Corrected atomic limit in the local-density approximation and the electronic structure of d impurities in Rb

Room-temperature ferromagnetism, half-metallicity and spin transport in monolayer CrSc2Te4-based magnetic tunnel junction devices

The discovery of novel two-dimensional (2D) half-metallic materials with a robust ferromagnetic (FM) order and a high Curie temperature (Tc) is attractive for the advancement of next-generation spintronic devices. Here, we propose a monolayer with stable 2D intrinsic FM half-metallicity, i.e., the CrSc2Te4 monolayer, which was constructed by intercalating a monolayer of 1T-CrTe2-type sandwiched between two layers of 2H-ScTe2 monolayers. Our calculations reveal that it exhibits exceptional dynamical, thermal, and mechanical stabilities accompanied by a robust half-metallicity characterized by a wide bandgap of 1.02 eV and FM ordering with a high Tc of 326 K. Notably, these properties remain intact in almost the entire range of the biaxial strain from -5% to 5%. Furthermore, our investigations demonstrate excellent spin transport capabilities, including an outstanding spin-filtering effect, and a remarkably high tunneling magnetoresistance ratio peaking at 6087.07%. The remarkable magnetic features of the 2D CrSc2Te4 monolayer with room temperature FM, intrinsic half-metallicity, and 100% spin-polarization make it a promising candidate for the next-generation high-performance spintronic nanodevices as well as high-density magnetic recording and sensors.

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Enhanced Cathode Performance in Pr0.5Sr0.5FeO3-δ of Perovskite Catalytic Materials via Doping with VB Subgroup Elements (V, Nb, and Ta)

Perovskite-style materials are cathode systems known for their stability in solid oxide fuel cells (SOFCs). Pr0.5Sr0.5FeO3-δ (PSF) exhibits excellent electrode performance in perovskite cathode systems at high temperatures. Via VB subgroup metals (V, Nb, and Ta) modifying the B-site, the oxidation and spin states of iron elements can be adjusted, thereby ultimately adjusting the cathode's physicochemical properties. Theoretical predictions indicate that PSF has poor stability, but the relative arrangement of the three elements on the B-site can significantly improve this material's properties. The modification of Nb has a large effect on the stability of PSF cathode materials, reaching a level of -2.746 eV. The surface structure of PSF becomes slightly more stable with an increase in the percentage of oxygen vacancy structures, but the structural instability persists. Furthermore, the differential charge density distribution and adsorption state density of the three modified cathode materials validate our adsorption energy prediction results. The initial and final states of the VB subgroup metal-doped PSF indicate that PSFN is more likely to complete the cathode surface adsorption reaction. Interestingly, XRD and EDX characterization are performed on the synthesized pure and Nb-doped PSF material, which show the orthorhombic crystal system of the composite theoretical model structure and subsequent experimental components. Although PSF exhibits strong catalytic activity, it is highly prone to decomposition and instability at high temperatures. Furthermore, PSFN, with the introduction of Nb, shows greater stability and can maintain its activity for the ORR. EIS testing clearly indicates that Nb most significantly improves the cathode. The consistency between the theoretical predictions and experimental validations indicates that Nb-doped PSF is a stable and highly active cathode electrode material with excellent catalytic activity.

Linear response theories for interatomic exchange interactions

The linear response is a perturbation theory establishing the relationship between given physical variable and the external field inducing this variable. A well-known example of the linear response theory in magnetism is the susceptibility relating the magnetization with the magnetic field. In 1987, Liechtensteinet alcame up with the idea to formulate the problem of interatomic exchange interactions, which would describe the energy change caused by the infinitesimal rotations of spins, in terms of this susceptibility. The formulation appears to be very generic and, for isotropic systems, expresses the energy change in the form of the Heisenberg model, irrespectively on which microscopic mechanism stands behind the interaction parameters. Moreover, this approach establishes the relationship between the exchange interactions and the electronic structure obtained, for instance, in the first-principles calculations based on the density functional theory. The purpose of this review is to elaborate basic ideas of the linear response theories for the exchange interactions as well as more recent developments. The special attention is paid to the approximations underlying the original method of Liechtensteinet alin comparison with its more recent and more rigorous extensions, the roles of the on-site Coulomb interactions and the ligand states, and calculations of antisymmetric Dzyaloshinskii-Moriya interactions, which can be performed alongside with the isotropic exchange, within one computational scheme. The abilities of the linear response theories as well as many theoretical nuances, which may arise in the analysis of interatomic exchange interactions, are illustrated on magnetic van der Walls materials CrX3(X=Cl, I), half-metallic ferromagnet CrO2, ferromagnetic Weyl semimetal Co3Sn2S2, and orthorhombic manganitesAMnO3(A=La, Ho), known for the peculiar interplay of the lattice distortion, spin, and orbital ordering.

