3d Transition Metal Compounds Research Articles

Three-dimensional pine-tree-like bimetallic sulfide with maximally exposed active sites by secondary structural restructuring for efficient electrocatalytic OER

Developing efficient, low-cost and bifunctional catalysts with predominant durability for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is an extraordinary challenge in the preparation of green hydrogen energy by electrochemical water splitting. Three-dimensional (3D) transition metal compounds have become a research hotspot as OER electrocatalysts, which can replace noble metal oxides such as RuO2 and IrO2 to reduce application costs. Herein, we synthesized a novel three-dimensional pine-tree-like bimetallic sulfide arrays on nickel foam (FeCoS/NF) using various optimization strategies such as morphology optimization, in situ growth and introduction of heterogeneous structures. The as-synthesized FeCoS/NF electrocatalyst only requires relatively low overpotential of 156 mV to achieve a current density of 20 mA cm−2 for OER, with a Tafel slope of only 37 mV dec−1. It also has a small charge transfer resistance, an electrochemical surface area and good electrochemical stability in alkaline electrolytes. The excellent performance of FeCoS/NF can be attributed to the synergistic effect and amorphous phase of FeCoS as well as the well-defined pine-tree-like array architecture with a large surface area, abundant active sites, and sufficient gas and electrolyte diffusion channels.

Superconductivity in a Layered Cobalt Oxychalcogenide Na2CoSe2O with a Triangular Lattice.

Unconventional superconductivity in bulk materials under ambient pressure is extremely rare among the 3d transition metal compounds outside the layered cuprates and iron-based family. It is predominantly linked to highly anisotropic electronic properties and quasi-two-dimensional (2D) Fermi surfaces. To date, the only known example of a Co-based exotic superconductor is the hydrated layered cobaltate, NaxCoO2·yH2O, and its superconductivity is realized in the vicinity of a spin-1/2 Mott state. However, the nature of the superconductivity in these materials is still a subject of intense debate, and therefore, finding a new class of superconductors will help unravel the mysteries of their unconventional superconductivity. Here, we report the discovery of superconductivity at ∼6.3 K in our newly synthesized layered compound Na2CoSe2O, in which the edge-shared CoSe6 octahedra form [CoSe2] layers with a perfect triangular lattice of Co ions. It is the first 3d transition metal oxychalcogenide superconductor with distinct structural and chemical characteristics. Despite its relatively low TC, this material exhibits very high superconducting upper critical fields, μ0HC2(0), which far exceeds the Pauli paramagnetic limit by a factor of 3-4. First-principles calculations show that Na2CoSe2O is a rare example of a negative charge transfer superconductor. This cobalt oxychalcogenide with a geometrical frustration among Co spins shows great potential as a highly appealing candidate for the realization of unconventional and/or high-TC superconductivity beyond the well-established Cu- and Fe-based superconductor families and opens a new field in the physics and chemistry of low-dimensional superconductors.

Electronic Configurations of 3d Transition-Metal Compounds Using Local Structure and Neural Networks.

Machine learning (ML) methods extract statistical relationships between inputs and results. When the inputs are solid-state crystal structures, structure-property relationships can be obtained. In this work, we investigate whether a simple neural network is able to learn the 3d orbital occupations for the transition-metal (TM) centers in crystalline inorganic solid-state compounds using only the local structure around the transition-metal centers described by rotationally invariant fingerprints based on spherical harmonics and one-hot elemental encoding. A multilayer neural network trained on density functional theory (DFT) results of about 1800 samples was developed and showed good performance in predicting the TM orbital occupations (for both spin channels). We study in detail how the local structure affects the predictions of the local properties and how they provide physical insights for the design of a future machine learning model for materials chemistry. The proposed ML method is illustrated in practical application by predicting local magnetic moments of the transition-metal atoms in a full set of inorganic structures with large unit cells. Although less accurate compared to the experimental data, the ML results compared well with the DFT results, suggesting the feasibility of electronic property prediction based only on structure input.

