Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Ferromagnetism in LaCoO3: relationship between the crystal structure, morphology and magnetic properties

It is known that in a weak magnetic field (H < 1 kOe) at T < TC ≈ 87 K, rare-earth cobalt oxide LaCoO3 has ferromagnetic properties. The physical mechanism of the emergence of ferromagnetism in LaCoO3 still remains unclear. We provided experimental evidence of the ferromagnetism and crystal lattice interconnection - the cause of ferromagnetic ordering is near-surface (defects) structural stresses. A theoretical model is proposed, within the framework of which the mechanism of the emergence of the magnetically ordered state in LaCoO3 is discussed.

Operando Studies Redirect Spatiotemporal Restructuration of Model Coordinated Oxides in Electrochemical Oxidation

Tetrahedral, pyramidal, and octahedral metal-oxygen coordinated ligands are fundamental components in all metal-oxide structures. Understanding the impacts of their spatiotemporal behaviors during electrochemical oxidation is crucial for diverse applications, yet remains unsolved due to challenges in designing model oxides and conducting operando characterizations. Herein, combining a suite of advanced operando characterizations and systematic computations, a link between oxygen-evolving performance and operational structural properties is established on model oxides. Compared with tetrahedral and octahedral structures, pyramidal structure is more susceptible to OH- attack due to its pristine unsaturated and asymmetric features and constant single-electron occupancy on the active z2 orbital during reaction, leading to surface-to-bulk restructuration into active amorphous high-valence CoOOHx with edge-sharing configurations. This is accompanied by ion leaching to create nanoscale space, following a leaching tendency of Sr2+ > Ba2+ > La3+ > Y3+. Operando soft X-ray absorption spectroscopy demonstrates a harder non-uniform dehydrogenation process over time (Co3+OOH → Co3+/4+OOHx → Co4+OO) because of the enhanced CoO covalency with higher energy barriers. Lattice oxygen participates in active CoOOHx formation but sacrifices stability. To address this activity-stability trade-off, an ion-tuning strategy is proposed to simultaneously enhance both activity and stability in electrode and device.

Investigations on the Origin of Topotactic Phase Transition of LaCoO3 Thin Films with In Situ XRD and Ambient Pressure Hard X-ray Photoelectron Spectroscopy

With the applications of in situ X-ray diffraction (XRD), electrical I-V measurement, and ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES), the characteristics of the topotactic phase transition of LaCoO3 (LCO) thin films are examined. XRD measurements show clear evidence of structural phase transition (SPT) of the LCO thin films from the perovskite (PV) LaCoO3 to the brownmillerite (BM) La2Co2O5 phases through the intermediate La3Co3O8 phase at a temperature of 350 °C under high-vacuum conditions, ∼10-5 mbar. The reverse SPT from BM to PV phases is also found under ambient pressure (>100 mbar) of air near 100 °C. Both observed SPTs in XRD are also identified in the electrical I-V measurements, i.e., the metallic PV phase to the insulating BM phase and vice versa. During the onset of SPTs, the bulk chemical and electronic states of LCO thin films are monitored with AP-HAXPES. The oxidation states in Co 2p spectra indicate that the oxygen vacancies are closely related to the SPT of LCO thin films. Also, the presence of enlarged band gap is observed as the SPT from PV to BM phases takes place, revealing the modified electronic properties of LCO due to the creation of oxygen vacancies. The analysis of valence band structures is further compared to the I-V measurements.

Tuning the Spin Transition and Carrier Type in Rare-Earth Cobaltates via Compositional Complexity

There is growing interest in material candidates with properties that can be engineered beyond traditional design limits. Compositionally complex oxides (CCO), often called high entropy oxides, are excellent candidates, wherein a lattice site shares more than four cations, forming single-phase solid solutions with unique properties. However, the nature of compositional complexity in dictating properties remains unclear, with characteristics that are difficult to calculate from first principles. Here, compositional complexity is demonstrated as a tunable parameter in a spin-transition oxide semiconductor La1- x(Nd, Sm, Gd, Y)x/4CoO3, by varying the population x of rare earth cations over 0.00≤ x≤ 0.80. Across the series, increasing complexity is revealed to systematically improve crystallinity, increase the amount of electron versus hole carriers, and tune the spin transition temperature and on-off ratio. At high a population (x = 0.8), Seebeck measurements indicate a crossover from hole-majority to electron-majority conduction without the introduction of conventional electron donors, and tunable complexity is proposed as new method to dope semiconductors. First principles calculations combined with angle resolved photoemission reveal an unconventional doping mechanism of lattice distortions leading to asymmetric hole localization over electrons. Thus, tunable complexity is demonstrated as a facile knob to improve crystallinity, tune electronic transitions, and to dope semiconductors beyond traditional means.