Issues on DFT+U calculations of organic diradicals

The diradical state is an important electronic state for understanding molecular functions and should be elucidated for the in silico design of functional molecules and their application to molecular devices. The density functional theory calculation with plane-wave basis and correction of the on-site Coulomb parameter U (DFT+U/plane-wave calculation) is a good candidate of high-throughput calculations of diradical-band interactions. However, it has not been investigated in detail to what extent the DFT+U/plane-wave calculation can be used to calculate organic diradicals with a high degree of accuracy. In the present study, using typical organic diradical molecules (bisphenalenyl molecules) as model systems, the discrepancy in the optimum U values between the two electronic states (open-shell singlet and triplet) that compose the diradical state is detected. The calculated results show that the reason for this U value discrepancy is the difference in electronic delocalisation due to π-conjugation between the open-shell singlet and triplet states, and that the effect of U discrepancy becomes large as diradical character decreases. This indicates that it is necessary to investigate the U value discrepancy with reference to the calculated results by more accurate methods or to experimental values when calculating organic diradicals with low diradical character. For this investigation, the local magnetic moments, unpaired beta electron numbers, and effective magnetic exchange integral values can be used as reference values. For the effective magnetic exchange integral values, the effects of U discrepancy are partially cancelled out. However, because the effects may not be completely offset, care should be taken when using the effective magnetic exchange integral value as a reference. Furthermore, a comparison of DFT+U and hybrid-DFT calculations shows that the DFT+U underestimates the HOMO-LUMO gap of bisphenalenyls, although a qualitative discussion of the gap is possible.

Perfect cubic metallo-borospherenes TM8B6 (TM = Ni, Pd, Pt) as superatoms following the 18-electron rule

Transition-metal (TM)-doped metallo-borospherenes exhibit unique structures and bonding in chemistry which have received considerable attention in recent years. Based on extensive global minimum searches and first-principles theory calculations, we predict herein the first and smallest perfect cubic metallo-borospherenes Oh TM8B6 (TM = Ni (1), Pd (2), Pt (3)) and Oh Ni8B6- (1-) which contain eight equivalent TM atoms at the vertexes of a cube and six quasi-planar tetra-coordinate face-capping boron atoms on the surface. Detailed canonical molecular orbital and adaptive natural density partitioning bonding analyses indicate that Oh TM8B6 (1/2/3) as superatoms possess nine completely delocalized 14c-2e bonds following the 18-electron principle (1S21P61D10), rendering spherical aromaticity and extra stability to the complex systems. Furthermore, Ni8B6 (1) can be used as building blocks to form the three-dimensional metallic binary crystal NiB (4) (Pm3̄m) in a bottom-up approach which possesses a typical CsCl-type structure with an octa-coordinate B atom located exactly at the center of the cubic unit cell. The IR, Raman, UV-vis and photoelectron spectra of the concerned clusters are computationally simulated to facilitate their experimental characterization.

First-principles investigation of the electronics, optical, mechanical, thermodynamics and thermoelectric properties of Na based Quaternary Heusler alloys (QHAs) NaHfXGe (X = Co, Rh, Ir)

In this study, we explored the electronic and thermoelectric (TE) properties of the Na-based Quaternary Heusler Alloys (QHAs) NaHfXGe (X = Co, Rh, Ir) using density functional theory (DFT). We performed the spin-polarized DFT calculations at the general gradient approximation (GGA) level and confirmed the ground state non-magnetic configuration of NaHfXGe. The mechanical and thermodynamical stabilities are analyzed and discussed to validate the stability by calculating the elastic constant and phonon dispersion curve. A thorough investigation on the electronic properties are carried out by performing the GGA, GGA+U, and GGA+SOC formalism where we report the semi-conducting characteristic of NaHfCoGe and NaHfRhGe QHAs. However, NaHfIrGe is predicted to be a non-magnetic metal. From the calculated optical properties we found that the most active optical absorption occurs within the vis-UV region withα>105 cm-1, therefore the studied QHAs are proposed to be a promising optoelectronic materials. The results of the thermodynamic properties have shown that NaHfXGe follows Debye's low-temperature specific heat law and the classical thermodynamics of the Dulong-Petit law at high temperatures. The calculated TE efficiency using GGA+SOC formalism atT= 1200 K are ZT∼1.22 and 0.57 for NaHfCoGe and NaHfRhGe, suggesting that these materials are potential TE materials to operate at high temperature.

Giant unilateral electric-field control of magnetic anisotropy in MgO/Rh2CoSb heterojunctions

A large voltage-controlled magnetic anisotropy (VCMA) effect is highly desirable for applications of voltage-torque magnetic random access memory. In this work, the dependence of magnetic anisotropy (MA) on the electric field in a MgO-based heterojunction consisting of a new Heusler alloy, Rh2CoSb, is studied using first-principles calculations. We find that the Rh-terminated MgO/Rh2CoSb heterojunction has a perpendicular MA and a giant VCMA coefficient of 7024 fJ V-1 m-1. Furthermore, the VCMA coefficient shows a characteristic of dependence on the electric-field direction. The origins of these behaviors are elucidated by orbital-resolved MA and second-order perturbation theoretical analysis. As the spin-down states of the in-plane orbital, dxy, are close to the Fermi level, the shift of these states induced by the electric field gives rise to significant changes of magnetic anisotropy energy, which is mainly responsible for the giant VCMA effect.

Two-dimensional Janus pentagonal MSeTe (M = Ni, Pd, Pt): promising water-splitting photocatalysts and optoelectronic materials

Inspired by the interesting and novel properties exhibited by Janus transition metal dichalcogenides (TMDs) and two-dimensional pentagonal structures, we here investigated the structural stability, mechanical, electronic, photocatalytic, and optical properties for a class of two-dimensional (2D) pentagonal Janus TMDs, namely penta-MSeTe (M = Ni, Pd, Pt) monolayers, by using density functional theory (DFT) combined with Hubbard's correction (U). Our results showed that these monolayers exhibit good structural stability, appropriate band structures for photocatalysts, high visible light absorption, and good photocatalytic applicability. The calculated electronic properties reveal that the penta-MSeTe are semiconductors with a bandgap range of 2.06-2.39 eV, and their band edge positions meet the requirements for water-splitting photocatalysts in various environments (pH = 0-13). We used stress engineering to seek higher solar-to-hydrogen (STH) efficiency in acidic (pH = 0), neutral (pH = 7) and alkaline (pH = 13) environments for penta-MSeTe from 0% to +8% biaxial and uniaxial strains. Our results showed that penta-PdSeTe stretched 8% along the y direction and demonstrates an STH efficiency of up to 29.71% when pH = 0, which breaks the theoretical limit of the conventional photocatalytic model. We also calculated the optical properties and found that they exhibit high absorption (13.11%) in the visible light range and possess a diverse range of hyperbolic regions. Hence, it is anticipated that penta-MSeTe materials hold great promise for applications in photocatalytic water splitting and optoelectronic devices.