Energetic coordination compounds of late 3d metals with 1N-(Nitromethyl)-5H-tetrazole, a ligand with astonishing properties from the rare class of N-nitromethyl azoles

The hardly accessible N-nitromethyl moiety is introduced onto the 1,5H-tetrazole scaffold, representing the fourth and fifth ever synthesized N-nitromethyl azoles. Both compounds 1-(Nitromethyl)-5H-tetrazole (2a) and 2-(Nitromethyl)-5H-tetrazole (2b) are thoroughly analyzed by low-temperature single-crystal X-ray diffraction experiments, complemented by elemental analysis, multinuclear (1H, 13C, 14N, 15N) NMR as well as IR spectroscopy. Sensitivity measurements towards external stimuli revealed two highly insensitive compounds. Additionally, 2a was applied as ligand for energetic coordination compounds of 3d transition metals (e.g. Cu, Fe, Ni, and Zn) in combination with oxidizing nitrate, chlorate and perchlorate anions. The synthesized ECCs were analyzed by low-temperature single-crystal X-ray diffraction experiments, elemental analysis and IR spectroscopy. The thermal behavior of all compounds was investigated by differential thermal analysis and the sensitivities towards impact and friction was measured. Due to the lack of information about the properties of N-nitromethyl azoles, the complete insensitivity towards external stimuli of 2a and 2b supports first hints of a sensitivity decreasing influence of the N-nitromethyl moiety, which is highly desired when designing new high explosives. Additionally, the functional group is able to introduce meltability into molecular scaffolds.

Density Functional Theory Plus Dynamical Mean Field Theory within the Framework of Linear Combination of Numerical Atomic Orbitals: Formulation and Benchmarks.

The combination of density functional theory with dynamical mean-field theory (DFT+DMFT) has become a powerful first-principles approach to tackle strongly correlated materials in condensed matter physics. The wide use of this approach relies on robust and easy-to-use implementations, and its implementation in various numerical frameworks will increase its applicability on the one hand and help crosscheck the validity of the obtained results on the other. In this work, we develop a formalism within the linear combination of numerical atomic orbital (NAO) basis set framework, which allows for merging of NAO-based DFT codes with DMFT quantum impurity solvers. The formalism is implemented by interfacing two NAO-based DFT codes with three DMFT impurity solvers, and its validity is testified by benchmark calculations for a wide range of strongly correlated materials, including 3d transition metal compounds, lanthanides, and actinides. Our work not only enables DFT+DMFT calculations using popular and rapidly developing NAO-based DFT code packages but also facilitates the combination of more advanced beyond-DFT methodologies available in these codes with the DMFT machinery.

α-RuCl3 and other Kitaev materials

Quantum spin liquids have been drawing much attention in recent years as a platform to develop future quantum technologies, such as topological quantum computing. In particular, Kitaev’s honeycomb model has provided a blueprint to realize a quantum spin liquid that has Majorana fermions as its elementary excitation. While numerous theoretical studies have shown intriguing properties of quantum spin liquids, an experimental realization remains elusive. The recent observation of the quantized thermal Hall effect in α-RuCl3 has brought us tantalizingly close to an experimental realization of Kitaev quantum spin liquids. However, various groups report conflicting results, indicating that the Kitaev quantum spin liquid phase might be very fragile and its properties strongly depend on the sample. Here, we present a short overview of the rise of α-RuCl3 as a prime candidate material for realizing Kitaev quantum spin liquids. There are already many excellent review papers on this topic, so the emphasis will be on the materials aspect, comparing different crystal growth methods and crystal morphologies. We also discuss current research attempts to find other candidate materials to realize Kitaev quantum spin liquids, mostly focused on 3d transition-metal compounds, such as transition-metal halides and layered cobalt compounds.

Electron-beam-evaporation-based multi-source oxide MBE as a synthesis method for high-quality and novel magnetic materials—beyond 3d transition metal compounds

Magnetism and magnetic materials are among the most extensively studied research topics in condensed matter physics. While many practical applications of magnetic materials have been already launched, including permanent magnet motors, magnetic heads in hard disc drives (HDDs), and magnetoresistive random access memories (MRAMs), further search for new magnetic materials has not lost its charm. Here, we show that a multi-electron gun MBE apparatus for oxides, equipped with a high precision rate control system enables the synthesis of well-known ferromagnetic oxides, e.g., SrRuO3 of unparalleled quality. Furthermore, this method also allows to fetch novel magnetic materials, e.g., Sr3OsO6, that are well outside of the synthesis conditions known from 3d transition metal oxides.

