Ruthenium(V) oxides from low-temperature hydrothermal synthesis

Activating and Identifying the Active Site of RuS2 for Alkaline Hydrogen Oxidation Electrocatalysis

Searching for highly efficient and economical electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is crucial for the development of alkaline polymer membrane fuel cells. Here, we report a valid strategy to active pyrite-type RuS2 for alkaline HOR electrocatalysis by introducing sulfur vacancies. The obtained S-vacancies modified RuS2-x exhibits outperformed HOR activity with a current density of 0.676 mA cm-2 and mass activity of 1.43 mA μg-1, which are 15-fold and 40-fold improvement than those of Ru catalyst. In situ Raman spectra demonstrate the formation of S-H bond during the HOR process, identifying the S atom of RuS2-x is the real active site for HOR catalysis. Density functional theory calculations and experimental results including in situ surface-enhanced infrared absorption spectroscopy suggest the introduction of S vacancies can rationally modify the p orbital of S atoms, leading to enhanced binding strength between the S sites and H atoms on the surface of RuS2-x, together with the promoted connectivity of hydrogen-bonding network and lowered water formation energy, contributes to the enhanced HOR performance.

Fast and Durable Alkaline Hydrogen Oxidation Reaction at the Electron-Deficient Ruthenium-Ruthenium Oxide Interface

The slow hydrogen oxidation reaction (HOR) kinetics under alkaline conditions remain a critical challenge for the practical application of alkaline exchange membrane fuel cells. Herein, Ru/RuO₂ in-plane heterostructures are designed with abundant active Ru-RuO₂ interface domains as efficient electrocatalysts for the HOR in alkaline media. The experimental and theoretical results demonstrate that interfacial Ru and RuO₂ domains at Ru-RuO₂ interfaces are the optimal H and OH adsorption sites, respectively, endowing the well-defined Ru(100)/RuO₂ (200) interface as the preferential region for fast alkaline hydrogen electrocatalysis. More importantly, the metallic Ru domains become electron deficient due to the strong interaction with RuO₂ domains and show substantially improved inoxidizability, which is vital to maintain durable HOR electrocatalytic activity. The optimal Ru/RuO₂ heterostructured electrocatalyst exhibits impressive alkaline HOR activity with an exchange current density of 8.86 mA cm^-2 and decent durability. The exceptional electrocatalytic performance of Ru/RuO₂ in-plane heterostructure can be attributed to the robust and multifunctional Ru-RuO₂ interfaces endowed by the unique metal-metal oxide domains.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y₂Ru₂O₇ alternative through partial A-site substitution of Sr²⁺ for Y³⁺, efficiently improving its intrinsic OER activity. The optimized Y_1.7Sr_0.3Ru₂O₇ candidate observes a highly intrinsic mass activity of 1018 A g_Ru^-1 at an overpotential of 300 mV with excellent durability in 0.5 M H₂SO₄ electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr²⁺ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Orbital Effects in Solids: Basics, Recent Progress, and Opportunities

The properties of transition metal compounds are largely determined by nontrivial interplay of different degrees of freedom: charge, spin, lattice, and also orbital ones. Especially rich and interesting effects occur in systems with orbital degeneracy. For example, they result in the famous Jahn-Teller effect, leading to a plethora of consequences for static and dynamic properties, including nontrivial quantum effects. In the present review, we discuss the main phenomena in the physics of such systems, paying central attention to the novel manifestations of those. After shortly summarizing the basic phenomena and their descriptions, we concentrate on several specific directions in this field. One of them is the reduction of effective dimensionality in many systems with orbital degrees of freedom due to the directional character of orbitals, with the concomitant appearance of some instabilities that lead in particular to the formation of dimers, trimers, and similar clusters in a material. The properties of such cluster systems, which are largely determined by their orbital structure, are discussed in detail, and many specific examples of those in different materials are presented. Another big field that has acquired special significance relatively recently is the role of the relativistic spin-orbit interaction. The mutual influence of this interaction and the more traditional Jahn-Teller physics is treated in detail in the second part of the review. In discussing all of these questions, special attention is paid to novel quantum effects.

