxide fuel cells for renewable energy generation requires solid electrolytes with high ionic conductivity, thermal stability, and chemical durability. Metal oxide perovskites of the form ABO3 are promising candidates, especially when doped with cations that incorporate interstitial oxygen or oxygen vacancies to enhance ionic conductivity. However, doping can lead to complex and poorly understood crystallographic structures. In this study, we investigate the effects of Fe3+ doping on the orthorhombic Pbnm CaTiO3 perovskite, forming Ca(Ti0.8Fe0.2)O3−δ, and demonstrate the complexity of its phase transitions at operational temperatures. Using synchrotron X-ray diffraction and thermogravimetric analysis, we show that this material undergoes significant oxygen uptake at elevated temperatures, leading to an irreversible expansion of the unit cell and changes in polyhedral coordination upon cooling. These structural modifications are attributed to the partial oxidation of Fe3+ to Fe4+, providing insight into the inconsistencies observed in previous studies of oxygen-deficient perovskites. Our findings highlight the critical role of thermal history and oxygen availability in determining the structural stability and long-term performance of perovskite-based solid electrolytes in solid oxide fuel cells.

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.

How Oxygen Deficiency and Its Ordering Control Electrical Conductivity in Sr2-x Ca x Fe2O6-δ Perovskites as Related to Water Oxidation Electrocatalysis.

We report on the analysis of oxygen vacancies (OVs) content and ordering of Sr2-x Ca x Fe2O6-δ perovskites and explain how OVs change the electrical conductivity and oxygen evolution catalytic activity of these compounds. The structure and OV content are tuned by controlling the A-site composition and the reaction atmosphere. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) are used to identify the crystal structures and to quantify bulk oxygen vacancy contents. Our analysis shows that OV content and crystal structure govern the electronic transport properties of the Sr2-x Ca x Fe2O6-δ system. The electrical conductivity of oxygen-deficient perovskites (ODPs) is significantly higher than those in brownmillerites (by at least 2 orders of magnitude). Seebeck coefficient measurements identified that the Sr-rich ODPs and brownmillerites are p-type semiconductors, while Ca-rich brownmillerites are either insulators (within the experimental temperature range) or p-type semiconductors at lower temperatures (<750 K). Electrical conductivity of p-type semiconductors (Sr-rich compounds) reduces with higher OV content, and in brownmillerites with x ≥ 1.25, a transition to n-type semiconductor is observed at temperatures above 750 K. Our analysis shows that the hole and electron concentrations are similar in these brownmillerites, indicating major contributions from ionic transport. Finally, we show how oxygen deficiency alters the electrical conductivity and catalytic activity of the Sr2-x Ca x Fe2O6-δ system in noncomplementary ways.

Enhanced oxygen evolution reaction activity of oxygen-deficient perovskite catalysts with A-site defects boosted by self-assembled silver nanoparticles

To enhance the competitiveness of perovskite electrocatalysts in oxygen evolution reaction (OER), it is vital to improve its catalytic activity and durability while controlling costs. Herein, we report a composite electrocatalyst of Ag/Sr0·8Ag0·2Co0·75Fe0·25O3-δ, which was designed by integrating internal structure-engineering, A-site defect, and in-situ Ag segregation. This electrocatalyst demonstrates an excellent OER performance in an alkaline electrolyte, surpassing the catalytic activity and durability of commercial RuO2 electrocatalysts. Various characterization techniques as well as density functional theory (DFT) calculations were performed to elucidate the synergistic coupling effects of Sr0·8Ag0·2Co0·75Fe0·25O3-δ with Ag nanoparticles (NPs) in the OER process. The robust interface anchoring Ag NPs on Sr0·8Ag0·2Co0·75Fe0·25O3-δ perovskite can induce ligand effects and interparticle cooperation, thereby promoting electron transfer and ion migration during OER. Meanwhile, partial Ag+ forms Ag NPs contributing to the formation of A-site defects, which results in an increase of oxygen vacancies and induces lattice expansion. A-site defects can optimize the occupancy of eg orbitals close to unity for B-site transition metals, thereby greatly enhancing OER catalytic activity. This work highlights an effective strategy to enhance OER activity of perovskite electrocatalysts.