Signature of Kondo hybridisation with an orbital-selective Mott phase in 4d Ca2−xSrxRuO4

AbstractThe heavy fermion state with Kondo-hybridisation (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localised electrons appropriate for Kondo pairing. Here, we report a substitution- and temperature-dependent angle-resolved photoemission study on 4d Ca2−xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localised γ-band induces the KH with an itinerant β-band, resulting in spectral weight suppression around the Fermi level. Our work demonstrates the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.

Structural reconstruction and anisotropic conductance in 4f-ferromagnetic monolayer

Two-dimensional magnets are promising for nanoscale spintronic applications. Currently, most available candidates are based on 3d transition metal compounds, with hexagonal or honeycomb lattice geometry. Here, a GdCl3 monolayer with 4f moments is theoretically studied, which can be exfoliated from its existing bulk. Its orthorhombic structure and hendecahedral ion cages are unique in two-dimensional. Furthermore, a significant structural reconstruction is caused by the implantation of Li atoms into its interstitial position, which also lead to ferromagnetism via a double-exchange-like process. Its highly anisotropic conductance may be peculiarly useful for nanoelectronics.

Resonant inelastic X-ray scattering as a probe of Jeff\u2009=\u20091/2 state in 3d transition-metal oxide

The state with effective total moment Jeff = 1/2 stabilized by the spin-orbit coupling is known to suppress Jahn-Teller distortions and may induce a strong exchange anisotropy. This in turn may lead to the formation of an elusive spin-liquid state in real materials. While recent studies have demonstrated that such a situation can be realized in 3d transition-metal compounds such as those based on Co2+ and Cu2+, diagnosis of Jeff = 1/2 state remains challenging. We show that resonant inelastic X-ray scattering is an effective tool to probe this state and apply it to CuAl2O4, material where Cu2+ ions were previously proposed to be in the Jeff = 1/2 state. Our results unambiguously demonstrate that, contrary to previous expectations, a competitive (to Jeff = 1/2) Jahn-Teller state realizes in this compound.

Rational Design of 3d Transition-Metal Compounds for Thermoelectric Properties by Using Periodic Trends in Electron-Correlation Modulation.

The electronic structures in solid-state transition-metal compounds can be represented by two parameters: the charge-transfer energy (Δ), which is the energy difference between the p-band of an anion and an upper Hubbard band contributed by transition-metal d-orbitals, and the onsite Coulomb repulsion energy (U), which represents the energy difference between lower and upper Hubbard bands composed of split d-orbitals in transition metals. These parameters can facilitate the classification of various types of electronic structures. In this study, the dependences of anion species (N3-, P3-, As3-, O2-, S2-, Se2-, Te2-, F-, Cl-, Br-, and I-) on Δ and U of 566 different binary and ternary 3d transition-metal compounds were investigated using ionic-model calculations. We were able to identify the systematic chemical trends in the variations in Δ and U values with the anion species of 11 different families of 3d transition-metal compounds in a comprehensive manner. The effective use of Δ-U diagrams given here, to facilitate the discovery and development of functional compounds, was demonstrated on thermoelectric compounds by classifying the thermoelectric properties of 3d transition-metal compounds and by predicting unrealized high-performance thermoelectric compounds.

Formation of ilmenite-type single-crystalline MgTiO3 thin films by pulsed-laser deposition

Ilmenite structure is a good playground to explore magnetism in honeycomb lattices owing to the existence of an intriguing variety of magnetism in 3d transition metal compounds. Because of the absence of magnetic ions, ilmenite-type MgTiO3 is a promising candidate for d0 transparent insulators to reveal general features of TiO6 honeycomb layers. In this study, we found an optimum growth condition to synthesize ilmenite-type single-crystalline MgTiO3 thin films on Al2O3(0001) substrates by pulsed-laser deposition. By increasing oxygen pressure from roughly 10−6 to 10−1 Torr, we obtained (0001)-oriented MgTiO3 thin films with suppression of segregation of a Mg2TiO4 phase. On the triangular lattice of Al2O3, twin-domain formation is suppressed with decreasing deposition rate, resulting in the synthesis of single-crystalline MgTiO3 thin films. The bandgap of the MgTiO3 film was evaluated to be about 4.4 eV by optical absorption spectra, which implies d0 transparent insulator.

