Sequential Spin State Transition and Intermetallic Charge Transfer in PbCoO3

Realization of Intrinsic Colossal Magnetoresistance in Pb(Pb1/3Hg2/3)3Mn4O12: An A Site-Ordered Quadruple Perovskite Oxide

Colossal magnetoresistance (CMR) effects have been extensively studied in ABO3 perovskite manganites where the Mn3+-O-Mn4+ double-exchange mechanism plays a pivotal role. However, A-site-ordered AA'3B4O12-type quadruple perovskite oxides exhibit significantly suppressed double exchange due to their extremely small B-O-B bond angles (≈140°), hindering the realization of intrinsic CMR effects. Here, we report the design and synthesis of a novel quadruple perovskite oxide Pb(Pb1/3Hg2/3)3Mn4O12 (PPHMO) characterized by an unusually increased Mn-O-Mn bond angle of up to 153°. This compound crystallizes into a cubic Im3̅ structure with the charge distribution Pb2+(Pb1/33.5+Hg2/32+)3Mn43.63+O12. A ferromagnetic phase transition is observed at the Curie temperature TC ≈ 120 K, accompanied by an insulator-to-metal transition. Furthermore, applying magnetic fields significantly reduces the resistivity, resulting in intrinsic CMR effects with an absolute MR value of 650% at 8 T, increasing to 2250% at 16 T near TC. The large intrinsic MR is thereby realized unprecedentedly in an A-site-ordered quadruple perovskite oxide. Related origins for the intrinsic CMR effects presented in the current PPHMO are discussed in detail.

Sequential Charge Transfer and Magnetic Ordering in Quasi-One-Dimensional Antiferromagnet CrSbS3

Charge transfer and magnetic ordering are two profound condensed matter phenomena that impart distinct structures and properties to materials. However, such two effects rarely occur successively within a single material. Here, we present a unique case of temperature-induced sequential charge transfer and magnetic ordering in antiferromagnetic CrSbS3 with quasi-one-dimensional structure. Intriguingly, the contrasting effects of charge transfer between Cr and [Sb2S6] units and antiferromagnetic transition on lattice expansion result in diverse thermal expansion behaviors of lattice parameters, which ultimately leads to an anti-Invar effect on the lattice volume. Neutron powder diffraction further unravels a C-type antiferromagnetic order. Moreover, pressure regulation can also induce similar volume collapse, implying potential valence and spin-state transitions. The multiple electronic transitions observed in CrSbS3 offer a valuable platform for deciphering the interweaving among charge, spin, and lattice degrees of freedom, and shed a new light for exploring novel quantum phenomena in more low-dimensional van der Waals materials.

Concurrent Pressure-Induced Superconductivity and Photoconductivity Transitions in PbSe0.5Te0.5

Concurrent superconductivity and negative photoconductivity (NPC) are rarely observed. Here, the discovery in PbSe0.5Te0.5 of superconductivity and photoconductivity transitions between positive photoconductivity (PPC) and NPC during compression is reported to ≈40 GPa and subsequent decompression, which are also accompanied by reversible structure transitions (3D Fm3 ¯ ${{\bar{3}}}$ m ⇌ 2D Pnma ⇌ 3D Pm3 ¯ ${{\bar{3}}}$ m). Superconductivity with a maximum Tc of ≈6.7 K coincides with NPC and structure transition of Pnma to Pm3 ¯ ${{\bar{3}}}$ m at ≈18 GPa and the latter phase is preserved down to ≈5 GPa with enhanced Tc of ≈6.9 K during decompression. The observations imply the simultaneous superconducting and photoconductive transitions are closely related to the metallic Pm3 ¯ ${{\bar{3}}}$ m phase. First-principles calculations suggest the enhanced p-p hybridization and charge transfer between Pb-5p and ligand-p orbitals near the Fermi surface play key roles in electron-phonon interaction of mediating the Cooper pairs in PbSe0.5Te0.5. Hall coefficient measurements reveal that photothermal effect enhances electron-phonon interplay, which decreases carrier concentration and mobility and results in the reversal of PPC-NPC. Structure-dependent superconductivity and NPC are jointly mediated by electron-phonon interplay, which is tunable through illumination or cooling at high-pressure. The findings shed light on the origin of superconductive and photoconductive transitions in versatile materials of lead chalcogenides.