DFT study of the brownmillerite-type strontium-based oxygen-deficient perovskites SrTMO2.5 (TM = Mn, Fe, Co and Cu)

DFT studies are performed to investigate the crystal structure and geometry, electronic and magnetic properties of brownmillerite-type strontium-based oxygen-deficient perovskites [Formula: see text] (TM = Mn, Fe, Co and Cu) in orthorhombic phase. The comparison of the calculated lattice parameters shows consistency with the experiments, and all these perovskites are thermodynamically stable as revealed by cohesive energy. The spin-dependent calculations of the electronic band profiles demonstrate the metallic behavior of all these deficient perovskites. The increase in electrical resistivity and electronic thermal conductivities with temperature also confirms the metallic behavior of these compounds. The optimized ground state energies in different magnetic phases and post-DFT treatment confirm that [Formula: see text] and [Formula: see text] are stable in C-type AFM, [Formula: see text] is in G-type AFM and [Formula: see text] in A-type AFM phase, respectively. The high specific heat of these deficient perovskites makes them good candidates for high-temperature technology. Based on the above physical properties, it is expected that these deficient perovskites are potential candidates for spintronics and magnetic storage devices.

Improvement in electrochemical performance of SrTiO3 with rGO via composite (SrTiO3/rGO) strategy fabricated via solvothermal approach

Developing sustainable and highly effective electrocatalysts for the oxygen evolution reaction (OER) is a significant research target for precise custody of water splitting to fulfill worldwide energy demands. This study focuses on the fabrication of SrTiO3/rGO-based composite via a solvothermal technique, which works as an electrocatalyst for evaluating the efficiency of OER in alkaline media. The material displays outstanding OER efficacy in 1 M KOH solution with its cubic structure validated by the X-ray diffraction (XRD) study. The electrochemical study reveals that the SrTiO3/rGO composite has a minimal overpotential (222 mV) compared to SrTiO3 (289 mV) and the commercial catalyst RuO2 (367 mV). The SrTiO3/rGO composite also has a minimal Tafel plot gradient of 37 mV dec−1 whereas RuO2 has a Tafel value of 72 mV dec−1. The current material has superior properties compared to RuO2, a commercial catalyst for the OER. The SrTiO3/rGO composite shows exceptional stability for 40 h. The low conductivity of oxygen-deficient SrTiO3 perovskite can be improved by incorporating carbon-based rGO into it, as demonstrated by the minimal Rct (0.07 Ω) obtained from electrochemical impedance spectroscopy (EIS) analysis and an enlarged surface area of 250 cm2, as evidenced by electrochemical surface area (ECSA). Therefore, our research indicates that a particular morphology of metal oxide can improve the efficiency of electrocatalysis when combined with reduced graphene oxide (rGO), suggesting its potential for generating effective and environmentally friendly energy. Hence, these findings may improve the smooth passage of electrons and provide a novel perspective to function as an adequate substitute for RuO2, a commercial catalyst.

Informatics-Based Learning of Oxygen Vacancy Ordering Principles in Oxygen-Deficient Perovskites.

Ordered oxygen vacancies (OOVs) in perovskites can exhibit long-range order and may be used to direct materials properties through modifications in electronic structures and broken symmetries. Based on the various vacancy patterns observed in previously known compounds, we explore the ordering principles of oxygen-deficient perovskite oxides with ABO2.5 stoichiometry to identify other OOV variants. We performed first-principles calculations to assess the OOV stability on a data set of 50 OOV structures generated from our bespoke algorithm. The algorithm employs uniform planar vacancy patterns on (111) pseudocubic perovskite layers and the approach proves effective for generating stable OOV patterns with minimal computational loads. We find as expected that the major factors determining the stability of OOV structures include coordination preferences of transition metals and elastic penalties resulting from the assemblies of polyhedra. Cooperative rotational modes of polyhedra within the OOV structures reduce elastic instabilities by optimizing the bond valence of A- and B cations. This finding explains the observed formation of vacancy channels along low-index crystallographic directions in prototypical OOV phases. The identified ordering principles enable us to devise other stable vacancy patterns with longer periodicity for targeted property design in yet to be synthesized compounds.

