Transition Metal Monoxides Research Articles

Sulfate salt assistant fabrication of Fe-doped Ni2P modified with SO42−/carbon as highly efficient oxygen evolution reaction electrocatalyst

The incorporation of oxyanion groups offers a greater potential for enhancing the activity of oxygen evolution reaction (OER) electrocatalysts compared to traditional metal cations doping, owing to their unique configurations and high electronegativity. However, the incorporation of oxyanion groups that differ from those derived from the oxidation of anions in transition metal monoxides poses significant challenges, thereby limiting further applications of oxyanion group modification approach. Herein, we present a novel sulfate salt assistant approach to fabricate Fe-doped Ni2P modified with SO42−/carbon (Fe-Ni2P-S/C) nanofibers as highly efficient OER electrocatalyst. The optimized Fe-Ni2P-S/C nanofibers display superb OER activity, requiring low overpotentials of 266, 323, and 357 mV at 100, 500, and 1000 mA cm−2, respectively. Theoretical calculations reveal that the co-adsorption of PO43− and SO42− on the surface of reconstructed electrocatalyst can reduce the energy barrier of rate-determining step, thereby resulting in enhanced OER activity. The present study emphasizes the crucial role played by anion groups in OER activity as well as proposes a novel approach for incorporating anion groups into electrocatalysts.

Optical spectroscopy of CrO and electronic states of the Cr group metal monoxides

All of the experimentally known electronic states of the Cr group metal monoxides (CrO, MoO, and WO) have been presented in the paper. The optical spectra of the CrO molecule have been investigated in the gas phase through a combination of the laser-induced fluorescence (LIF) excitation and single-vibronic-level (SVL) emission spectroscopy in the supersonic expansion. The rotational constants of the vibronic electronic states, including X5Π−1 (v″ = 0–3), B5Π−1 (v′ = 0–10), and B5Π1 (v′ = 1, 5), and the vibrational constants of the spin–orbit components X5Π−1,0,1 have been obtained. The molecular constants of the MoO and WO molecules have been summarized by reviewing the previous spectroscopic studies, and a comprehensive energy level diagram of the Cr group metal monoxides has been constructed. By comparing the electronic configurations, bond lengths, and vibrational frequencies of all the transition metal monoxides in the ground electronic state, the significance of the relativistic effect in the bonding of the 5d transition metal monoxides has been discussed. The related spectroscopic data of the CrO molecule are available at https://doi.org/10.57760/sciencedb.j00113.00085.

Vertex effects in describing the ionization energies of the first-row transition-metal monoxide molecules.

The GW approximation is considered to be the simplest approximation within Hedin's formulation of many-body perturbation theory. It is expected that some of the deficiencies of the GW approximation can be overcome by adding the so-called vertex corrections. In this work, the recently implemented G0W0Γ0 (1) scheme, which incorporates the vertex effects by adding the full second-order self-energy correction to the GW self-energy, is applied to a set of first-row transition-metal monoxide (TMO) anions. Benchmark calculations show that results obtained by G0W0Γ0 (1) on top of the B3LYP hybrid functional starting point (SP) are in good agreement with experiment data, giving a mean absolute error of 0.13eV for a testset comprising the ionization energies (IEs) of 27 outer valence molecular orbitals (MOs) from nine TMO anions. A systematic SP-dependence investigation by varying the ratio of the exact exchange (EXX) component in the PBE0-type SP reveals that, for G0W0Γ0 (1), the best accuracy is achieved with 20% EXX. Further error analysis in terms of the orbital symmetry characteristics (i.e., σ, π, or δ) in the testset indicates the best amount of EXX in the SP for G0W0Γ0 (1) calculations is independent of MO types, and this is in contrast with the situation in G0W0 calculations, where the best EXX ratio varies for different classes of MOs. Despite its success in describing the absolute IE values, we, however, found that G0W0Γ0 (1) faces difficulties in describing the energy separations between certain states of interest, worsening the already underestimated G0W0 predictions.