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation.

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO2). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO2, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO2 in their reduced states. The Ni(0) d10 species associates with CO2 to form a putative Aresta-type Ni-η2-CO2 complex, where electron transfer to CO2 through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d9 intermediate binding CO2 can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d7-to-d9 configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO2 activation for complexes based on redox-innocent ligands.

Chemical accuracy with σ-functionals for the Kohn-Sham correlation energy optimized for different input orbitals and eigenvalues.

Recently, a new type of orbital-dependent functional for the Kohn-Sham (KS) correlation energy, σ-functionals, was introduced. Technically, σ-functionals are closely related to the well-known direct random phase approximation (dRPA). Within the dRPA, a function of the eigenvalues σ of the frequency-dependent KS response function is integrated over purely imaginary frequencies. In σ-functionals, this function is replaced by one that is optimized with respect to reference sets of atomization, reaction, transition state, and non-covalent interaction energies. The previously introduced σ-functional uses input orbitals and eigenvalues from KS calculations with the generalized gradient approximation (GGA) exchange-correlation functional of Perdew, Burke, and Ernzerhof (PBE). Here, σ-functionals using input orbitals and eigenvalues from the meta-GGA TPSS and the hybrid-functionals PBE0 and B3LYP are presented and tested. The number of reference sets taken into account in the optimization of the σ-functionals is larger than in the first PBE based σ-functional and includes sets with 3d-transition metal compounds. Therefore, also a reparameterized PBE based σ-functional is introduced. The σ-functionals based on PBE0 and B3LYP orbitals and eigenvalues reach chemical accuracy for main group chemistry. For the 10966 reactions from the highly accurate W4-11RE reference set, the B3LYP based σ-functional exhibits a mean average deviation of 1.03 kcal/molcompared to 1.08 kcal/molfor the coupled cluster singles doubles perturbative triples method if the same valence quadruple zeta basis set is used. For 3d-transition metal chemistry, accuracies of about 2 kcal/molare reached. The computational effort for the post-self-consistent evaluation of the σ-functional is lower than that of a preceding PBE0 or B3LYP calculation for typical systems.

Towards Kitaev spin liquid in 3d transition metal compounds

This paper reviews the current progress on searching the Kitaev spin liquid state in 3[Formula: see text] electron systems. Honeycomb cobaltates were recently proposed as promising candidates to realize the Kitaev spin liquid state, due to the more localized wavefunctions of [Formula: see text] ions compared with that of [Formula: see text] and [Formula: see text] ions, and also the easy tunability of the exchange Hamiltonian in favor of Kitaev interaction. Several key parameters that have large impacts on the exchange constants, such as the charge-transfer gap and the trigonal crystal field, are identified and discussed. Specifically, tuning crystal field effect by means of strain or pressure is emphasized as an efficient phase control method driving the magnetically ordered cobaltates into the spin liquid state. Experimental results suggesting the existence of strong Kitaev interactions in layered honeycomb cobaltates are discussed. Finally, the future research directions are briefly outlined.

Kitaev Spin Liquid in 3d Transition Metal Compounds.

We study the exchange interactions and resulting magnetic phases in the honeycomb cobaltates. For a broad range of trigonal crystal fields acting on Co^{2+} ions, the low-energy pseudospin-1/2 Hamiltonian is dominated by bond-dependent Ising couplings that constitute the Kitaev model. The non-Kitaev terms nearly vanish at small values of trigonal field Δ, resulting in spin liquid ground state. Considering Na_{3}Co_{2}SbO_{6} as an example, we find that this compound is proximate to a Kitaev spin liquid phase, and can be driven into it by slightly reducing Δ by ∼20 meV, e.g., via strain or pressure control. We argue that, due to the more localized nature of the magnetic electrons in 3d compounds, cobaltates offer the most promising search area for Kitaev model physics.

First-principles-based calculation of branching ratio for 5d, 4d, and 3dtransition metal systems

A new first-principles computation scheme to calculate ‘branching ratio’ has been applied to various 5d, 4d, and 3d transition metal elements and compounds. This recently suggested method is based on a theory which assumes the atomic core hole interacts barely with valence electrons. While it provides an efficient way to calculate the experimentally measurable quantity without generating spectrum itself, its reliability and applicability should be carefully examined especially for the light transition metal systems. Here we select 36 different materials and compare the calculation results with experimental data. It is found that our scheme well describes 5d and 4d transition metal systems whereas, for 3d materials, the difference between the calculation and experiment is quite significant. It is attributed to the neglect of core–valence interaction whose energy scale is comparable with the spin–orbit coupling of core p orbitals.