Influence of Alkaline-Earth-Metal Substitutions on the Bismuth Ruthenate Structure: Bi2-xA'xRu2O6O'1-y (A' = Mg, Ca, Sr)

Solid solutions with the formula of Bi_2-xA'_xRu₂O_7-y (A' = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) have been synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group Fd3̅m with complex A/A' cation coordination environments. The Bi cation is found to be off-center from the ideal position because of a lone-pair distortion, while the positions of the substituted A' cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 Å. Interestingly, this is one of only two known pyrochlores with Mg²⁺ located in an 8-coordinated site. The average Ru oxidation state for each substitution is found to increase, and charge compensates for the lower divalent A' substitution. The solid solutions show temperature-independent resistance across the series with small changes in magnitude that scale with the amount of substitution, while displaying Pauli paramagnetic behavior throughout.

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

Perovskite oxides with general composition ABO₃ are a large group of inorganic materials that can contain a variety of cations from all parts of the Periodic Table and that have diverse properties of application in fields ranging from electronics, energy storage to photocatalysis. Solvothermal synthesis routes to these materials have become increasingly investigated in the past decade as a means of direct crystallisation of the solids from solution. These methods have significant advantages leading to adjustment of crystal form from the nanoscale to the micron-scale, the isolation of compositions not possible using conventional solid-state synthesis and in addition may lead to scalable processes for producing materials at moderate temperatures. These aspects are reviewed, with examples taken from the past decade's literature on the solvothermal synthesis of perovskites with a systematic survey of B-site cations, from transition metals in Groups 4-8 and main group elements in Groups 13, 14 and 15, to solid solutions and heterostructures. As well as hydrothermal reactions, the use of various solvents and solution additives are discussed and some trends identified, along with prospects for developing control and predictability in the crystallisation of complex oxide materials.

Structures of mixed manganese ruthenium oxides (Mn1-xRux)O2 crystallised under acidic hydrothermal conditions

A synthesis method for the preparation of mixed manganese-ruthenium oxides is presented along with a detailed characterisation of the solids produced. The use of 1 M aqueous sulfuric acid mediates the redox reaction between KRuO4, KMnO4 and Mn2+ to form ternary oxides. At reaction temperature of 100 °C the products are mixtures of α-MnO2 (hollandite-type) and β-MnO2 (rutile-type), with some evidence of Ru incorporation in each from their expanded unit cell volumes. At reaction temperature of 200 °C solid-solutions β-Mn1-xRuxO2 are formed and materials with x ≤ 0.6 have been studied. The amount of Ru included in the oxide is greater than expected from the ratio of metals used in the synthesis, as determined by elemental analysis, implying that some Mn remains unreacted in solution. Powder X-ray diffraction (XRD) shows that while the unit cell volume expands in a linear manner, following Vegard's law, the tetragonal lattice parameters, and the a/c ratio, do not follow the extrapolated trends: this anisotropic behaviour is consistent with the different local coordination of the metals in the end members. Powder XRD patterns show increased peak broadening with increasing ruthenium content, which is corroborated by electron microscopy that shows nanocrystalline material. X-ray absorption near-edge spectra show that the average oxidation state of Mn in the solid solutions is reduced below +4 while that of Ru is increased above +4, suggesting some redistribution of charge. Analysis of the extended X-ray absorption fine structure provides complementary local structural information, confirming the formation of a solid solution, while X-ray photoelectron spectroscopy shows that the surface oxidation states of both Ru and Mn are on average lower than +4, suggesting a disordered surface layer may be present in the materials.