Thermal and chemical expansion of layered oxygen-deficient double perovskites

Layered oxygen-deficient double perovskites (ODP) based on the rare-earth elements (REE), barium and 3d-metals (Fe, Co, Cu etc.) are characterized by high values of electrical conductivity and high electrochemical activity in oxygen reduction reaction, and are considered as prospective cathode materials for intermediate-temperature solid oxide fuel cells (SOFC) on the base of proton- and oxygen-ion conducting solid electrolytes (SE). Effective cathode materials should be thermomechanically compatible with materials of SE, which tаkes place when the values of their thermal expansion coefficients (TEC) are close to each other. Due to this the study of thermal expansion of ODP as well as the isotation of different contributions in it (thermal, chemical, spin etc.), is of considerable interest. In this work using dilatometric method the thermal expansion of NdBa1–xSrxFeCo0,5Cu0,5O6−δ (0.0 ≤ х ≤ 1.0) (NBSFCC) ODP was studied using dilatometric method. It was established that the values of average linear thermal expansion coefficient (LTEC) (α) of the samples sharply increased from (15.1–16.2) · 10–6 K–1 at Т &lt; 630–920 K to (18.9–23.5) 10–6 K–1 at Т &gt; 630–920 K due to the evolution of weaklybonded oxygen from the samples. Values of α in the low-temperature region increase with increasing of values of their oxygen nonstoichiometry index (δ), and in the high-temperature one increase with the x increasing due to the increment of chemical contribution in the samples expansion. Based of the results of dilatometry, thermogravimetry, and iodometry, the thermal and chemical contributions in the expansion оn NBSFCC were isolated, and the effect of crystal structure, cationic and anionic composition of NBSFCC ODP on the values of their thermal and linear chemical expansion coefficient (LCEC, αδ ) was investigated. It was found, that LCEC values of the samples sharply increased from (8.6–11.8) · 10–3 at (х &lt; 0.5) to (12.6–15.8) · 10–3 at (х &gt; 0.5) when transition from ordered tetragonal (х &lt; 0.5) to disordered cubic (х &gt; 0.5) phase took place. It was shown, that dependences of LTEC and LCEC of NBSFCC phases on their crystal structure and chemical compositions obtained in this work are in good accordance with the analogous dependences determined earlier for the ODP of other types.

Comparative Thermal Insulation Nature of Ca2FeMnO6−δ and Sr2FeMnO6−δ

In this study, we investigate the utility of Ca2FeMnO6-δ and Sr2FeMnO6-δ as materials with low thermal conductivity, finding potential applications in thermoelectrics, electronics, solar devices, and gas turbines for land and aerospace use. These compounds, characterized as oxygen-deficient perovskites, feature distinct vacancy arrangements. Ca2FeMnO6-δ adopts a brownmillerite-type orthorhombic structure with ordered vacancy arrangement, while Sr2FeMnO6-δ adopts a perovskite cubic structure with disordered vacancy distribution. Notably, both compounds exhibit remarkably low thermal conductivity, measuring below 0.50 Wm−1K−1. This places them among the materials with the lowest thermal conductivity reported for perovskites. The observed low thermal conductivity is attributed to oxygen vacancies and phonon scattering. Interestingly as SEM images show the smaller grain size, our findings suggest that creating vacancies and lowering the grain size or increasing the grain boundaries play a crucial role in achieving such low thermal conductivity values. This characteristic enhances the potential of these materials for applications where efficient heat dissipation, safety, and equipment longevity are paramount.

Thermal Conductivity of CaSrFe2O6-δ

The thermal conductivity of CaSrFe2O6-δ, an oxygen-deficient perovskite, is a critical parameter for understanding its thermal transport properties and potential applications in energy conversion and electronic devices. In this study, we present an investigation of the thermal conductivity of CaSrFe2O6-δ at room temperature for its thermal insulation property study. Experimental measurement was conducted using a state-of-the-art thermal characterization technique, Thermtest thermal conductivity meter. The thermal conductivity of CaSrFe2O6-δ was found to be 0.574W/m/K, exhibiting a notable thermal insulation property.

Study of Thermal Insulation Nature of A2Fe2O6-δ (A and A’ = Ca, Sr)

Materials with low thermal conductivity have been used in thermoelectrics, electronics, solar devices, and land base and aerospace gas turbines to prevent heat dissipation and provide safety and longevity of equipment. We report Ca2Fe2O6-δ, and Sr2Fe2O6-δ for their low thermal conductivities. These compounds are vacancy-ordered oxygen-deficient perovskites but with different vacancy arrangements. Ca2Fe2O6-δ has a brownmillerite type structure while Sr2Fe2O6-δ has a different structure.