Single-Atom Catalysts and Dual-Atom Catalysts for CO2 Electroreduction: Competition or Cooperation?

Developing highly active and selective electrocatalysts for electrochemical reduction of CO2 can reduce environmental pollution and mitigation of greenhouse gas emission. Owing to maximal atomic utilization, the atomically dispersed catalysts are broadly adopted in CO2 reduction reaction (CO2 RR). Dual-atom catalysts (DACs), with more flexible active sites, distinct electronic structures, and synergetic interatomic interactions compared to single-atom catalysts (SACs), may have great potential to enhance catalytic performance. Nevertheless, most of the existing electrocatalysts have low activity and selectivity due to their high energy barrier. Herein, 15 electrocatalysts are explored with noble metallic (Cu, Ag, and Au) active sites embedded in metal-organic hybrids (MOHs) for high-performance CO2 RR and studied the relationship between SACs and DACs by first-principles calculation. The results indicated that the DACs have excellent electrocatalytic performance, and the moderate interaction between the single- and dual-atomic center can improve catalytic activity in CO2 RR. Four among the 15 catalysts, including (CuAu), (CuCu), Cu(CuCu), and Cu(CuAu) MOHs inherited a capability of suppressing the competitive hydrogen evolution reaction with favorable CO overpotential. This work not only reveals outstanding candidates for MOHs-based dual-atom CO2 RR electrocatalysts but also provides new theoretical insights into rationally designing 2D metallic electrocatalysts.

Active Learning the High-Dimensional Transferable Hubbard U and V Parameters in the DFT + U + V Scheme

Density functional theory (DFT) is a powerful quantum mechanical computational tool to perform electronic structure calculations for materials. Few DFT methods can ensure accuracy and efficiency simultaneously. DFT + U + V is an alternative effective approach to overcome this drawback. However, the accuracy sensitively depends on the self-consistent estimation of the high-dimensional onsite and intersite Hubbard interaction U and V terms. We propose Bayesian optimization using a dropout (BOD) algorithm, one type of active learning method, to optimize U and V terms. The DFT + U + V with U/V obtained by BOD can produce improved electronic properties for diverse bulk materials of comparable quality to the hybrid functionals with lower computational cost compared to the linear response approach. Note that the band gaps calculated by BOD are somewhat different from that of hybrid functionals by simply applying the same U/V parameters as in the case of surface slabs and interfaces, which suggests that the transferability of U/V from the bulk models to slabs and interfaces is not as well as expected. BOD is extended to calculate the U/V parameters for slabs and interfaces and reach similar results as bulk solids. Moreover, we find that the U/V are reasonably transferable between surface slabs and interfaces with different thicknesses under various effects of quantum confinement, which contributes to fast access to the electronic properties of large-scale systems with higher accuracy.

Enhanced anodic catalytic performance in PrFeO3-δ of perovskite materials via Co-doping with Sr and VB subgroup metals (V, Nb, Ta)

The anodic catalytic capability of PrFeO3-δ is restricted by the Fe-site element type in the perovskite material structure due to its low electrical conductivity of electrons. Here, we present a strategy for tuning the Fe-site element type via Sr and VB subgroup metals (V, Nb, Ta) co-doping to enhance the anodic catalytic performance of PrFeO3-δ anode materials. Our calculations show that Sr and Nb co-doping has suitable hydrogen adsorption energy for PrFeO3-δ anode materials, and its adsorption energy is adjusted to -0.717 eV, which is more suitable to absorb the hydrogen molecule than other high-profile perovskite anode materials. Meanwhile, after the doped surface is adsorbed by hydrogen molecules, the bond length lengthens until it breaks, and one of the broken hydrogen atoms moves directly above the surface oxygen atom, which is beneficial for accelerating the anodic catalytic reaction. Thus, the Pr0.5Sr0.5Fe0.875Nb0.125O3-δ material is a promising perovskite anode catalyst. Interestingly, the stability of PrFeO3-δ is significantly affected by the oxygen vacancy content; the structural stability of the undoped system can be maintained via Sr and Nb co-doping to avoid decomposition, which provides new thinking to maintain the high stability of perovskite ferrite materials. Furthermore, we find that relative to the PrFeO3-δ, the Pr0.5Sr0.5Fe0.875Nb0.125O3-δ surface of hydrogen adsorption has obvious charge transfer and upward shift of the d-band center. Our anodic catalytic theoretical work shows that Sr and Nb co-doping can effectively enhance the catalytic performance of the PrFeO3-δ ferrite materials.