A new tilt and an old twist on the nickel arsenide structure-type: synthesis and characterisation of the quaternary transition-metal cyanamides A2MnSn2(NCN)6 (A = Li and Na)

In this work, we describe the synthesis and structure of the quaternary transition-metal cyanamides Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6. These phases crystallise isotypically in layered structures, with P3[combining macron]1m symmetry, that comprise hexagonal close-packed arrays of NCN2- anions with metal cations in 5/6 of the octahedral holes, thereby reflecting low-symmetry modifications of the hierarchical [NiAs]-type MNCN structure. The distinct coordination requirements of the metal cations template an ordered decoration across the octahedral sites with corundum-like [Sn2(NCN)3]2+ layers alternating with [A2Mn(NCN)3]2- layers which resemble a portion of the Li2Zr(NCN)3 structure. This motif is also mirrored in the form of the NCN2- anions which adopt N-C[triple bond, length as m-dash]N2- cyanamide shapes with clear single- and triple-bond character. Distortion-mode analysis reveals the importance of K1 octahedral twist and K2 cyanamide tilt displacements in stabilising these phases, the latter of which is only accessible because of the extended nature of the NCN2- anion. These are the first examples of non-binary transition-metal cyanamides to be discovered and this study highlights how the additional flexibility of the NCN2- anion affords a novel structure-type not observed in oxide chemistry.

Spin waves and spin-state transitions in a ruthenate high-temperature antiferromagnet

Ruthenium compounds serve as a platform for fundamental concepts such as spin-triplet superconductivity¹, Kitaev spin liquids^2-5 and solid-state analogues of the Higgs mode in particle physics^6,7. However, basic questions about the electronic structure of ruthenates remain unanswered, because several key parameters (including Hund's coupling, spin-orbit coupling and exchange interactions) are comparable in magnitude and their interplay is poorly understood, partly due to difficulties in synthesizing large single crystals for spectroscopic experiments. Here we introduce a resonant inelastic X-ray scattering (RIXS)^8,9 technique capable of probing collective modes in microcrystals of 4d electron materials. We observe spin waves and spin-state transitions in the honeycomb antiferromagnet SrRu₂O₆ (ref. ¹⁰) and use the extracted exchange interactions and measured magnon gap to explain its high Néel temperature^11-16. We expect that the RIXS method presented here will enable momentum-resolved spectroscopy of a large class of 4d transition-metal compounds.

Sr(M,Te)2O6 (M = Cr, Mn, Fe, Co, Ni): A Magnetically Dilute Family of Honeycomb Tellurates

We report the synthesis and characterization of five new honeycomb tellurates with the PbSb₂O₆ structure type. These materials, SrM_2/3Te_4/3O₆ (M = Cr, Mn, Fe) and SrM_1/2Te_3/2O₆ (M = Co, Ni), are prepared by conventional solid state synthesis. Rietveld refinements of powder X-ray and neutron diffraction data reveal that these materials crystallize in the P3̅1 m space group, in which M and Te are octahedrally coordinated and randomly distributed over the honeycomb sublattice. In three of these materials (M = Cr, Mn, Fe), the transition metal takes a 3+ oxidation state, and hence, charge neutrality necessitates that the honeycomb sublattice is 66% occupied by nonmagnetic Te. In the remaining two of these materials (M = Co, Ni), the transition metal takes a 2+ oxidation state, requiring that 75% of the honeycomb lattice is occupied by nonmagnetic Te. Thus, the honeycomb sublattice is highly diluted, as only 33% or 25% of the sites are occupied by a magnetic ion. These occupation values fall well-below the percolation threshold for the honeycomb lattice, p _c = 70%. Accordingly, magnetic susceptibility measurements reveal no magnetic order or spin freezing down to 1.8 K.

Hydrothermal Synthesis of Pyrochlore-Type Pentavalent Bismuthates Ca2Bi2O7 and Sr2Bi2O7

The pyrochlore-type Ca₂Bi₂O₇ and Sr₂Bi₂O₇ have been synthesized from a low-temperature hydrothermal route using NaBiO₃·nH₂O as a starting material. The crystal structures of these compounds were refined using synchrotron powder X-ray diffraction data. The cell parameters were found to be a = 10.75021 (5) Å and 10.94132 (6) Å for Ca₂Bi₂O₇ and Sr₂Bi₂O₇, respectively. Density functional theory calculations showed the metallic band structure, but the negligible mixing of O2 2p bands with the A-site alkaline-earth-metal states and weak overlap with the conduction bands result in the semiconducting behavior.