High-Pressure Synthesis of Quadruple Perovskite Oxide CaCu3Cr2Re2O12 with a High Ferrimagnetic Curie Temperature

An AA'3B2B'2O12-type quadruple perovskite oxide of CaCu3Cr2Re2O12 was synthesized at 18 GPa and 1373 K. Both an A- and B-site ordered quadruple perovskite crystal structure was observed, with the space group Pn-3. The valence states are verified to be CaCu32+Cr23+Re25+O12 by bond valence sum calculations and synchrotron X-ray absorption spectroscopy. The spin interaction among Cu2+, Cr3+, and Re5+ generates a ferrimagnetic transition with the Curie temperature (TC) at about 360 K. Moreover, electric transport properties and specific heat data suggest the presence of a half-metallic feature for this compound. The present study provides a promising quadruple perovskite oxide with above-room-temperature ferrimagnetism and possible half-metallic properties, which shows potential in the usage of spintronic devices.

Targeted Spin-State Regulation to Boost Oxygen Reduction Reaction

Catalytic reactions are known to be significantly affected by spin states and their variations during reaction processes, yet the mechanisms behind them remain not fully understood, thus preventing the rational optimization of catalysis. Here, we explore the relationship between the spin states of active sites and their catalytic performance, taking the oxygen reduction reaction as an example. We demonstrate that the catalytic performance is spin-state-dependent and can be improved by adjusting spin states during the catalytic process. To this end, we further investigate the possibility of altering the spin states of transition metals through the application of external fields, such as adsorbed species. By studying the influence of the strength of adsorbed ligands on spin states and its impact on catalytic performance, our results show that optimal catalytic performance is achieved when the strength of the external field is neither too strong nor too weak, forming a volcano-like relationship between the catalytic performance and the external field strength. Our findings can have far-reaching implications for the rational design of high-performance catalysis.

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.

Suppression of Pressure-Induced Phase Transitions in a Monoclinically Distorted LiNbO3-Type CuNbO3 by Preference for a CuO3 Triangular Coordination Environment

Pressure-induced phase transitions in a monoclinically distorted LiNbO₃-type CuNbO₃ with triangularly coordinated Cu and octahedrally coordinated Nb were experimentally and computationally investigated. Phase transitions into GdFeO₃-type or NaIO₃-type structures generally observed in LiNbO₃-type compounds below 30 GPa were not detected in CuNbO₃ even at the maximum experimental pressure, 32.4 GPa. Our density functional theory calculations revealed that the phase transition is suppressed by the preference for the CuO₃ triangular coordination environment, which reduces the total internal energy. This study clarifies that the change in the coordination environment of given ions can affect the pressure-induced phase transition.

Emergent physical properties of perovskite-type oxides prepared under high pressure

The perovskite ABO₃ family demonstrates a wide variety of structural evolutions and physical properties and is arguably the most important family of complex oxides. Chemical substitutions of the A- and/or B-site and modulation of oxygen content can effectively regulate their electronic behaviors and multifunctional performances. In general, the BO₆ octahedron represents the main unit controlling the electronic and magnetic properties while the A-site ion is often not involved. However, a series of unconventional perovskite materials have been recently synthesized under high pressure, such as the s-d level controlled Pb-based perovskite family and quadruple perovskite oxides containing transition metal ions at the A-site. In these compounds, the intersite A-B correlations play an important role in electronic behaviors and further induce many emergent physical properties.

5f Covalency Synergistically Boosting Oxygen Evolution of UCoO4 Catalyst

Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO₄ with alternative CoO₆ and 5f-related UO₆ octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. UCoO₄ exhibits a low overpotential of 250 mV at 10 mA cm^–2, surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co²⁺ ion in pristine UCoO₄ was converted to high-valence Co^3+/4+, while U⁶⁺ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co^3+/4+ was synergistically enhanced by the covalent bonding of U⁶⁺-5f in the U 5f–O 2p–Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

High-Pressure Synthesis and Magnetism of the 4H-BaMnO3 Single Crystal and Its 6H-Type Polymorph

A 4H-type BaMnO₃ single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P6₃/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO₆ octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at T_N ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P6₃/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO₆ dimers are separated by MnO₆ octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at T_N1 ≈ 220 K and T_N2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO₃.