Correct and Accurate Polymorphic Energy Ordering of Transition-Metal Monoxides Obtained from Semilocal and Onsite-Hybrid Exchange-Correlation Approximations

The relative energetic stability of the structural phases of common antiferromagnetic transition-metal oxides (MnO, FeO, CoO, and NiO) within the semilocal and hybrid density functionals are fraught with difficulties. In particular, MnO is known to be the most difficult case for almost all common semilocal and hybrid density approximations. Here, we show that the meta-generalized gradient approximation (meta-GGA) constructed from the cuspless hydrogen model and Pauli kinetic energy density (MGGAC) can lead to the correct phase as the ground-state of MnO. The relative energy differences of zinc blende (zb) and rock salt (rs) structures as computed using MGGAC are found to be in nice agreement with those obtained from high-level correlation methods like the random phase approximation or quantum Monte Carlo techniques. Besides, we have also applied the onsite hybrid functionals (closely related to DFT+U) based on GGA and meta-GGA functionals, and it is shown that a relatively high amount of Hartree–Fock exchange is necessary to obtain rs as the ground-state phase. Our present investigation suggests the semilocal MGGAC and onsite hybrids, both being computationally cheap, as methods of choice for the calculation of the relative stability of antiferromagnetic transition-metal oxides having potential applications in solid-state physics and structural chemistry.

Magnetic Bands Producing a Monoclinic Magnetic Structure in NiO, FeO, MnO, and a Tetragonal One in CoO

In a foregoing paper, the author reported evidence that the multi-spin-axis magnetic structure proposed in 1964 by van Laar is realized in antiferromagnetic CoO. Within the nonadiabatic Heisenberg model, this tetragonal body-centered structure is generated by atomic-like electrons in a special magnetic band of CoO, a mechanism that may emerge only because the nonadiabatic Heisenberg model goes beyond the adiabatic approximation. The present paper compares the band structures of the transition-metal monoxides NiO, CoO, FeO, and MnO, and shows that only CoO possesses a magnetic band which may produce the tetragonal magnetic structure proposed by van Laar. The magnetic bands of the other monoxides, NiO, FeO, and MnO, are clearly related to the monoclinic base-centered magnetic structure experimentally observed in these materials.

DFT+U within the framework of linear combination of numerical atomic orbitals.

We present a formulation and implementation of the density functional theory (DFT)+U method within the framework of linear combination of numerical atomic orbitals (NAO). Our implementation not only enables single-point total energy and electronic-structure calculations but also provides access to atomic forces and cell stresses, hence allowing for full structure relaxations of periodic systems. Furthermore, our implementation allows one to deal with non-collinear spin texture, with the spin-orbit coupling (SOC) effect treated self-consistently. The key aspect behind our implementation is a suitable definition of the correlated subspace when multiple atomic orbitals with the same angular momentum are used, and this is addressed via the "Mulliken charge projector" constructed in terms of the first (most localized) atomic orbital within the d/f angular momentum channel. The important Hubbard U and Hund J parameters can be estimated from a screened Coulomb potential of the Yukawa type, with the screening parameter either chosen semi-empirically or determined from the Thomas-Fermi screening model. Benchmark calculations are performed for four late transition metal monoxide bulk systems, i.e., MnO, FeO, CoO, and NiO, and for the 5d-electron compounds IrO2. For the former type of systems, we check the performance of our DFT+U implementation for calculating bandgaps, magnetic moments, electronic band structures, as well as forces and stresses; for the latter, the efficacy of our DFT+U+SOC implementation is assessed. Systematic comparisons with available experimental results, especially with the results from other implementation schemes, are carried out, which demonstrate the validity of our NAO-based DFT+U formalism and implementation.

Microscopic Magnetic Origin of Rhombohedral Distortion in NiO

Numerous investigations have been conducted to explore the structural phase transition in antiferromagnetic 3d transition metal monoxides accompanied by appearance of magnetic phase transition. However, how the spins induce distortion in the high symmetric structure has not yet been fully understood. In this study, the monoxide NiO is used as an example to investigate what lowers the structural symmetry. By comparing two different magnetic structures, our results reveal that the spin–lattice coupling is responsible for such a structural distortion. Then, a spin–lattice model, including the strain component, is constructed to simulate the transition procedure. Moreover, the results from the first-principles calculations are used to compare with our model results. Both first-principles calculations and model simulations clarify the structural phase transition caused by a unique magnetic arrangement.

Accurate energies of transition metal atoms, ions, and monoxides using selected configuration interaction and density-based basis-set corrections.

The semistochastic heat-bath configuration interaction method is a selected configuration interaction plus perturbation theory method that has provided near-full configuration interaction (FCI) levels of accuracy for many systems with both single- and multi-reference character. However, obtaining accurate energies in the complete basis-set limit is hindered by the slow convergence of the FCI energy with respect to basis size. Here, we show that the recently developed basis-set correction method based on range-separated density functional theory can be used to significantly speed up basis-set convergence in SHCI calculations. In particular, we study two such schemes that differ in the functional used and apply them to transition metal atoms and monoxides to obtain total, ionization, and dissociation energies well converged to the complete-basis-set limit within chemical accuracy.