Precise electronic structure modulation on MXene-based single atom catalysts for high-performance electrocatalytic CO2 reduction reaction: A first-principle study

Electrocatalytic CO₂ reduction reaction (CO₂RR) to CO is a logical approach to achieve a carbon-neutral cycle. In this work, a series of Ti₂CO₂ and O vacancy containing Ti₂CO₂ MXene-based transition metal (TM) single atom catalysts (SACs), including TM-Ti₂CO₂ and TM-O_v-Ti₂CO₂, are explored for high-performance CO₂RR. Sc/Ti/V/Cr-Ti₂CO₂ and Ni-O_v-Ti₂CO₂ are screened out with limiting potential (U_L) more positive than -0.50 V. Ni-O_v-Ti₂CO₂ is a candidate catalyst for CO₂RR to CO, considering its activity with U_L of -0.27 eV, and the selectivity relevant to hydrogen evolution reaction and HCOOH production. Meanwhile, a novel activity descriptor of TM-Ti-O group valence state is proposed according to that TMs work in synergy with coordinated Ti and O atoms and a level of around 0.64 e^- benefits to CO₂RR. This work highlights oxygen vacancy containing Ti₂CO₂-based Ni SAC as a promising catalyst for CO₂RR, and provides a feasible electronic structure design principle for guiding the design of MXene-based SACs for CO₂RR.

Computational Design and Theoretical Properties of WC3N6, an H-Free Melaminate and Potential Multifunctional Material

By means of first-principles theory, existence, synthetic conditions, and structural as well as physicochemical properties have been predicted for the first hydrogen-free melaminate salt of the composition WC₃N₆. We find at least two energetically favorable polymorphs adopting space groups P1 and P3, both of which are layer-like porous materials. In addition to sizable Madelung fields stabilizing saltlike WC₃N₆, the complex C₃N₆^6- anions are connected via perfectly optimized W-N bonds, forming WN₅ in the P1 and WN₆ coordination polyhedra in the P3 polymorphs. The band gaps of the P1 and P3 phases are HSE-predicted as 2.25 and 1.21 eV, respectively, significantly smaller than those of g-C₃N₄ and WO₃. Moreover, both phases have suitable band-edge potentials that may provide sufficient driving force for photocatalytic water splitting; at least for the P1 phase, there is also a reasonable chance for reduced electron-hole recombination. In addition, the polymorphs's large optical absorption coefficients should greatly enhance the photocatalytic performance. WC₃N₆ defines a new class of compounds and has unique structural characteristics, mirrored from its electrical and optical properties, and it should provide another chemical path for preparing efficient photocatalysts and optoelectronic devices.

Heptacoordinate transition-metal-decorated metallo-borospherenes and multiple-helix metallo-boronanotubes

The recent discovery of lanthanide-metal-decorated metallo-borospherenes LM₃B₁₈^- (LM = La, Tb) marks the onset of a new class of boron-metal binary nanomaterials. Using the experimentally observed or theoretically predicted borospherenes as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel chiral metallo-borospherenes C₂ Ni₆ ∈ B₃₉^- (1), C₁ Ni₆ ∈ B₄₁⁺ (3), C₂ Ni₆ ∈ B₄₂²⁺ (4), C₂ Ni₆ ∈ B₄₂ (5), and C₂ Ni₈ ∈ B₅₆ (6) as the global minima of the systems decorated with quasi-planar heptacoordinate Ni (phNi) centers in η⁷-B₇ heptagons on the cage surfaces, which are found to be obviously better favoured in coordination energies than hexacoordinate Ni centers in previously reported D_2d Ni₆ ∈ B₄₀ (2). Detailed bonding analyses indicate that these phNi-decorated metallo-borospherenes follow the σ + π double delocalization bonding pattern, with two effective (d-p)σ coordination bonds formed between each phNi and its η⁷-B₇ ligand, rendering spherical aromaticity and extra stability to the systems. The structural motif in elongated axially chiral Ni₆ ∈ B₄₂²⁺ (4), Ni₆ ∈ B₄₂ (5), and Ni₈ ∈ B₅₆ (6) can be extended to form the metallic phNi-decorated boron double chain (BDC) double-helix Ni₄ ∈ B₂₈ (2, 0) (P4̄m2) (8), triple-helix Ni₆ ∈ B₄₂ (3, 0) (P3̄m1) (9), and quadruple-helix Ni₈ ∈ B₅₆ (4, 0) (P4mm) (10) metallo-boronanotubes, which can be viewed as quasi-multiple-helix DNAs composed of interconnected BDCs decorated with phNi centers in η⁷-B₇ heptagons on the tube surfaces in the atomic ratio of Ni : B = 1 : 7.

Understanding the different effects of 4d-transition metals on the performance of Li-rich cathode Li2MnO3 by first-principles

The poor cycling performance of Li-rich cathode Li₂MnO₃, a promising cathode for next-generation Li-ion batteries, limits its commercial applications. Transition metal (TM) doping is widely applied to optimize the electrochemical performance of Li₂MnO₃, where the d valence electrons of the TM play a crucial role. Nevertheless, the rule of the doping effect of TM with various numbers of d electrons has not been well summarized. In this work, 4d-TMs (Zr, Nb, Mo, Ru and Rh) are selected as dilute doping elements for Li₂MnO₃ to evaluate their effect on the performance of Li₂MnO₃ through first-principles calculations. The calculations indicate that as the number of 4d electrons increases, the doped TM transforms from an electrochemically inert state (Zr and Nb) to an electrochemically active state (Mo, Ru and Rh) in Li₂MnO₃. Meanwhile, the orbital hybridization between the 4d electrons of the TM and the 2p electrons of O becomes stronger from Zr to Rh, which promotes the co-oxidation of the TM and O for charge compensation and alleviates the excessive oxidation of O, thus enhancing the stability of O. Moreover, the oxidation of the doped TM and lattice Mn during charging can trigger a decrease in the initial average delithiation potential. Although the 4d-TMs exhibit slight promoting or inhibiting effects on Li diffusion, no obvious rule related to the number of d electrons has been found. Our work highlights the rule of the doping effect of TMs with different 4d electrons on the electrochemical performance of Li₂MnO₃ and would facilitate a better design of Li₂MnO₃ cathode materials.