Direct Identification of O─O Bond Formation Through Three-Step Oxidation During Water Splitting by Operando Soft X-ray Absorption Spectroscopy

Anionic redox allows the direct formation of O─O bonds from lattice oxygens and provides higher catalytic in the oxygen evolution reaction (OER) than does the conventional metal ion mechanism. While previous theories have predicted and experiments have suggested the possible O─O bond, it has not yet been directly observed in the OER process. In this study, operando soft X-ray absorption spectroscopy (sXAS) at the O K-edge and the operando Raman spectra is performed on layered double CoFe hydroxides (LDHs) after intercalation with [Cr(C2O4)3]3-, and revealed a three-step oxidation process, staring from Co2+ to Co3+, further to Co4+ (3d6L), and ultimately leading to the formation of O─O bonds and O2 evolution above a threshold voltage (1.4 V). In contrast, a gradual oxidation of Fe is observed in CoFe LDHs. The OER activity exhibits a significant enhancement, with the overpotential decreasing from 300 to 248 mV at 10 mA cm-2, following the intercalation of [Cr(C2O4)3]3- into CoFe LDHs, underscoring a crucial role of anionic redox in facilitating water splitting.

Operando Soft X-ray Absorption of LaMn1-x Co x O3 Perovskites for CO Oxidation

We employed operando soft X-ray absorption spectroscopy (XAS) to monitor the changes in the valence states and spin properties of LaMn1-x Co x O3 catalysts subjected to a mixture of CO and O2 at ambient pressure. Guided by simulations based on charge transfer multiplet theory, we quantitatively analyze the Mn and Co 2p XAS as well as the oxygen K-edge XAS spectra during the reaction process. The Mn sites are particularly sensitive to the catalytic reaction, displaying dynamics in their oxidation state. When Co doping is introduced (x ≤ 0.5), Mn oxidizes from Mn2+ to Mn3+ and Mn4+, while Co largely maintains a valence state of Co2+. In the case of LaCoO3, we identify high-spin and low-spin Co3+ species combined with Co2+. Our investigation underscores the importance to consider the spin and valence states of catalyst materials under operando conditions.

Suppressing Structure Delamination for Enhanced Electrochemical Performance of Solid Oxide Cells

Reversible solid oxide cells (rSOCs) have significant potential as efficient energy conversion and storage systems. Nevertheless, the practical application of their conventional air electrodes, such as La0.8Sr0.2MnO3-δ (LSM), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), and PrBa0.8Ca0.2Co2O5+δ (PBCC), remains unsatisfactory due to interface delamination during prolonged electrochemical operation. Using micro-focusing X-ray absorption spectroscopy (µ-XAS), a decrease (increase) in the co-valence state from the electrode surface to the electrode/electrolyte interface is observed, leading to the above delamination. Utilizing the one-pot method to incorporate an oxygen-vacancy-enriched CeO2 electrode into these air electrodes, the uniform distribution of the Co valence state is observed, alleviating the structural delamination. PBCC-CeO2 electrodes exhibited a degradation rate of 0.095 mV h-1 at 650 °C during a nearly 500-h test as compared with 0.907 mV h-1 observed during the 135-h test for PBCC. Additionally, a remarkable increase in electrolysis current density from 636 to 934 mA cm-2 under 1.3 V and a maximum power density from 912 to 989 mW cm-2 upon incorporating CeO2 into PBCC is also observed. BSCF-CeO2 and LSM-CeO2 also show enhanced electrochemical performance and prolonged stability as compared to BSCF and LSM. This work offers a strategy to mitigate the structural delamination of conventional electrodes to boost the performance of rSOCs.

Comparing the Impacts of Strain Types on Oxygen-Vacancy Formation in a Perovskite Oxide via Nanometer-Scale Strain Fields

The utilization of an in-plane lattice misfit in an oxide epitaxially grown on another oxide with a different lattice parameter is a well-known approach to induce strains in oxide materials. However, achieving a sufficiently large misfit strain in this heteroepitaxial configuration is usually challenging, unless the thickness of the grown oxide is kept well below a critical value to prevent the formation of misfit dislocations at the interface for relaxation. Instead of adhering to this conventional approach, here, we employ nanometer-scale large strain fields built around misfit dislocations to examine the effects of two distinct types of strains─tension and compression─on the generation of oxygen vacancies in heteroepitaxial LaCoO3 films. Our atomic-level observations, coupled with local electron-beam irradiation, clarify that the in-plane compression notably suppresses the creation of oxygen vacancies, whereas the formation of vacancies is facilitated under tensile strain. Demonstrating that the defect generation can considerably vary with the type of strain, our study highlights that the experimental approach adopted in this work is applicable to other oxide systems when investigating the strain effects on vacancy formation.

Spectroscopic evidence of spin-state excitation in d-electron correlated semiconductor FeSb2

Iron antimonide (FeSb2) has been investigated for decades due to its puzzling electronic properties. It undergoes the temperature-controlled transition from an insulator to an ill-defined metal, with a cross-over from diamagnetism to paramagnetism. Extensive efforts have been made to uncover the underlying mechanism, but a consensus has yet to be reached. While macroscopic transport and magnetic measurements can be explained by different theoretical proposals, the essential spectroscopic evidence required to distinguish the physical origin is missing. In this paper, through the use of X-ray absorption spectroscopy and atomic multiplet simulations, we have observed the mixed spin states of 3d 6 configuration in FeSb2. Furthermore, we reveal that the enhancement of the conductivity, whether induced by temperature or doping, is characterized by populating the high-spin state from the low-spin state. Our work constitutes vital spectroscopic evidence that the electrical/magnetical transition in FeSb2 is directly associated with the spin-state excitation.