A full configuration interaction quantum Monte Carlo study of ScO, TiO, and VO molecules.

Accurate ab initio calculations of 3d transition metal monoxide molecules have attracted extensive attention because of their relevance in physical and chemical science as well as theoretical challenges in treating strong electron correlation. Meanwhile, recent years have witnessed the rapid development of the full configuration interaction quantum Monte Carlo (FCIQMC) method to tackle electron correlation. In this study, we carry out FCIQMC simulations to ScO, TiO, and VO molecules and obtain accurate descriptions of 13 low-lying electronic states (ScO 2Σ+, 2Δ, 2Π; TiO 3Δ, 1Δ, 1Σ+, 3Π, 3Φ; VO 4Σ-, 4Φ, 4Π, 2Γ, 2Δ), including states that have significant multi-configurational character. The FCIQMC results are used to assess the performance of several other wave function theory and density functional theory methods. Our study highlights the challenging nature of the electronic structure of transition metal oxides and demonstrates FCIQMC as a promising technique going forward to treat more complex transition metal oxide molecules and materials.

Pressure-induced yttrium oxides with unconventional stoichiometries and novel properties

The oxygenic motifs (e.g., ${\mathrm{O}}^{2--}$, ${{\mathrm{O}}_{2}}^{2\text{--}}$ ${{\mathrm{O}}_{2}}^{2\text{--}}$, and ${{\mathrm{O}}_{2}}^{\text{--}}$) that are present in compounds have a substantial effect on their electronic structure and behavior. Herein, first-principles swarm-intelligence crystal structural searches reveal that the reaction between Y and O under high pressure leads to the formation of novel compounds with unique properties. Several O-rich Y-O compounds (e.g., ${\mathrm{YO}}_{2}$, ${\mathrm{Y}}_{2}{\mathrm{O}}_{5}$, and ${\mathrm{YO}}_{3}$) emerge as being stable. It is shown that the oxygenic motifs found within the stable species depend upon the oxygen content and pressure (e.g., ${\mathrm{O}}^{2\text{--}}$ in YO and ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, the coexistence of ${\mathrm{O}}^{2\text{--}}$ and ${{\mathrm{O}}_{2}}^{2\text{--}}$ in ${\mathrm{YO}}_{2}$ and ${\mathrm{Y}}_{2}{\mathrm{O}}_{5}$, ${{\mathrm{O}}_{2}}^{2\text{--}}$ in $Pm\text{\ensuremath{-}}3$ ${\mathrm{YO}}_{3}$, and ${\mathrm{O}}^{2\text{--}}$ in Cmcm ${\mathrm{YO}}_{3}$), and are accompanied by a transition in the electronic structure from superconducting to metallic to semiconducting. Notably, the Cmcm symmetry ${\mathrm{YO}}_{3}$ phase, consisting of a 13-fold coordinated face-sharing polyhedron with 15 faces, can be classified as a transition metal (TM) superoxide. The long sought-after bulk yttrium monoxide (YO) is shown to become stable at high pressure. NaCl-type YO is superconducting with a critical temperature $({T}_{c})$ of 13.0 K at 25 GPa, becoming the TM monoxide with the highest known ${T}_{c}$. Our work will inspire future studies exploring the chemistry and properties of O-rich TM oxides at high pressure.

Single-crystalline epitaxial TiO film: A metal and superconductor, similar to Ti metal.

Titanium monoxide (TiO), an important member of the rock salt 3d transition-metal monoxides, has not been studied in the stoichiometric single-crystal form. It has been challenging to prepare stoichiometric TiO due to the highly reactive Ti2+ We adapt a closely lattice-matched MgO(001) substrate and report the successful growth of single-crystalline TiO(001) film using molecular beam epitaxy. This enables a first-time study of stoichiometric TiO thin films, showing that TiO is metal but in proximity to Mott insulating state. We observe a transition to the superconducting phase below 0.5 K close to that of Ti metal. Density functional theory (DFT) and a DFT-based tight-binding model demonstrate the extreme importance of direct Ti-Ti bonding in TiO, suggesting that similar superconductivity exists in TiO and Ti metal. Our work introduces the new concept that TiO behaves more similar to its metal counterpart, distinguishing it from other 3d transition-metal monoxides.