Quantum Hall phase in graphene engineered by interfacial charge coupling

The quantum Hall effect can be substantially affected by interfacial coupling between the host two-dimensional electron gases and the substrate, and has been predicted to give rise to exotic topological states. Yet the understanding of the underlying physics and the controllable engineering of this interaction remains challenging. Here we demonstrate the observation of an unusual quantum Hall effect, which differs markedly from that of the known picture, in graphene samples in contact with an antiferromagnetic insulator CrOCl equipped with dual gates. Two distinct quantum Hall phases are developed, with the Landau levels in monolayer graphene remaining intact at the conventional phase, but largely distorted for the interfacial-coupling phase. The latter quantum Hall phase is even present close to the absence of a magnetic field, with the consequential Landau quantization following a parabolic relation between the displacement field and the magnetic field. This characteristic prevails up to 100 K in a wide effective doping range from 0 to 10¹³ cm^-2.

Computational prediction of new magnetic materials

The discovery of new magnetic materials is a big challenge in the field of modern materials science. We report the development of a new extension of the evolutionary algorithm USPEX, enabling the search for half-metals (materials that are metallic only in one spin channel) and hard magnetic materials. First, we enabled the simultaneous optimization of stoichiometries, crystal structures, and magnetic structures of stable phases. Second, we developed a new fitness function for half-metallic materials that can be used for predicting half-metals through an evolutionary algorithm. We used this extended technique to predict new, potentially hard magnets and rediscover known half-metals. In total, we report five promising hard magnets with high energy product (|BH|_MAX), anisotropy field (H_a), and magnetic hardness (κ) and a few half-metal phases in the Cr-O system. A comparison of our predictions with experimental results, including the synthesis of a newly predicted antiferromagnetic material (WMnB₂), shows the robustness of our technique.

Highly efficient overall urea electrolysis via single-atomically active centers on layered double hydroxide

Anodic urea oxidation reaction (UOR) is an intriguing half reaction that can replace oxygen evolution reaction (OER) and work together with hydrogen evolution reaction (HER) toward simultaneous hydrogen fuel generation and urea-rich wastewater purification; however, it remains a challenge to achieve overall urea electrolysis with high efficiency. Herein, we report a multifunctional electrocatalyst termed as Rh/NiV-LDH, through integration of nickel-vanadium layered double hydroxide (LDH) with rhodium single-atom catalyst (SAC), to achieve this goal. The electrocatalyst delivers high HER mass activity of 0.262 A mg^-1 and exceptionally high turnover frequency (TOF) of 2.125 s^-1 at an overpotential of 100 mV. Moreover, exceptional activity toward urea oxidation is addressed, which requires a potential of 1.33 V to yield 10 mA cm^-2, endorsing the potential to surmount the sluggish OER. The splendid catalytic activity is enabled by the synergy of the NiV-LDH support and the atomically dispersed Rh sites (located on the Ni-V hollow sites) as evidenced both experimentally and theoretically. The self-supported Rh/NiV-LDH catalyst serving as the anode and cathode for overall urea electrolysis (1 mol L^-1 KOH with 0.33 mol L^-1 urea as electrolyte) only requires a small voltage of 1.47 V to deliver 100 mA cm^-2 with excellent stability. This work provides important insights into multifunctional SAC design from the perspective of support sites toward overall electrolysis applications.

Influence of Excess Charge on Water Adsorption on the BiVO4(010) Surface

We present a combined computational and experimental study of the adsorption of water on the Mo-doped BiVO₄(010) surface, revealing how excess electrons influence the dissociation of water and lead to hydroxyl-induced alterations of the surface electronic structure. By comparing ambient pressure resonant photoemission spectroscopy (AP-ResPES) measurements with the results of first-principles calculations, we show that the dissociation of water on the stoichiometric Mo-doped BiVO₄(010) surface stabilizes the formation of a small electron polaron on the VO₄ tetrahedral site and leads to an enhanced concentration of localized electronic charge at the surface. Our calculations demonstrate that the dissociated water accounts for the enhanced V⁴⁺ signal observed in ambient pressure X-ray photoelectron spectroscopy and the enhanced signal of a small electron polaron inter-band state observed in AP-ResPES measurements. For ternary oxide surfaces, which may contain oxygen vacancies in addition to other electron-donating dopants, our study reveals the importance of defects in altering the surface reactivity toward water and the concomitant water-induced modifications to the electronic structure.

Large Spin Coherence Length and High Photovoltaic Efficiency of the Room Temperature Ferrimagnet Ca2 FeOsO6 by Strain Engineering

The influence of epitaxial strain on the electronic, magnetic, and optical properties of the distorted double perovskite Ca₂ FeOsO₆ is studied. These calculations show that the compound realizes a monoclinic structure with P2₁ /n space group from -6% to +6% strain. While it retains ferrimagnetic ordering with a net magnetic moment of 2 μ_B per formula unit at low strain, it undergoes transitions into E-antiferromagnetic and C-antiferromagnetic phases at -5% and +5% strain, respectively. It is shown that spin frustration reduces the critical temperature of the ferrimagnetic ordering from the mean field value of 600-350 K, in excellent agreement with the experimental value of 320 K. It is also shown that the critical temperature can be tuned efficiently through strain and that the spin coherence length surpasses that of Sr₂ FeMoO₆ under tensile strain. An indirect-to-direct bandgap transition is observed at +5% strain. Localization of the valence and conduction states on different transition metal sublattices enables efficient electron-hole separation upon photoexcitation. The calculated spectroscopic limited maximum efficiency of up to 33% points to excellent potential of Ca₂ FeOsO₆ in solar cell applications.