Electronic Spin Alignment within Homologous NiS2/NiSe2 Heterostructures to Promote Sulfur Redox Kinetics in Lithium-Sulfur Batteries

The catalytic activation of the Li-S reaction is fundamental to maximize the capacity and stability of Li-S batteries (LSBs). Current research on Li-S catalysts mainly focuses on optimizing the energy levels to promote adsorption and catalytic conversion, while frequently overlooking the electronic spin state influence on charge transfer and orbital interactions. Here, hollow NiS2/NiSe2 heterostructures encapsulated in a nitrogen-doped carbon matrix (NiS2/NiSe2@NC) are synthesized and used as a catalytic additive in sulfur cathodes. The NiS2/NiSe2 heterostructure promotes the spin splitting of the 3d orbital, driving the Ni3+ transformation from low to high spin. This high spin configuration raises the electronic energy level and activates the electronic state. This accelerates the charge transfer and optimizes the adsorption energy, lowering the reaction energy barrier of the polysulfides conversion. Benefiting from these characteristics, LSBs based on NiS2/NiSe2@NC/S cathodes exhibit high initial capacity (1458 mAh·g⁻1 at 0.1C), excellent rate capability (572 mAh·g⁻1 at 5C), and stable cycling with an average capacity decay rate of only 0.025% per cycle at 1C during 500 cycles. Even at high sulfur loadings (6.2 mg·cm⁻2), high initial capacities of 1173 mAh·g⁻1 (7.27 mAh·cm⁻2) are measured at 0.1C, and 1058 mAh·g⁻1 is retained after 300 cycles.

Boosting the methanol selectivity in CO2 hydrogenation over a MOF-derived CuZn@CN catalyst via Rb incorporation

The highly selective hydrogenation of CO2 to methanol has been achieved through the simultaneous utilization of alkali metals and Co as promoters over Cu-Zn@CN catalysts derived from MOF. Rb facilitates the dissociation of CO2 in the aqueous phase at relatively mild conditions to yield methanol with a selectivity of 89%.

Spin State Disproportionation in Insulating Ferromagnetic LaCoO3 Epitaxial Thin Films

The origin of insulating ferromagnetism in epitaxial LaCoO3 films under tensile strain remains elusive despite extensive research efforts are devoted. Surprisingly, the spin state of its Co ions, the main parameter of its ferromagnetism, is still to be determined. Here, the spin state in epitaxial LaCoO3 thin films is systematically investigated to clarify the mechanism of strain-induced ferromagnetism using element-specific X-ray absorption spectroscopy and dichroism. Combining with the configuration interaction cluster calculations, it is unambiguously demonstrated that Co3+ in LaCoO3 films under compressive strain (on LaAlO3 substrate) is practically a low-spin state, whereas Co3+ in LaCoO3 films under tensile strain (on SrTiO3 substrate) have mixed high-spin and low-spin states with a ratio close to 1:3. From the identification of this spin state ratio, it is inferred that the dark strips observed by high-resolution scanning transmission electron microscopy indicate the position of Co3+ high-spin state, i.e., an observation of a spin state disproportionation in tensile-strained LaCoO3 films. This consequently explains the nature of ferromagnetism in LaCoO3 films. The study highlights the importance of spin state degrees of freedom, along with thin-film strain engineering, in creating new physical properties that do not exist in bulk materials.

Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties, such as valence, spin, and orbital degrees of freedom, as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and can be subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultrahigh-conductivity, nonmagnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce the emergence of a net moment on the surrounding transition metal octahedral sites. These highly localized moments communicate through the itinerant metal states, which trigger the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultrahigh-conductivity film and will be critical to applications across spintronics.

Room-temperature valence transition in a strain-tuned perovskite oxide

Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr_1-xCa_xCoO₃). Here, we show that in thin films of the complex perovskite (Pr_1-yY_y)_1-xCa_xCoO_3-δ, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. The technological implications of this result are accompanied by fundamental prospects, as complete strain control of the electronic ground state is demonstrated, from ferromagnetic metal under tension to nonmagnetic insulator under compression, thereby exposing a potential novel quantum critical point.

Positive temperature coefficient of the thermal conductivity above room temperature in a perovskite cobaltite

The thermal conductivity above room temperature is investigated for LaCoO₃-based materials showing spin-state and insulator-metal crossovers. A positive temperature coefficient (PTC) of the thermal conductivity is observed during the insulator-metal crossover around 500 K. Our analysis indicates that the phononic thermal transport is also enhanced in addition to the electronic contribution as the insulator-metal crossover takes place. The enhancement of the phononic component is ascribed to the reduction of the incoherent local lattice distortion coupled with the spin/orbital state of each Co³⁺ ion, which is induced by the enhanced spin-state fluctuation between low and excited spin-states. Moreover, fine tunability for the PTC of the thermal conductivity is demonstrated via doping hole-type carriers into LaCoO₃. The observed enhancement ratio of the thermal conductivity κ _T (773 K) / κ _T (323 K) = 2.6 in La_0.95Sr_0.05CoO₃ is the largest value among oxide materials which exhibit a PTC of their thermal conductivity above room temperature. The thermal rectification ratio is estimated to reach 61% for a hypothetical thermal diode consisting of La_0.95Sr_0.05CoO₃ and LaGaO₃, the latter of which is a typical band insulator. These results indicate that utilizing spin-state and orbital degrees of freedom in strongly correlated materials is a useful strategy for tuning thermal transport properties, especially for designing thermal diodes.