Emergence of quantum critical charge and spin-state fluctuations near the pressure-induced Mott transition in MnO, FeO, CoO, and NiO

We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent $\mathrm{DFT}+\mathrm{dynamical}$ mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of relative weights of the different valence states and spin-state configurations. Employing the concept of fluctuating valence in a correlated solid, we demonstrate that in MnO, FeO, and CoO a Mott insulator-metal transition and collapse of the local moments is accompanied by a sharp crossover of the spin-state and valence configurations. Our microscopic explanation of the magnetic collapse differs from the accepted picture and points out a remarkable dynamical coexistence (frustration) of the high-, intermediate-, and low-spin states. In particular, in MnO, the magnetic collapse is found to be driven by the appearance of the intermediate-spin state (IS), competing with the low-spin (LS) state; in FeO, we observe a conventional high-spin to low-spin (HS-LS) crossover. Most interestingly, in CoO, we obtain a remarkable (dynamical) coexistence of the HS and LS states, i.e., a HS-LS frustration, up to high pressure. Our results demonstrate the importance of quantum fluctuations of the valence and spin states for the understanding of quantum criticality of the Mott transitions.

Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials

We present in full detail a newly developed formalism enabling density functional perturbation theory (DFPT) calculations from a $\mathrm{DFT}+U$ ground state. The implementation includes ultrasoft pseudopotentials and is valid for both insulating and metallic systems. It aims at fully exploiting the versatility of DFPT combined with the low-cost $\mathrm{DFT}+U$ functional. This allows us to avoid computationally intensive frozen-phonon calculations when $\mathrm{DFT}+U$ is used to eliminate the residual electronic self-interaction from approximate functionals and to capture the localization of valence electrons, e.g., on $d$ or $f$ states. In this way, the effects of electronic localization (possibly due to correlations) are consistently taken into account in the calculation of specific phonon modes, Born effective charges, dielectric tensors, and in quantities requiring well converged sums over many phonon frequencies, as phonon density of states and free energies. The new computational tool is applied to two representative systems, namely CoO, a prototypical transition metal monoxide and ${\mathrm{LiCoO}}_{2}$, a material employed for the cathode of Li-ion batteries. The results show the effectiveness of our formalism to capture in a quantitatively reliable way the vibrational properties of systems with localized valence electrons.

Discovery of space aromaticity in transition–metal monoxide crystal Nb3O3 enabled by octahedral Nb6 structural units

An octahedral Nb6 structural unit with space aromaticity is identified for the first time in a transition–metal monoxide crystal Nb3O3 by ab initio calculations. The strong Nb–Nb metallic bonding facilitates the formation of stable octahedral Nb6 structural units and the release of delocalization energy. Moreover, the Nb atoms in continuously connected Nb6 structural units share their electrons with each other in a continuous space of framework, so that the electrons are uniformly distributed. The newly discovered aromaticity in the octahedral Nb6 structural units extends the range of aromatic compounds and broadens our vision in structural chemistry.

Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO

Recently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by Chen et al (2018 Phys. Rev. Mater. 2 073803) and Cui et al (2018 J. Phys. Chem. Lett. 9 2338), respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the GW method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps.

Practical GW scheme for electronic structure of 3d-transition-metal monoxide anions: ScO-, TiO-, CuO-, and ZnO.

The GW approximation to many-body perturbation theory is a reliable tool for describing charged electronic excitations, and it has been successfully applied to a wide range of extended systems for several decades using a plane-wave basis. However, the GW approximation has been used to test limited spectral properties of a limited set of finite systems (e.g., frontier orbital energies of closed-shell sp molecules) only for about a decade using a local-orbital basis. Here, we calculate the quasiparticle spectra of closed- and open-shell molecular anions with partially and completely filled 3d shells (shallow and deep 3d states, respectively), ScO-, TiO-, CuO-, and ZnO-, using various levels of GW theory, and compare them to experiments to evaluate the performance of the GW approximation on the electronic structure of small molecules containing 3d transition metals. We find that the G-only eigenvalue self-consistent GW scheme with W fixed to the PBE level (GnW0@PBE), which gives the best compromise between accuracy and efficiency for solids, also gives good results for both localized (d) and delocalized (sp) states of 3d-transition-metal oxide molecules. The success of GnW0@PBE in predicting electronic excitations in these systems reasonably well is likely due to the fortuitous cancellation effect between the overscreening of the Coulomb interaction by PBE and the underscreening by the neglect of vertex corrections. Together with the absence of the self-consistent field convergence error (e.g., spin contamination in open-shell systems) and the GW multisolution issue, the GnW0@PBE scheme gives the possibility to predict the electronic structure of complex real systems (e.g., molecule-solid and sp-d hybrid systems) accurately and efficiently.