Hydrothermal synthesis and structure determination of a new calcium iron ruthenium hydrogarnet

A new calcium iron ruthenium hydrogarnet with the approximate composition Ca3(Ru2− x Fe x )(FeO4)2− y (H4O4)1+ y (x=1, y≈0.35) has been obtained by hydrothermal synthesis under oxidizing alkaline conditions. The compound crystallizes in the cubic space group Ia3̅d (No. 230) with a lattice parameter of a=12.4804(4) Å (T=100 K) and Z=8. The octahedral site of the garnet structure is equally occupied by Ru and Fe, whereas the tetrahedral site is partially occupied by Fe only. A partial substitution of the oxide anions by hydroxide ions is necessary for charge balancing, corresponding to the so-called hydrogarnet defects. The presence of hydroxide groups is proven by infrared spectroscopy. 57Fe Mössbauer spectroscopic data provide evidence for two different Fe3+ coordination environments as well as a magnetic ordering of two iron substructures with the respective ordering temperature above room temperature. The crystal composition was verified by energy-dispersive X-ray spectroscopy and the thermal behavior of the calcium iron ruthenate was studied by difference thermal analysis.

Alkaline-Earth Rhodium Hydroxides: Synthesis, Structures, and Thermal Decomposition to Complex Oxides

The rhodium(III) hydrogarnets Ca₃Rh₂(OH)₁₂ and Sr₃Rh₂(OH)₁₂ crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl₃·3H₂O and either Ca(OH)₂ or Sr(OH)₂ in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO₂ is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH)₆, are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. ²³Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. Ca₃Rh₂(OH)₁₂ yields CaRh₂O₄ and CaO above 650 °C, from which phase-pure CaRh₂O₄ is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. Sr₃Rh₂(OH)₁₂ decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite Sr₆Rh₅O₁₅, which contains mixed-valent Rh^3+/4+, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH)₆, a complex set of decomposition events takes place via transient phases.

Mechanistic insights into hydrodeoxygenation of phenol on bimetallic phosphide catalysts

Different binding modes of the reactant on different catalysts determine the hydrodeoxygenation selectivity.

Localized-itinerant dichotomy and unconventional magnetism in SrRu2O6

Electron correlations tend to generate local magnetic moments that usually order if the lattices are not too frustrated. The hexagonal compound SrRu₂O₆ has a relatively high Neel temperature but small local moments, which seem to be at odds with the nominal valence of Ru⁵⁺ in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${t}_{2g}^{3}$$\end{document}t2g3 configuration. Here, we investigate the electronic property of SrRu₂O₆ using density functional theory (DFT) combined with dynamical-mean-field theory (DMFT). We find that the strong hybridization between Ru d and O p states results in a Ru valence that is closer to +4, leading to the small ordered moment ~1.2 μ_B. While this is consistent with a DFT prediction, correlation effects are found to play a significant role. The local moment per Ru site remains finite ~2.3 μ_B in the whole temperature range investigated. Due to the lower symmetry, the t_2g manifold is split and the quasiparticle weight is renormalized significantly in the a_1g state, while the renormalization in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${e}_{g}^{{\rm{^{\prime} }}}$$\end{document}eg′ states is about a factor of 2–3 weaker. Our theoretical Neel temperature ~700 K is in reasonable agreement with experimental observations. SrRu₂O₆ is a unique system in which localized and itinerant electrons coexist with the proximity to an orbitally-selective Mott transition within the t_2g sector.