Mo-Incorporated Magnetite Fe3 O4 Featuring Cationic Vacancies Enabling Fast Lithium Intercalation for Batteries

Transition metal oxides (TMOs) as high-capacity electrodes have several drawbacks owing to their inherent poor electronic conductivity and structural instability during the multi-electron conversion reaction process. In this study, the authors use an intrinsic high-valent cation substitution approach to stabilize cation-deficient magnetite (Fe₃ O₄ ) and overcome the abovementioned issues. Herein, 5 at% of Mo⁴⁺ -ions are incorporated into the spinel structure to substitute octahedral Fe³⁺ -ions, featuring ≈1.7 at% cationic vacancies in the octahedral sites. This defective Fe_2.93 ▫_0.017 Mo_0.053 O₄ electrode shows significant improvements in the mitigation of capacity fade and the promotion of rate performance as compared to the pristine Fe₃ O₄ . Furthermore, physical-electrochemical analyses and theoretical calculations are performed to investigate the underlying mechanisms. In Fe_2.93 ▫_0.017 Mo_0.053 O₄ , the cationic vacancies provide active sites for storing Li⁺ and vacancy-mediated Li⁺ migration paths with lower energy barriers. The enlarged lattice and improved electronic conductivity induced by larger doped-Mo⁴⁺ yield this defective oxide capable of fast lithium intercalation. This is confirmed by a combined characterization including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and density functional theory (DFT) calculation. This study provides a valuable strategy of vacancy-mediated reaction to intrinsically modulate the defective structure in TMOs for high-performance lithium-ion batteries.

New Undisputed Evidence and Strategy for Enhanced Lattice-Oxygen Participation of Perovskite Electrocatalyst through Cation Deficiency Manipulation

Oxygen evolution reaction (OER) is a key half-reaction in many electrochemical transformations, and efficient electrocatalysts are critical to improve its kinetics which is typically sluggish due to its multielectron-transfer nature. Perovskite oxides are a popular category of OER catalysts, while their activity remains insufficient under the conventional adsorbate evolution reaction scheme where scaling relations limit activity enhancement. The lattice oxygen-mediated mechanism (LOM) has been recently reported to overcome such scaling relations and boost the OER catalysis over several doped perovskite catalysts. However, direct evidence supporting the LOM participation is still very little because the doping strategy applied would introduce additional active sites that may mask the real reaction mechanism. Herein, a dopant-free, cation deficiency manipulation strategy to tailor the bulk diffusion properties of perovskites without affecting their surface properties is reported, providing a perfect platform for studying the contribution of LOM to OER catalysis. Further optimizing the A-site deficiency achieves a perovskite candidate with excellent intrinsic OER activity, which also demonstrates outstanding performance in rechargeable Zn-air batteries and water electrolyzers. These findings not only corroborate the key role of LOM in OER electrocatalysis, but also provide an effective way for the rational design of better catalyst materials for clean energy technologies.

Energy Level Alignment at the Cobalt Phosphate/Electrolyte Interface: Intrinsic Stability vs Interfacial Chemical Reactions in 5 V Lithium Ion Batteries

The intrinsic stability of the 5 V LiCoPO₄-LiCo₂P₃O₁₀ thin-film (carbon-free) cathode material coated with MoO₃ thin layer is studied using a comprehensive synchrotron electron spectroscopy in situ approach combined with first-principle calculations. The atomic-molecular level study demonstrates fully reversible electronic properties of the cathode after the first electrochemical cycle. The polyanionic oxide is not involved in chemical reactions with the fluoroethylene-containing liquid electrolyte even when charged to 5.1 V vs Li⁺/Li. The high stability of the cathode is explained on the basis of the developed energy level model. In contrast, the chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO₃ in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to external electrolyte oxidation via charge transfer from the relevant electron level to the MoO₃ valence band state and internal electrolyte oxidation via proton transfer to the solvents. This study provides a deeper insight into the development of both a doping strategy to enhance the electronic conductivity of high-voltage cathode materials and an efficient surface coating against unfavorable interfacial chemical reactions.