3D Printing of Multiscale Ti64-Based Lattice Electrocatalysts for Robust Oxygen Evolution Reaction

Electrically assisted water splitting is an endurable strategy for hydrogen production, but the sluggish kinetics of oxygen evolution reaction (OER) extremely restrict the large-scale production of hydrogen. Developing highly efficient and non-precious catalytic materials is essential to accelerate the sluggish kinetics of OER. However, currently used catalyst supports, such as copper foam, suffer from inferior corrosion resistance and structural stability, resulting in the disabled functionality of 3D conductive networks. To this end, a novel 3D freestanding electrode with corrosion-resistant and robust Ti-6Al-4V titanium alloy lattice as the catalyst support is designed via a 3D printing technology of selective laser melting. After the coating of core-shell Cu(OH)2@CoNi carbonate hydroxides (CoNiCH) on the designed lattice, a unique micro/nano-sized hierarchical porous structure is formed, which endows the electrocatalyst with a promising electrocatalytic activity (a low overpotential of 355 mV at 30 mA cm^-2 and Tafel slope of 125.3 mV dec^-1 ). Computational results indicate that the CoNiCH exhibits optimized electron transfer and the catalytic activity of the Ni site is higher than that of the Co site in the CoNiCH. Therefore, the integration of robust catalyst supports and highly active materials opens up an avenue for reliable and high-performance OER electrocatalysts.

Regulating the spin state of single-atom doped covalent triazine frameworks for efficient nitrogen fixation

Covalent triazine frameworks (CTFs), served as a versatile platform, can form expedient metal-N single-atom coordination sites as promising catalytic centers. To seek out excellent candidate catalysts of M/CTFs (M = Transition metal) for nitrogen reduction reaction (NRR), a "five-step" strategy involving spin states has been established for hierarchical high-throughput screening and reveals strong coordination ability of the CTFs, outstanding conductivity of the M/CTFs, effective adsorption and activation of N₂* attributed to the electron transfer and orbital hybridization between the M/CTFs and N₂*. Among the potential candidates, the Cr/CTF is screened out to be an excellent one for nitrogen fixation, which can not only inhibit hydrogen evolution reaction (HER) greatly but also has good thermodynamic stability (E_b =  -4.40 eV), narrow band gap (E_g = 0.03 eV), moderate adsorption energy (E_a =  -0.84 eV), large activation energy (ΔG_N2* = -0.71 eV) and a theoretical Faradaic efficiency of 100%. The spin state has been confirmed to be an important descriptor of catalytic activity and the two-state reactivity (TSR) is validated to exist in the NRR. Reaction mechanism with different spin states of Cr/CTF has been demonstrated to give a great impact on the nitrogen fixation, providing solid theoretical support for the design of more efficient NRR catalysts.

Alloying two-dimensional NbSi2N4: a new strategy to realize half-metallic antiferromagnets

Finding two-dimensional (2D) materials with both 100% spin polarization and zero net magnetic moment is essential for next-generation spintronics. Half-metallic antiferromagnets (HMAFs) are ideal materials to satisfy these exigent needs, but such a system has never been found among 2D inorganic materials. In this paper, we theoretically demonstrate that intrinsic 2D HMAFs can be realized by alloying Nb with Mn in 2D septuple-atomic-layer NbSi₂N₄. By continuously incorporating Mn, the stronger Mn-N hybridization relative to Nb-N induces a metal to half-metal to semiconductor transition. The competitive coupling between the Nb-d itinerant electron spin and the Nb-Mn d-d direct interaction drives the ferromagnetic to antiferromagnetic phase transition. For the first time in 2D inorganic materials, the exact cancellation of local magnetic moments and band gap opening in one spin channel is obtained simultaneously at a Nb/Mn ratio of 3 : 1, as demonstrated by our first-principles calculations. The present results would not only inspire materials design of more 2D HMAFs in the future but also impel the advancement of next-generation antiferromagnetic spintronic devices.

First-Principles Study on Possible Half-Metallic Ferrimagnetism in Double Perovskites Pb2XX'O6 (X = Ti, Zr, Hf, V, Nb and Ta, X' = Tc, Ru, Os and Rh)

Pb-based double perovskite compounds with chemical formula Phey have abundant physical properties in the spintronic field. Among all the features, the spin interaction of half-metallic (HM) is regarded as an important performance measure because of its high potential in spintronic devices. In this research study, we calculate density of state (DOS) to investigate possible half-metal candidates by executing structural optimization based on the method of generalized gradient approximation (GGA) and strong correlation effect (GGA + U). Furthermore, following the earlier methods by calculating and comparing energy difference of various compounds with the four initial magnetic states: ferromagnetic, ferrimagnetic, antiferromagnetic and nonmagnetic, we can determine which magnetic state is more stable. Results indicate that there are 13 possible ferrimagnetic HM candidates in these combinations, including Pb₂NbTcO₆, Pb₂TaTcO₆, Pb₂TiRuO₆, Pb₂ZrRuO₆, Pb₂HfRuO₆, Pb₂VRuO₆, Pb₂NbRuO₆, Pb₂TadRuO₆, Pb₂ZrOsO₆, Pb₂HfOsO₆, Pb₂VOsO₆, Pb₂ZrRhO₆ and Pb₂HfRhO₆ under GGA and GGA + U schemes. The stability of analysis by analyzing the energy gap illustrates that all 13 possible candidates are half metals and ferrimagnetic states, so our studies could provide guidelines for scientists to fabricate new double perovskites in future.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi₂ monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi₂ monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T _C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi₂ monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi₂ monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

La@[La5&B30]0/−/2−: endohedral trihedral metallo-borospherenes with spherical aromaticity

Extensive first-principles theory calculations predict the perfect endohedral metallo-borospherene D3h La@[La5&B30] (1) and its monoanion Cs La@[La5&B30]− (2) and dianion D3h La@[La5&B30]2− (3) which appear to be spherically aromatic in nature.