Chemical state effects on the Auger transitions in Cr, Fe, and Cu compounds

As a sophisticated surface elemental analysis tool, electron-excited Auger electron spectroscopy offers excellent spatial resolution (<10 nm), high surface sensitivity (∼2 nm in depth), and efficient detection (about 0.1 atm%) of all elements heavier than helium, which makes scanning Auger nanoprobe especially suitable for small feature analysis. However, elemental composition can generally only be done in a semi-quantitative way. In addition, the chemical-state identification based on peak position often fails due to the complex surface environment. In order to improve the accuracy of quantitative Auger analysis, we systematically investigated the influence of the oxidation states and primary beam on the sensitivity factors for Auger transitions in 3d transition metal Cr/Fe/Cu compounds. Relativistic Dirac-Fock-Slater calculations were performed to illustrate the electronic structure, electron-impacted ionization, and Auger transition dynamics at an atomistic and electronic level. It was found that a new set of chemical state-based sensitivity factors are required for highly accurate quantification, because the chemical states of an element directly affect all four major components in its sensitivity factor, i.e., ionization cross section, backscattering factor, Auger yield, and mean free path. In this study, the chemical state and environment can be affected by the oxidation state, the number and types of coordinating atoms, the spin state, and magnetic environment; the focus here will be on the oxidation state. The electron configurations of different oxidation states directly affect the Coulomb and Breit interactions differently in Auger transitions. The intensity ratios and the energy gaps between different Auger transitions were found to change with oxidation state in these 3d transition metal compounds. These findings may provide a new guide to improve the accuracy of Auger quantification and help the oxidation state identification in complex materials.

An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research.

X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e., synchrotron x-ray light sources. In this paper, we report the design and performance of an improved XAFS and XES spectrometer based on the general conceptual design of Seidler et al. [Rev. Sci. Instrum. 85, 113906 (2014)]. New developments include reduced mechanical degrees of freedom, much-increased flux, and a wider Bragg angle range to enable extended x-ray absorption fine structure (EXAFS) measurement and analysis for the first time with this type of modern laboratory XAFS configuration. This instrument enables a new class of routine applications that are incompatible with the mission and access model of the synchrotron light sources. To illustrate this, we provide numerous examples of x-ray absorption near edge structure (XANES), EXAFS, and XES results for a variety of problems and energy ranges. Highlights include XAFS and XES measurements of battery electrode materials, EXAFS of Ni with full modeling of results to validate monochromator performance, valence-to-core XES for 3d transition metal compounds, and uranium XANES and XES for different oxidation states. Taken en masse, these results further support the growing perspective that modern laboratory-based XAFS and XES have the potential to develop a new branch of analytical chemistry.

Predicting diamond-like Co-based chalcogenides as unconventional high temperature superconductors

We predict Co-based chalcogenides with a diamond-like structure can host unconventional high temperature superconductivity (high-Tc). The essential electronic physics in these materials stems from the Co layers with each layer being formed by vertex-shared CoA4 (A=S, Se, Te) tetrahedra complexes, a material genome proposed recently by us to host potential unconventional high-Tc close to a d7 filling configuration in 3d transition metal compounds. We calculate the magnetic ground states of different transition metal compounds with this structure. It is found that (Mn, Fe, Co)-based compounds all have a G-type antiferromagnetic (AFM) insulating ground state while Ni-based compounds are paramagnetic metal. The AFM interaction is the largest in the Co-based compounds as the three t2g orbitals all strongly participate in AFM superexchange interactions. The abrupt quenching of the magnetism from the Co to Ni-based compounds is very similar to those from Fe to Co-based pnictides in which a C-type AFM state appears in the Fe-based ones but vanishes in the Co-based ones. This behavior can be considered as an electronic signature of the high-Tc gene. Upon doping, as we predicted before, this family of Co-based compounds favor a strong d-wave pairing superconducting state.