High proton conduction by full hydration in highly oxygen deficient perovskite

We report high proton conductivity (10 mS cm−1 at 235 °C) of stable BaSc0.8W0.2O2.8, which is attributed to (1) high proton concentration due to full hydration and large amount of oxygen vacancies and (2) high proton mobility due to reduced proton trapping.

Integration of SrFeO3-δ with reduced graphene oxide nanoribbon for symmetric/asymmetric capacitive charge storage

Oxygen-deficient perovskite oxides are attractive electrode materials due to their tendency to store charge through anion intercalation mechanism. Herein, SrFeO3-δ (SFO) is synthesized and studied for supercapacitor application. In order to have better stability and electrolytic accessibility, SFO is immoblized over holey-reduced graphene oxide nanoribbons (rGONR) in different ratios by varying rGONR content viz. 20, 40, 60 and 80 wt%. The hybrid material comprising 60 wt% rGONR (SFO/rGONR60) exhibits maximum specific capacitance (Csp) of 1143.7 F g−1 at 5 A g−1 with good cycling stability of ∼90.3% capacitance retention over 5000 galvanic charge-discharge (GCD) cycles at 20 A g−1. The high electrochemical performance of SFO/rGONR60 hybrid is attributed to its good electronic conductivity, improved ionic diffusion and the presence of enormous active sites. Asymmetric cell (K-doped δ-MnO2||SFO/rGONR60), on operating up to 1.5 V, can deliver high energy (E) and power (P) densities of 76 Wh kg−1 and 1463 W kg−1, respectively, which is greater than that of symmetric (SFO/rGONR60||SFO/rGONR60) cell. This research helps in understanding the relationship between oxygen-deficient perovskite oxide and rGONR in charge storage and cell performance.

Electrocatalytic Properties of Oxygen-Deficient Perovskites Ca3Fe3-xMnxO8 (x = 1-2) for the Hydrogen Evolution Reaction.

We have demonstrated a systematic trend in the electrocatalytic activity for the hydrogen evolution reaction (HER) and its correlations with transition-metal type, structural order, and electrical conductivity. The materials studied in this work, Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3), Ca3Fe1.5Mn1.5O8, and Ca3Fe2MnO8, belong to the family of oxygen-deficient perovskites and show a gradual increase in the ordering of oxygen vacancies. Ca3FeMn2O8 (CaFe1/3Mn2/3O3-1/3) contains randomly distributed oxygen vacancies, which begin to order in Ca3Fe1.5Mn1.5O8, and are fully ordered in Ca3Fe2MnO8. The gradual increase in the structural order is associated with a systematic enhancement of the electrocatalytic activity for HER in acidic conditions, Ca3FeMn2O8 < Ca3Fe1.5Mn1.5O8 < Ca3Fe2MnO8. While the improvement of the HER activity is also associated with an increase in the Fe content, we have shown that the type of structural order plays a more important role. We demonstrated this effect by control experiments on an analogous material where all Mn was substituted by Fe, leading to a different type of structural order and showing an inferior HER activity compared to the above three materials. Furthermore, electrical conductivity studies in a wide range of temperatures, 25-800 °C, indicate that the trend in the electrical conductivity is the same as that of the HER activity. These findings reveal several important structure-property relationships and highlight the importance of synergistic effects in enhancing the electrocatalytic properties.

Orbitally driven spin reorientation in Mn-doped YBaCuFeO5

The oxygen-deficient layered perovskite YBaCuFeO5 (YBCFO) is a rare type-II multiferroic material where ferroelectricity, driven by incommensurate spiral magnetic order, is believed to be achievable up to temperatures higher than room temperature. A cycloidal spiral order rather than a helical spiral order is an essential ingredient for the existence of ferroelectricity in this material. Motivated by a recent experimental study on Mn-doped YBCFO where the spiral plane was observed to cant more towards the crystallographic c-axis upon Mn doping at Fe sites than in the parent compound, we performed a detailed theoretical investigation using density functional theory calculations to understand the mechanism behind such spin reorientation. Our total energy calculations, within the generalized gradient approximation plus U plus spin–orbit coupling approach, reveal that Fe/Cu spin moments indeed align more towards the c-axis in Mn-doped YBCFO than in the parent compound. The largest exchange interaction (Cu–Cu in the a−b plane) is observed to decrease systematically with Mn doping concentration, reflecting the lowering of the transition temperature seen in experiments. Further, the inter-bilayer Cu–Mn exchange interaction becomes ferromagnetic in Mn-doped YBCFO, whereas the corresponding Cu–Fe exchange is antiferromagnetic in the parent compound, giving rise to frustration in the commensurate magnetic order. Most importantly, our electronic structure calculations reveal that in Mn-doped YBCFO because of hybridization with the Mn dz2 orbital, the highest occupied Cu orbital becomes the dz2 orbital as opposed to the dx2−y2 orbital in the parent compound. Therefore, we believe that the occupancy of the out-of-plane-oriented Cu dz2 orbital in place of the planar dx2−y2 orbital along with the frustrating exchange interaction drives the spins to align along the c-axis in Mn-doped YBCFO.