Enhanced C–H bond activation by tuning the local environment of surface lattice oxygen of MoO3

The lattice oxygen on transition metal oxides serves as a critical active site in the dehydrogenation of alkanes, whose activity is determined by electronic properties and environmental structures. Hydrogen affinity has been used as a universal descriptor to predict C–H bond activation, while the understanding of the environmental structure is ambiguous due to its complexity. This paper describes a combined theoretical and experimental study to reveal the activity of lattice oxygen species with different local structures, taking Mo-based oxides and C–H bond activation of low-carbon alkanes as model catalytic systems. Our theoretical work suggests that oxygen species with convex curvature are more active than those with concave curvature. Theoretically, we propose an interpretative descriptor, the activation deformation energy, to quantify the surface reconstruction induced by adsorbates with various environmental structures. Experimentally, a Mo-based polyoxometalate with the convex curvature structure shows nearly five times the initial activity than single-crystal molybdenum oxide with the concave one. This work provides theoretical guidance for designing metal oxide catalysts with high activity.

Tuning of the environmental structure near the lattice oxygen of molybdenum oxides can form a favorable spatial structure to enhance the intrinsic activity for C–H bond activation.

Understanding complex multiple sublattice magnetism in double double perovskites

Understanding magnetism in multiple magnetic sublattice system, driven by the interplay of varied nature of magnetic exchanges, is on one hand challenging and on other hand intriguing. Motivated by the recent synthesis of AA[Formula: see text]BB[Formula: see text]O[Formula: see text] double double perovskites with multiple magnetic ions both at A- and B-sites, we investigate the mechanism of magnetic behavior in these interesting class of compounds. We find that the magnetism in such multiple sublattice compounds is governed by the interplay and delicate balance between two distinct mechanisms, (a) kinetic energy-driven multiple sublattice double exchange mechanism and (b) the conventional super-exchange mechanism. The derived spin Hamiltonian based on first-principles calculations is solved by classical Monte Carlo technique which reproduces the observed magnetic properties. Finally, the influence of off-stoichiometry, as in experimental samples, is discussed. Some of these double double perovskite compounds are found to possess large total magnetic moment and also are found to be half-metallic with reasonably high transition temperature, which raises the hope of future applications of these large magnetic moment half-metallic oxides in spintronics and memory devices.

A computational investigation of the adsorption of small copper clusters on the CeO2(110) surface

We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cu_n (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO₂(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cu_n cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuO_x/CeO₂-nanorod catalysts in many reactions. We demonstrate that CeO₂(110) is highly reducible upon Cu_n adsorption, with electron transfer from Cu_n clusters, and that a Cu_n cluster grows along the long bridge sites until Cu₃, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO₂(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu₃ to Cu₄ transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu₄ adsorption reveal a Cu bilayer rich in Cu⁺ species at the Cu-O interface.

A local-orbital density functional formalism for a many-body atomic Hamiltonian: Hubbard-Hund's coupling and DFT + U functional

In the conventional DFT + U approach, the mean field solution of the Hubbard Hamiltonian associated with thedorf(iσ) electrons of a transition metal atom is used to define the DFT + U potential acting on theiσ-electrons. In this work, we go beyond that mean field solution by analyzing the correlation energy and potential for a multi-level atom described by a Kanamori Hamiltonian connected to different channels representing the environment. As a first step, we analyze the many-body solution of our model, using a local-orbital density functional formalism that takes as independent variables the orbital occupancies,n_iσ, of the atomic orbitals; accordingly, we present the corresponding density functional solution describing the correlation energy and potential as a function ofn_iσ. Then, we use this analysis to introduce a DFT + U potential extending previous proposals to materials with arbitrarily high correlation. In particular, we find that this potential mainly screens the conventional mean field potential contribution, and also yields new terms associated with the number of atomic electrons. Our results show that the atomic correlation effects enhance the role played by the intra-atomic exchange interaction and favor the formation of magnetic solutions.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Theoretical insights into the mechanism of oxygen evolution reaction (OER) on pristine BiVO4(001) and BiVO4(110) surfaces in acidic medium both in the gas and solution (water) phases

Monoclinic scheelite bismuth vanadate is an efficient photocatalyst for water splitting. In this paper, we perform DFT + Ucalculations to investigate the structural, electronic, and optical properties, water adsorption and the oxygen evolution reaction processes on BiVO₄(001) and BiVO₄(110) surfaces in acidic medium both in the gas and solution (water) phases. The structural, electronic, optical, and water adsorption properties reveal that BiVO₄(001) surface is energetically more stable than BiVO₄(110) surface in vacuum. On other hand, the water oxidation mechanisms reveal that BiVO₄(110) surface in water and in strained form in vacuum is energetically more stable than BiVO₄(001) surface in water and in strained form in vacuum bothU = 0 and 2.1 V. The free energy of adsorption for all systems atU = 2.1 V reduce about 2 times than that atU = 0 V. Such analyzes provide important insights into the role of different facets on BiVO₄surface for photocatalytic reactions.