Rietveld Refinement and X-ray Absorption Study on the Bonding States of Lanthanum-Based Perovskite-Type Oxides La1−xCexCoO3

Metal-oxygen bonding of the Ce-doped LaCoO3 system remains largely unexplored despite extensive studies on its magnetic properties. Here, we investigate the structure and local structure of nanoscale La1−xCexCoO3, with x = 0, 0.2, and 0.4, using the Rietveld refinement and synchrotron X-ray absorption techniques, complemented by topological analysis of experimental electron density and electron energy distribution. The Rietveld refinement results show that LaCoO3 subject to Ce addition is best interpretable by a model of cubic symmetry in contrast to the pristine LaCoO3, conventionally described by either a monoclinic model or a rhombohedral model. Ce4+/Co2+ are more evidently compatible dopants than Ce3+ for insertion into the main lattice. X-ray absorption data evidence the partially filled La 5d-band of the pristine LaCoO3 in accordance with the presence of La–O bonds with the shared-type atomic interaction. With increasing x, the increased Ce spectroscopic valence and enhanced La–O ionic bonding are noticeable. Characterization of the local structures around Co species also provides evidence to support the findings of the Rietveld refinement analysis.

Main Descriptors To Correlate Structures with the Performances of Electrocatalysts

Traditional trial and error approaches to search for hydrogen/oxygen redox catalysts with high activity and stability are typically tedious and inefficient. There is an urgent need to identify the most important parameters that determine the catalytic performance and so enable the development of design strategies for catalysts. In the past decades, several descriptors have been developed to unravel structure-performance relationships. This Minireview summarizes reactivity descriptors in electrocatalysis including adsorption energy descriptors involving reaction intermediates, electronic descriptors represented by a d-band center, structural descriptors, and universal descriptors, and discusses their merits/limitations. Understanding the trends in electrocatalytic performance and predicting promising catalytic materials using reactivity descriptors should enable the rational construction of catalysts. Artificial intelligence and machine learning have also been adopted to discover new and advanced descriptors. Finally, linear scaling relationships are analyzed and several strategies proposed to circumvent the established scaling relationships and overcome the constraints imposed on the catalytic performance.

Double-Exchange-Induced in situ Conductivity in Nickel-Based Oxyhydroxides: An Effective Descriptor for Electrocatalytic Oxygen Evolution

Motivated by in silico predictions that Co, Rh, and Ir dopants would lead to low overpotentials to improve OER activity of Ni-based hydroxides, we report here an experimental confirmation on the altered OER activities for a series of metals (Mo, W, Fe, Ru, Co, Rh, Ir) doped into γ-NiOOH. The in situ electrical conductivity for metal doped γ-NiOOH correlates well with the trend in enhanced OER activities. Density functional theory (DFT) calculations were used to rationalize the in situ conductivity of the key intermediate states of metal doped γ-NiOOH during OER. The simultaneous increase of OER activity with intermediate conductivity was later rationalized by their intrinsic connections to the double exchange (DE) interaction between adjacent metal ions with various d orbital occupancies, serving as an indicator for the key metal-oxo radical character, and an effective descriptor for the mechanistic evaluation and theoretical guidance in design and screening of efficient OER catalysts.

Structural Behavior and Spin-State Features of BaAl2O4 Scaled through Tuned Co3+ Doping

Pure and Co³⁺-doped BaAl₂O₄ [Ba(Al_1-xCo_x)₂O₄, x = 0, 0.0077, 0.0379] powder samples were prepared by a facile hydrothermal route. Elemental analyses by static secondary ion mass spectrometry (SIMS), X-ray absorption spectroscopy (XAS) measurements at the Co K-edge, and X-ray diffraction studies were fully correlated, thus addressing a complete description of the structural complexity of Co³⁺-doped BaAl₂O₄ powder. Powder X-ray diffraction (PXRD) patterns indicated that prepared samples were nanocrystalline with a hexagonal P6₃ symmetry. The X-ray absorption near-edge structure (XANES) measurements revealed the presence of cobalt in a +3 oxidation state, while the rarely documented, tetrahedral symmetry around Co³⁺ was extracted from the extended X-ray absorption fine structure (EXAFS) oscillation patterns. Rietveld structure refinements showed that Co³⁺ preferentially substitutes Al³⁺ at tetrahedral Al3 sites of the BaAl₂O₄ host lattice, whereas the (Al3)O₄ tetrahedra remain rather regular with Co³⁺-O distances ranging from 1.73(9) to 1.74(9) Å. The underlying magneto-structural features were unraveled through axial and rhombic zero-field splitting (ZFS) terms. The increased substitution of Al³⁺ by Co³⁺ at Al3 sites leads to an increase of the axial ZFS terms in Co³⁺-doped BaAl₂O₄ powder from 10.8 to 26.3 K, whereas the rhombic ZFS parameters across the series change in the range from 2.7 to 10.4 K, showing a considerable increase of anisotropy together with the values of the anisotropic g-tensor components flowing from 1.7 to 2.5. We defined the line between the Co³⁺ doping limit and influenced magneto-structural characteristics, thus enabling the design of strategy to control the ZFS terms' contributions to magnetic anisotropy within Co³⁺-doped BaAl₂O₄ powder.