Influence of Te-Incorporated LaCoO3 on Structural, Morphology and Magnetic Properties for Multifunctional Device Applications.

A high perovskite activity is sought for use in magnetic applications. In this paper, we present the simple synthesis of (2.5% and 5%) Tellurium-impregnated-LaCoO3 (Te-LCO), Te and LaCoO3 (LCO) by using a ball mill, chemical reduction, and hydrothermal synthesis, respectively. We also explored the structure stability along with the magnetic properties of Te-LCO. Te has a rhombohedral crystal structure, whereas Te-LCO has a hexagonal crystal system. The reconstructed Te was imbued with LCO that was produced by hydrothermal synthesis; as the concentration of the imbuing agent grew, the material became magnetically preferred. According to the X-ray photoelectron spectra, the oxidation state of the cobaltite is one that is magnetically advantageous. As a result of the fact that the creation of oxygen-deficient perovskites has been shown to influence the mixed (Te4+/2-) valence state of the incorporated samples, it is abundantly obvious that this process is of utmost significance. The TEM image confirms the inclusion of Te in LCO. The samples start out in a paramagnetic state (LCO), but when Te is added to the mixture, the magnetic state shifts to a weak ferromagnetic one. It is at this point that hysteresis occurs due to the presence of Te. Despite being doped with Mn in our prior study, rhombohedral LCO retains its paramagnetic characteristic at room temperature (RT). As a result, the purpose of this study was to determine the impacts of RT field dependency of magnetization (M-H) for Te-impregnated LCO in order to improve the magnetic properties of RT because it is a low-cost material for advanced multi-functional and energy applications.

When copper is not enough: Advantages and drawbacks of using copper in de-NOx reactions over lanthanum manganite perovskite structures

Using a Cu-doped lanthanum manganite perovskite structure (LaCu0.5Mn0.5O3-δ), we show how Pd addition following different synthesis approaches (sol-gel vs. impregnation) allows to overcome the oxidation limitation of Cu in realistic de-NOx reaction mixtures involving NO, CO, O2 and water. The beneficial interplay of Cu with the oxygen-deficient perovskite resulting from in situ exsolution during humid de-NOx treatments is strongly connected with the metallic Cu state enhancing the N2O chemistry. The beneficial catalytic properties of metallic Cu can be preserved by adjustment of the Pd amount, as evidenced by combined in situ X-ray photoelectron spectroscopy and catalytic experiments following the main reactions of the de-NOx reaction network, including water-gas shift reaction and CO oxidation. The concept of impregnation with a noble metal and simultaneous exsolution with Cu yields a reactive Cu(Pd)/perovskite interface and provides a first step towards the economization of the noble metal content at the surface in future works.

Thermal Conductivity of CaSrFe0.75Co0.75Mn0.5O6-δ

CaSrFe0.75Co0.75Mn0.5O6-δ, an oxygen-deficient perovskite, had been reported for its better electrocatalytic properties of oxygen evolution reaction. It is essential to investigate different properties such as the thermal conductivity of such efficient functional materials. The thermal conductivity of CaSrFe0.75Co0.75Mn0.5O6-δ is a critical parameter for understanding its thermal transport properties and potential applications in energy conversion and electronic devices. In this study, the authors present an investigation of the thermal conductivity of CaSrFe0.75Co0.75Mn0.5O6-δ at room temperature for its thermal insulation property study. Experimental measurement was conducted using a state-of-the-art thermal characterization technique, Thermtest thermal conductivity meter. The thermal conductivity of CaSrFe0.75Co0.75Mn0.5O6-δ was found to be 0.724 W/m/K at 25 °C, exhibiting a notable thermal insulation property i.e., low thermal conductivity.