Exceptionally Robust Face-Sharing Motifs Enable Efficient and Durable Water Oxidation

Corner-sharing oxides usually suffer from structural reconstruction during the bottleneck oxygen-evolution reaction (OER) in water electrolysis. Therefore, introducing dynamically stable active sites in an alternative structure is urgent but challenging. Here, 1D 5H-polytype Ba₅ Bi_0.25 Co_3.75 FeO_{14- δ} oxide with face-sharing motifs is identified as a highly active and stable candidate for alkaline OER. Benefiting from the stable face-sharing motifs with three couples of combined bonds, Ba₅ Bi_0.25 Co_3.75 FeO_{14- δ} can maintain its local structures even under high OER potentials as evidenced by fast operando spectroscopy, contributing to a negligible performance degradation over 110 h. Besides, the higher Co valence and smaller orbital bandgap in Ba₅ Bi_0.25 Co_3.75 FeO_{14- δ} endow it with a much better electron transport ability than its corner-sharing counterpart, leading to a distinctly reduced overpotential of 308 mV at 10 mA cm^-2 in 0.1 m KOH. Further mechanism studies show that the short distance between lattice-oxygen sites in face-sharing Ba₅ Bi_0.25 Co_3.75 FeO_{14- δ} can accelerate the deprotonation step (*OOH + OH^-  = *OO + H₂ O + e^- ) via a steric inductive effect to promote lattice-oxygen participation. In this work, not only is a new 1D face-sharing oxide with impressive OER performance discovered, but also a rational design of dynamic stable and active sites for sustainable energy systems is inaugurated.

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.

Unraveling Nanoscale Cobalt Oxide Catalysts for the Oxygen Evolution Reaction: Maximum Performance, Minimum Effort

The oxygen evolution reaction (OER) is a key bottleneck step of artificial photosynthesis and an essential topic in renewable energy research. Therefore, stable, efficient, and economical water oxidation catalysts (WOCs) are in high demand and cobalt-based nanomaterials are promising targets. Herein, we tackle two key open questions after decades of research into cobalt-assisted visible-light-driven water oxidation: What makes simple cobalt-based precipitates so highly active-and to what extent do we need Co-WOC design? Hence, we started from Co(NO₃)₂ to generate a precursor precipitate, which transforms into a highly active WOC during the photocatalytic process with a [Ru(bpy)₃]²⁺/S₂O₈^2-/borate buffer standard assay that outperforms state of the art cobalt catalysts. The structural transformations of these nanosized Co catalysts were monitored with a wide range of characterization techniques. The results reveal that the precipitated catalyst does not fully change into an amorphous CoO_x material but develops some crystalline features. The transition from the precipitate into a disordered Co₃O₄ material proceeds within ca. 1 min, followed by further transformation into highly active disordered CoOOH within the first 10 min. Furthermore, under noncatalytic conditions, the precursor directly transforms into CoOOH. Moreover, fast precipitation and isolation afford a highly active precatalyst with an exceptional O₂ yield of 91% for water oxidation with the visible-light-driven [Ru(bpy)₃]²⁺/S₂O₈^2- assay, which outperforms a wide range of carefully designed Co-containing WOCs. We thus demonstrate that high-performance cobalt-based OER catalysts indeed emerge effortlessly from a self-optimization process favoring the formation of Co(III) centers in all-octahedral environments. This paves the way to new low-maintenance flow chemistry OER processes.

Pressure-Driven Sequential Lattice Collapse and Magnetic Collapse in Transition-Metal-Intercalated Compounds FexNbS2

Volume collapse under high pressure is an intriguing phenomenon involving subtle interplay between lattice, spin, and charge. The two most important causes of volume collapse are lattice collapse (low-density to high-density) and magnetic collapse (high-spin to low-spin). Herein we report the pressure-driven sequential volume collapses in partially intercalated Fe_xNbS₂ (x = 1/4, 1/3, 1/2, 2/3). Because of the distinct interlayer atomic occupancy, the low-iron-content samples exhibit both lattice and magnetic collapses under compression, whereas the high-iron-content samples exhibit only one magnetic collapse. Theoretical calculations indicate that the low-pressure volume collapses for x = 1/4 and x = 1/3 are lattice collapses, and the high-pressure volume collapses for all four samples are magnetic collapses. The magnetic collapse involving the high-spin to low-spin crossover of Fe²⁺ has also been verified by in situ X-ray emission measurements. Integrating two distinct volume collapses into one material provides a rare playground of lattice, spin, and charge.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO₃ (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO₃ remain unclear. Herein, we reveal that PbFeO₃ crystallizes into an unusual 2a_p × 6a_p × 2a_p orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb²⁺_0.5Pb⁴⁺_0.5FeO₃ lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb²⁺ and Pb⁴⁺) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe³⁺ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.