Thermodynamic model of the coupled valence and spin state transition in cobaltates

A class of cobalt-based oxides exhibits a peculiar type of transition, entangling valence and spin state degrees of freedom of 4f and 3d elements. It constitutes one of the most spectacular illustrations of the interplay between charge, spin and lattice degrees of freedom in strongly correlated materials. In this work, we present a thermodynamic model capable to reproduce the main features of this transition. Our approach is based on the minimization of a free energy combining the contributions of two sublattices and the interaction between them. The coupling energies introduced in the model are related to well-known chemical pressure effects in the perovskite structure. The results of this model are compared to experimental data derived from x-ray absorption spectroscopy.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

Two Spin-State Crystallizations in LaCoO_{3}

We report a magnetostriction study of a perovskite LaCoO_{3} above 100 T using our state-of-the-art strain gauge to investigate an interplay between electron correlation and spin crossover. There has been a controversy regarding whether two novel phases in LaCoO_{3} at high magnetic fields result from crystallizations or Bose-Einstein condensation during spin crossover as manifestations of localization and delocalization in spin states, respectively. We show that both phases are crystallizations rather than condensations, and the two crystallizations are different, based on the observations that the two phases exhibit as magnetostriction plateaux with distinct heights. The crystallizations of spin states have emerged manifesting the localizations and interactions in spin crossover with large and cooperative lattice changes.

Roles of Re-entrant cluster glass state and spin-lattice coupling in magneto-dielectric behavior of giant dielectric double perovskite La1.8Pr0.2CoFeO6

La based Co-Fe combined double perovskite (La1.8Pr0.2CoFeO6) was synthesized and the dielectric (zero-field and in-field), magnetic, x-ray absorption and Raman spectroscopy measurements have been investigated for La1.8Pr0.2CoFeO6double perovskite. The existence of re-entrant cluster glass state is observed. The magneto-dielectric (MD) is found in two temperature regions (25-80 K and 125-275 K). It has been demonstrated that the observed MD at low and high temperatures are respectively due to the spin freezing and the spin-lattice coupling. Furthermore, the very large dielectric constant and the low loss suggest that La1.8Pr0.2CoFeO6is very important from the application point of view.

Simple descriptor derived from symbolic regression accelerating the discovery of new perovskite catalysts

Symbolic regression (SR) is an approach of interpretable machine learning for building mathematical formulas that best fit certain datasets. In this work, SR is used to guide the design of new oxide perovskite catalysts with improved oxygen evolution reaction (OER) activities. A simple descriptor, μ/t, where μ and t are the octahedral and tolerance factors, respectively, is identified, which accelerates the discovery of a series of new oxide perovskite catalysts with improved OER activity. We successfully synthesise five new oxide perovskites and characterise their OER activities. Remarkably, four of them, Cs0.4La0.6Mn0.25Co0.75O3, Cs0.3La0.7NiO3, SrNi0.75Co0.25O3, and Sr0.25Ba0.75NiO3, are among the oxide perovskite catalysts with the highest intrinsic activities. Our results demonstrate the potential of SR for accelerating the data-driven design and discovery of new materials with improved properties.

Voltage- and time-dependent valence state transition in cobalt oxide catalysts during the oxygen evolution reaction

The ability to determine the electronic structure of catalysts during electrochemical reactions is highly important for identification of the active sites and the reaction mechanism. Here we successfully applied soft X-ray spectroscopy to follow in operando the valence and spin state of the Co ions in Li₂Co₂O₄ under oxygen evolution reaction (OER) conditions. We have observed that a substantial fraction of the Co ions undergo a voltage-dependent and time-dependent valence state transition from Co³⁺ to Co⁴⁺ accompanied by spontaneous delithiation, whereas the edge-shared Co-O network and spin state of the Co ions remain unchanged. Density functional theory calculations indicate that the highly oxidized Co⁴⁺ site, rather than the Co³⁺ site or the oxygen vacancy site, is mainly responsible for the high OER activity.

Effect of Multiplicity Fluctuation in Cobalt Ions on Crystal Structure, Magnetic and Electrical Properties of NdCoO3 and SmCoO3

The structural, magnetic, electrical, and dilatation properties of the rare-earth NdCoO₃ and SmCoO₃ cobaltites were investigated. Their comparative analysis was carried out and the effect of multiplicity fluctuations on physical properties of the studied cobaltites was considered. Correlations between the spin state change of cobalt ions and the temperature dependence anomalies of the lattice parameters, magnetic susceptibility, volume thermal expansion coefficient, and electrical resistance have been revealed. A comparison of the results with well-studied GdCoO₃ allows one to single out both the general tendencies inherent in all rare-earth cobaltites taking into account the lanthanide contraction and peculiar properties of the samples containing Nd and Sm.

Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3

Spin state transitions and intermetallic charge transfers can essentially change material structural and physical properties while excluding external chemical doping. However, these two effects have rarely been found to occur sequentially in a specific material. In this article, we show the realization of these two phenomena in a perovskite oxide PbCoO₃ with a simple ABO₃ composition under high pressure. PbCoO₃ possesses a peculiar A- and B-site ordered charge distribution Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with insulating behavior at ambient conditions. The high spin Co²⁺ gradually changes to low spin with increasing pressure up to about 15 GPa, leading to an anomalous increase of resistance magnitude. Between 15 and 30 GPa, the intermetallic charge transfer occurs between Pb⁴⁺ and Co²⁺ cations. The accumulated charge-transfer effect triggers a metal-insulator transition as well as a first-order structural phase transition toward a Tetra.-I phase at the onset of ∼20 GPa near room temperature. On further compression over 30 GPa, the charge transfer completes, giving rise to another first-order structural transformation toward a Tetra.-II phase and the reentrant electrical insulating behavior.