Predication of screened hybrid functional on transition metal monoxides: From Mott insulator to charge transfer insulator

A systematic study on structural, electronic and magnetic properties of transition metal monoxides (TMOs with TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) is performed using the screened hybrid functional, also so-called Heyd-Scuseria-Ernzerhof hybrid functional (HSE). Our results demonstrate that the screened hybrid HSE functional could give reasonable predictions for structural, magnetic and electronic properties of transition metal monoxides when compared with available experimental data. ScO and TiO are predicted to be metals, VO, CrO, MnO, and FeO to be Mott insulators, CoO, NiO and CuO to be charge transfer insulators and ZnO for band insulator. The magnetic moment of the first-row transition metals presents a volcano shape, the moment of Sc and Zn is zero, while MnO locates at the summit. The interactions of d electrons in ScO and TiO are not as strong as in later monoxides. For later transition metal monoxides (MnO, FeO, CoO and NiO), they are typically strongly-correlated compounds, and HSE functional presents a much better performance in predictions of electronic properties. This work not only provides a good reference for exchange-correlation functional development, but the systematic study on magnetic and electronic properties paves the way for designing functional electromagnetic and photocatalysis materials of transition metal oxides.

Magnetic force theory combined with quasi-particle self-consistent GW method

We report a successful combination of magnetic force linear response theory with quasiparticle self-consistent GW method. The self-consistently determined wavefunctions and eigenvalues can just be used for the conventional magnetic force calculations. While its formulation is straightforward, this combination provides a way to investigate the effect of GW self-energy on the magnetic interactions which can hardly be quantified due to the limitation of current GW methodology in calculating the total energy difference in between different magnetic phases. In ferromagnetic 3d elements, GW self-energy slightly reduces the d bandwidth and enhances the interactions while the same long-range feature is maintained. In antiferromagnetic transition-metal monoxides, QSGW significantly reduces the interaction strengths by enlarging the gap. Orbital-dependent magnetic force calculations show that the coupling between eg and the nominally-empty 4s orbital is noticeably large in MnO which is reminiscent of the discussion for cuprates regarding the role of Cu-4s state. This combination of magnetic force theory with quasiparticle self-consistent GW can be a useful tool to study various magnetic materials.

DFT+DMFT calculations of the complex band and tunneling behavior for the transition metal monoxides MnO, FeO, CoO, and NiO

We report complex band structure (CBS) calculations for the four late transition metal monoxides, MnO, FeO, CoO and NiO, in their paramagnetic phase. The CBS is obtained from density functional theory plus dynamical mean field theory (DMFT) calculations to take into account correlation effects. The so-called $\beta$ parameters, governing the exponential decay of the transmission probability in the non-resonant tunneling regime of these oxides, are extracted from the CBS. Different model constructions are examined in the DMFT part of the calculation. The calculated $\beta$ parameters provide theoretical estimation for the decay length in the evanescent channel, which would be useful for tunnel junction applications of these materials.

Ab-initio investigation of electronic structures of α-BiFeO3 with different exchange-correlation functionals

The electronic structures of α-BiFeO3 are calculated by using a full-potential linearized-augmented-plane-wave method. We employed the local-density approximation (LDA) with the modified Becke-Johnson (mBJ) exchange potential and the LDA + U method. The indirect (direct) bandgap of 2.24 (2.44) eV obtained by LDA + U method is in good agreement with an experiment, while the mBJ potential produces the indirect bandgap of 2.55 eV, and the direct bandgap is slightly larger than the indirect one. The discrepancy between the experimental x-ray spectra and the calculated Fe-3d and O-2p density of states were revealed to be due to the effects of the core hole. The core-hole effects are also responsible for the smaller bandgap in x-ray spectroscopy than the optical spectroscopy. The calculated valence-charge density and the bonding character obtained by LDA + U method also provides the stronger ionic character of the compound than the mBJ potential. Although the mBJ method is very efficient one, it is still very time consuming compared to the LDA + U method. The most suitable exchange-correlation potential for α-BiFeO3 is the LDA + U. Therefore, it is better to use the LDA + U method for the electronic-structure calculations of BiFeO3 compound not only for reducing the calculational time but also for better description of bandgaps and some physical properties. From the similar calculations carried out for transition-metal monoxide system it was found that the inadequacy of using the mBJ potential for the description of the localized 3d-states is rather universal.