Lessons learned from first-principles calculations of transition metal oxides

Transition metal oxides (TMOs) are an important class of materials with diverse applications, ranging from memristors to photoelectrochemical cells. First-principles calculations are critical for understanding these complex materials at an atomic level and establishing relationships between atomic and electronic structures, particularly for probing quantities difficult or inaccessible to experiment. Here, we discuss computational strategies used to understand TMOs by focusing on two examples, a photoanode material, BiVO₄, and an oxide for low-power electronics, La_1-xSr_xCoO₃. We highlight key aspects required for the modeling of TMOs, namely, the descriptions of how oxygen vacancies, extrinsic doping, the magnetic state, and polaron formation impact their electronic and atomic structures and, consequently, many of the observed properties.

Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis

Structure-activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO₃ epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO₂ surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.

Evaluation of redox potentials of cathode materials of alkali-ion batteries using extended DFT+U+U↑↓ method: The role of interactions between the electrons with opposite spins

Limitations of the DFT+U approach (e.g., in Dudarev's formulation) applied for accurate evaluation of redox potentials of cathode materials of alkali-ion batteries with U parameters calculated via the linear response (LR) method are discussed. In contrast to our previous studies, where redox potentials of several cathode materials have been calculated in a good agreement with experiment (e.g., NaMnO₂, LiFePO₄, and LiTiS₂), herein, we analyze other cathode materials, such as LiNiO₂ and Ni- and V-containing phosphates for which this method provides much underestimated redox voltages. We ascribe this limited predictive power of the DFT+U method, parameterized via LR, to the absence of corrections of Coulomb interactions between the electrons with opposite spins. Using the recently proposed extended DFT+U+U_↑↓ functional, which includes the aforementioned corrections, we show how redox potentials of Ni- and V-based compounds could be calculated in a much better agreement with experiment, also proposing a procedure of parameterization of such calculations. Thus, our extended method allows us to calculate redox potentials of several important materials more accurately while retaining good agreement with experiment for structures where the standard DFT+U method also accurately predicts electrochemical properties.

Strong valley splitting ind0two-dimensional SnO induced by magnetic proximity effect

Strong magnetic interfacial coupling in van der Waals heterostructures is important for designing novel electronic devices. Besides the most studied transition metal dichalcogenides (TMDCs) materials, we demonstrate that the valley splitting can be activated in two-dimensional tetragonald⁰metal oxide, SnO, via the magnetic proximity effect by EuBrO. In SnO/EuBrO, the valley splitting of SnO can reach ∼46 meV, which is comparable to many TMDCs and equivalent to an external magnetic field of 800 T. In addition, the valley splitting can be further enhanced by adjusting interlayer distance and applying uniaxial strains. A design principle of new spintronic device based on this unique electronic structure of SnO/EuBrO has been proposed. Our findings indicate that SnO is a promising material for future valleytronics applications.

DFT studies of selective oxidation of propene on the MoO3(010) surface

Selective oxidation of propene to acrolein and acrylic acid has been applied in industry for many years. In this work, the density functional theory plus U (DFT+U) method was performed to study the hydrogen abstraction of propene on the MoO3(010) surface. From the most stable chemisorbed propene (di-σ propene), the allyl intermediate is difficult to produce on a perfect MoO3(010) surface because of the high barrier. In general, the barriers of the second hydrogen abstraction are much lower than those of the first one. The conclusion from our slab model calculations is consistent with the experimental results. It is found that the (3 + 2) mechanism exhibits lower barriers than the (5 + 2) mechanism. Oxygen defects facilitate the first dehydrogenation significantly, and π-allyl converts to σ-allyl favorably on defects, in agreement with a previous experimental study. The present study indicates that increasing the surface oxygen defects might be an effective way to promote the activity of MoO3 to propene oxidation.

Modulation of the electronic properties and photocatalytic performance of black phase monolayer GeSe by noble metal doping

Possible doping positions of noble metal atoms on the surface of monolayer GeSe.

Improvement of d-d interactions in density functional tight binding for transition metal ions with a ligand field model: assessment of a DFTB3+U model on nickel coordination compounds

To improve the description of interactions among the localized d, f electrons in transition metals, we have introduced a ligand-field motivated contribution into the Density Functional Tight Binding (DFTB) model. Referred to as DFTB3+U, the approach treats the d, f electron repulsions with rotationally invariant orbital-orbital interactions and a Hartree-Fock model; this represents a major conceptual improvement over the original DFTB3 approach, which treats the d, f-shell interactions in a highly averaged fashion without orbital level of description. The DFTB3+U approach is tested using a series of nickel compounds that feature Ni(ii) and Ni(iii) oxidation states. By using parameters developed with the original DFTB3 Hamiltonian and empirical +U parameters (F0/2/4 Slater integrals), we observe that the DFTB3+U model indeed provides substantial improvements over the original DFTB3 model for a number of properties of the nickel compounds, including the population and spin polarization of the d-shell, nature of the frontier orbitals, ligand field splitting and the energy different between low and high spin states at OPBE optimized structures. This proof-of-concept study suggests that with self-consistent parameterization of the electronic and +U parameters, the DFTB3+U model can develop into a promising model that can be used to efficiently study reactive events involving transition metals ion condensed phase systems. The methodology can be integrated with other approximate QM methods as well, such as the extended tight binding (xTB) approach.