Electronic parameters in cobalt-based perovskite-type oxides as descriptors for chemocatalytic reactions

Perovskite-type transition metal (TM) oxides are effective catalysts in oxidation and decomposition reactions. Yet, the effect of compositional variation on catalytic efficacy is not well understood. The present analysis of electronic characteristics of B-site substituted LaCoO₃ derivatives via in situ X-ray absorption spectroscopy (XAS) establishes correlations of electronic parameters with reaction rates: TM t_2g and e_g orbital occupancy yield volcano-type or non-linear correlations with NO oxidation, CO oxidation and N₂O decomposition rates. Covalent O 2p-TM 3d interaction, in ultra-high vacuum, is a linear descriptor for reaction rates in NO oxidation and CO oxidation, and for N₂O decomposition rates in O₂ presence. Covalency crucially determines the ability of the catalytically active sites to interact with surface species during the kinetically relevant step of the reaction. The nature of the kinetically relevant step and of surface species involved lead to the vast effect of XAS measurement conditions on the validity of correlations.

Design of efficient catalysts requires understanding the decisive electronic parameters for catalytic efficacy and their dependence on elemental composition. Here, the authors report covalency as suitable descriptor of perovskite-type transition metal oxides as chemo-catalysts.

The spin–orbit–phonon coupling and crystalline elasticity of LaCoO3 perovskite

Based on an integrated study of magnetic susceptibility, specific heat, and thermal expansion of single-crystal LaCoO₃ free from cobalt and oxygen vacancies, two narrow spin gaps are identified before and after the phonon softening of gap size ΔE ∼ 0.5 meV in a CoO₆-octahedral crystal electric field (CEF) and the thermally activated spin gap Q ∼ 25 meV, respectively. Significant excitation of Co³⁺ spins from a low-spin (LS) to a high-spin (HS) state is confirmed by the thermal activation behavior of spin susceptibility χ^S of energy gap Q ∼ 25 meV, which follows a two-level Boltzmann distribution to saturate at a level of 50% LS/50% HS statistically above ∼200 K, without the inclusion of a postulated intermediate spin (IS) state. A threefold increase in the thermal expansion; coefficient (α) across the same temperature range as that of thermally activated HS population growth is identified, which implies the non-trivial spin–orbit–phonon coupling caused the bond length of Co^3+(LS↔HS)–O fluctuation and the local lattice distortion. The unusually narrow gap of ΔE ∼ 0.5 meV for the CoO₆ octahedral CEF between e_g–t_2g indicates a more isotropic negative charge distribution within the octahedral CEF environment, which is verified by the Electron Energy Loss Spectroscopy (EELS) study to show nontrivial La–O covalency.

Considering the before and after phonon softening, the gap in a CoO₆-octahedral crystal electric fields (CEF) and the thermally activated spin gap, were observed of ∼0.5 meV and Q ∼ 25 meV in defect-free LaCoO₃ single crystal, respectively.

Strong Orbital Polarization in a Cobaltate-Titanate Oxide Heterostructure

Through a combination of experimental measurements and theoretical modeling, we describe a strongly orbital-polarized insulating ground state in an (LaTiO_{3})_{2}/(LaCoO_{3})_{2} oxide heterostructure. X-ray absorption spectra and ab initio calculations show that an electron is transferred from the titanate to the cobaltate layers. The charge transfer, accompanied by a large octahedral distortion, induces a substantial orbital polarization in the cobaltate layer of a size unattainable via epitaxial strain alone. The asymmetry between in-plane and out-of-plane orbital occupancies in the high-spin cobaltate layer is predicted by theory and observed through x-ray linear dichroism experiments. Manipulating orbital configurations using interfacial coupling within heterostructures promises exciting ground-state engineering for realizing new emergent electronic phases in metal oxide superlattices.

Tunable pH-dependent oxygen evolution activity of strontium cobaltite thin films for electrochemical water splitting

Understanding the oxygen evolution reaction (OER) dependence on the reaction environment pH is important to find alternative strategies to define an optimal pH value for high electrocatalytic activity. SrCoO_2.5 films with the brownmillerite phase are investigated in this study for their strain effects on the OER activity, with particular regard to the pH dependence. Pulsed laser deposited films with different thicknesses and, thus, strain conditions, are characterized in terms of long range and near-order structural properties and electrochemical OER activity. By comparison, more strained thinner films have smaller OER current at lower pH conditions, but higher sensitivity to the environment pH. Spectroscopic measurements allow us to correlate such behaviors to the Co 3d-O 2p hybridization effects of the CoO₆ octahedral sites, which lead to a variation of the 3d level electronic occupation. At the same time, density functional theory calculations show that the oxygen vacancy channels of the CoO₄ tetrahedral sites are stable with respect to the strain effects. These results provide new perspectives to manipulate the pH dependent OER activity through the strain effects, useful for designing water splitting-based devices with optimized performances.