Ru atom-modified covalent triazine framework as a robust electrocatalyst for selective alcohol oxidation in aqueous electrolytes

This work demonstrates that a single Ru atom-modified covalent triazine framework (Ru-CTF) has selectivity for the electrooxidation of benzyl alcohol in water over the oxygen evolution reaction. Additionally, Ru-CTF displayed higher stability than an immobilized Ru-organometallic complex due to the covalently cross-linked structure of CTF.

Three-dimensional magnetic interactions in quasi-two-dimensional PdAs2O6

Millimeter-sized PdAs₂O₆ single crystals are grown using the vapor transport technique. The magnetic order at [Formula: see text] K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd²⁺ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) [Formula: see text]/Pd²⁺ using the fitted magnetic form factor of Pd²⁺ . A weak λ-type anomaly around T _N was observed in specific heat and the magnetic entropy change across T _N is 1.72 J mol^-1 K.This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range [Formula: see text] 250 K. The comparison with SrRu₂O₆ suggests that the magnetic interactions in PdAs₂O₆ are dominated by Pd-(O-[Formula: see text]-O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. The comparison with NiAs₂O₆ suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

Itinerant Antiferromagnetism in RuO_{2}

Bulk rutile RuO_{2} has long been considered a Pauli paramagnet. Here we report that RuO_{2} exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05μ_{B} as evidenced by polarized neutron diffraction. Density functional theory plus U (DFT+U) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Néel temperature and small itinerant moments make RuO_{2} unique among ruthenate compounds and among oxide materials in general.

Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO6 (M = Cr, Mn, Fe)

New layered honeycomb tellurates, BiM(III)TeO₆ (M = Cr, Mn, Fe) were synthesized and characterized. BiM(III)TeO₆ (M = Cr, Fe) species crystallize in a trigonal space group, P3̅1c (No. 163), of edge-sharing M³⁺/Te⁶⁺O₆ octahedra, which form honeycomb-like double layers in the ab plane with Bi³⁺ cations located between the layers. Interestingly, the structure of BiMnTeO₆ is similar to those of the Cr/Fe analogues, but with monoclinic space group, P2₁/c (No. 14), attributed to the strong Jahn-Teller distortion of Mn³⁺ cations. The crystal structure of BiM(III)TeO₆ is a superstructure of PbSb₂O₆-related materials (ABB'O₆). The Cr³⁺ and Fe³⁺ cations are ordered 80% and 90%, respectively, while the Mn³⁺ ions are completely ordered on the B-site of the ABB'O₆ structure. BiCrTeO₆ shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of -59.85 K, while BiFeTeO₆ and BiMnTeO₆ show sharp AFM transitions at ∼11 K with θ of -27.56 K and at ∼9.5 K with θ of -17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering.

PbMn(IV)TeO6: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO6, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P6̅2m (No. 189), and consists of edge-sharing (Mn(4+)/Te(6+))O6 trigonal prisms that form honeycomb-like two-dimensional layers with Pb(2+) ions between the layers. The structural difference between PbMnTeO6, with disordered/trigonal prisms of Mn(4+)/Te(6+), versus the similar chiral SrGeTeO6 (space group P312), with long-range order of Ge(4+) and Te(6+) in octahedral coordination, is attributed to a difference in the electronic effects of Ge(4+) and Mn(4+). Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at T(N) ≈ 20 K and a frustration parameter (|θ|/T(N)) of ∼2.16.

Controlling the crystallisation of oxide materials by solvothermal chemistry: tuning composition, substitution and morphology of functional solids

Three aspects in the synthesis of oxides under solvothermal conditions are reviewed: materials discovery, substitutional chemistry and crystal habit control.

Ba4Ru3O10.2(OH)1.8: a new member of the layered hexagonal perovskite family crystallised from water

Two chemical reagents in water at 200 °C yield a complex barium ruthenate with a new 8H perovskite stacking sequence.

New honeycomb iridium(v) oxides: NaIrO3 and Sr3CaIr2O9

Two new honeycomb Ir⁵⁺ iridates are the first examples of a J = 0 state on a honeycomb lattice.