Exceptional oxygen evolution reactivities on CaCoO3 and SrCoO3

We investigated the roles of covalent bonding, separation of surface oxygen, and electrolyte pH on the oxygen evolution reaction (OER) on transition metal oxides by comparing catalytic onset potentials and activities of CaCoO₃ and SrCoO₃. Both cubic, metallic perovskites have similar Co^IV intermediate spin states and onset potentials, but a substantially smaller lattice parameter and shorter surface oxygen separation make CaCoO₃ a more stable catalyst with increased OER activity. The onset potentials are similar, occurring where H⁺ is removed from surface -OH^-, but two competing surface reactions determine the catalytic activity. In one, the surface -O^- is attacked by electrolyte OH^- to form the surface -OOH^-; in the other, two -O^- form a surface peroxide ion and an oxygen vacancy with electrolyte OH^- attacking the oxygen vacancy. The second pathway can be faster if the surface oxygen separation is smaller.

Investigation of multi-mode spin-phonon coupling and local B-site disorder in Pr2CoFeO6 by Raman spectroscopy and correlation with its electronic structure by XPS and XAS studies

Electronic structure of Pr₂CoFeO₆ (at 300 K) was investigated by x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy techniques. All three cations, i.e. Pr, Co and Fe were found to be trivalent in nature. XPS valance band analysis suggested the system to be insulating in nature. The analysis suggested that Co³⁺ ions exist in low spin state in the system. Moreover, Raman spectroscopy study indicated the random distribution of the B-site ions (Co/Fe) triggered by same charge states. In temperature-dependent Raman study, the relative heights of the two observed phonon modes exhibited anomalous behaviour near magnetic transition temperature T _N ~ 270 K, thus indicating towards interplay between spin and phonon degrees of freedom in the system. Furthermore, clear anomalous softening was observed below T _N which confirmed the existence of strong spin-phonon coupling occurring for at least two phonon modes of the system. The line width analysis of the phonon modes essentially ruled out the role of magnetostriction effect in the observed phonon anomaly. The investigation of the lattice parameter variation across T _N (obtained from the temperature-dependent neutron diffraction measurements) further confirmed the existence of the spin-phonon coupling.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Nanoscale ferroelastic twins formed in strained LaCoO3 films

The coexistence and coupling of ferroelasticity and magnetic ordering in a single material offers a great opportunity to realize novel devices with multiple tuning knobs. Complex oxides are a particularly promising class of materials to find multiferroic interactions due to their rich phase diagrams, and are sensitive to external perturbations. Still, there are very few examples of these systems. Here, we report the observation of twin domains in ferroelastic LaCoO₃ epitaxial films and their geometric control of structural symmetry intimately linked to the material's electronic and magnetic states. A unidirectional structural modulation is achieved by selective choice of substrates having twofold rotational symmetry. This modulation perturbs the crystal field-splitting energy, leading to unexpected in-plane anisotropy of orbital configuration and magnetization. These findings demonstrate the use of structural modulation to control multiferroic interactions and may enable a great potential for stimulation of exotic phenomena through artificial domain engineering.

Influence of Fe Substitution into LaCoO3 Electrocatalysts on Oxygen-Reduction Activity

The development of commercially friendly and stable catalysts for oxygen reduction reaction (ORR) is critical for many energy conversion systems such as fuel cells and metal-air batteries. Many Co-based perovskite oxides such as LaCoO₃ have been discovered as the stable and active ORR catalysts, which can be good candidates to replace platinum (Pt). Although researchers have tried substituting various transition metals into the Co-based perovskite catalysts to improve the ORR performance, the influence of substitution on the ORR mechanism is rarely studied. In this paper, we explore the evolution of ORR mechanism after substituting Fe into LaCoO₃, using the combination of X-ray photoelectron spectroscopy, high-resolution X-ray microscopy, X-ray diffraction, surface-sensitive soft X-ray absorption spectroscopy characterization, and electrochemical tests. We observed enhanced catalytic activities and increased electron transfer numbers during the ORR in Co-rich perovskite, which are attributed to the optimized e_g filling numbers and the stronger hybridization of transition metal 3d and oxygen 2p bands. The discoveries in this paper provide deep insights into the ORR catalysis mechanism on metal oxides and new guidelines for the design of Pt-free ORR catalysts.

Charge transfer-induced O p-band center shift for an enhanced OER performance in LaCoO3 film

Charge transfer between LaCoO₃ and La_0.67Sr_0.33MnO₃ interface shortens the distance between O p-band center and the Fermi level.

The influence of oxygen vacancy on the electronic and optical properties of ABO3−δ (A = La, Sr, B = Fe, Co) perovskites

ABO_3−δ (A = La, Sr, B = Fe, Co) perovskites are useful in a wide range of applications, including their recent exploration for application in high-temperature optical oxygen sensing for energy conversion devices such as solid oxide fuel cells.