Hubbard-U Corrections Research Articles

Theoretical Study of the Magnetic Properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H Compounds

We present theoretical investigations examining the electronic and magnetic properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H compounds, including magneto-crystalline anisotropy, magnetic moments, exchange-coupling parameters, and Curie temperatures. The spin-polarized fully relativistic Korringa–Kohn–Rostoker (SPR-KKR) band structure method has been employed, using the coherent potential approximation (CPA) to deal with substitutional disorder. Hubbard-U correction was applied to the local spin density approximation (LSDA+U) in order to account for the significant correlation effects arising from the 4f electronic states of Sm. According to our calculations, the total magnetic moments increases with H addition, in agreement with experimental data. Adding one H atom in the near-neighbor environment of the Fe 8j site reduces the magnetic moments of Fe 8j and enhances the magnetic moment of Fe 8f. For every investigated alloy, the site-resolved spin magnetic moments of Fe on the 8i, 8j, and 8f sites exhibit the same magnitude sequence, with msFe (8i) > msFe (8j) > msFe (8f). While the addition of H has a positive impact on magneto-crystalline anisotropy energy (MAE), the increase in Mo concentration is detrimental to MAE. The computed exchange-coupling parameters reveal the highest values between the closest Fe 8i spins, followed by Fe 8i and Fe 8j spins, for all investigated alloys. The Curie temperature of the alloys under investigation is increased by decreasing the Mo concentration or by H addition, which is qualitatively consistent with experimental findings.

The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: A theoretical investigation

The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0–2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (µ0Ms = 1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (µ0Ha = 12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders its use in permanent magnet applications. The effect of V-doping on magnetization, magnetocrystalline anisotropy energy, and Curie temperature is investigated by electronic band structure calculations. The spin-polarized fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method, which employs the coherent potential approximation (CPA) to deal with substitutional disorder, has been used. The Hubbard-U correction to local spin density approximation (LSDA + U) was used to account for the large correlation effects due to the 4f electronic states of Sm. The computed magnetic moments and magnetocrystalline anisotropy energies were compared with existing experimental data to validate the theoretical approach’s reliability. The exchange-coupling parameters from the Heisenberg model were used for obtaining the mean-field estimated Curie temperature. The magnetic anisotropy energy was separated into contributions from transition metals and Sm, and its relationships with the local environment, interatomic distances, and valence electron delocalization were analyzed. The suitability of the hypothetical SmFe10CoV alloy for permanent magnet manufacture was assessed using the calculated anisotropy field, magnetic hardness, and intrinsic magnetic properties.

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction.

Calculation of transition temperatures T1/2 for thermally driven spin-crossover in condensed phases is challenging, even with sophisticated state-of-the-art density functional approximations. The first issue is the accuracy of the adiabatic crossover energy difference ΔEHL between the low- and high-spin states of the bistable metal-organic complexes. The other is the proper inclusion of entropic contributions to the Gibbs free energy from the electronic and vibrational degrees of freedom. We discuss the effects of treatments of both contributions upon the calculation of thermochemical properties for a set of 20 spin-crossover materials using a Hubbard-U correction obtained from a reference ensemble spin-state. The U values obtained from a simplest bimolecular representation may overcorrect, somewhat, the ΔEHL values, hence giving somewhat excessive reduction of the T1/2 results with respect to their U = 0 values in the crystalline phase. We discuss the origins of the discrepancies by analyzing different sources of uncertainties. By use of a first-coordination-sphere approximation and the assumption that vibrational contributions from the outermost atoms in a metal-organic complex are similar in both low- and high-spin states, we achieve T1/2 results with the low-cost, widely used PBE generalized gradient density functional approximation comparable to those from the more costly, more sophisticated r2SCAN meta-generalized gradient approximation. The procedure is promising for use in high-throughput materials screening, because it combines rather low computational effort requirements with freedom from user manipulation of parameters.

Electronic behavior of Ag-doped YBa2Cu3O7-δ using Hubbard-U correction method

The ability to accurately describe the electronic properties in the highly correlated materials YBa2-xAgxCu3O7-δ was queried. Previous calculations revealed that the electronic bandgap was estimated much lower than the established experimental value when standard density functional theory, DFT was used in the computation of pure YBa2Cu3O7-δ. The electronic properties of Ag-doped at Ba-site samples such as the bandgap, total and partial density of states, electron density, and Mulliken population, were computationally calculated using DFT with the Hubbard-U correction method, DFT+U and compared to the pure samples. The calculation was conducted using CASTEP code in Material Studio software applying both LDA and range of GGA exchange-correlation functionals. 2 × 2 × 1 supercell symmetry was applied to enlarge the single cell crystal structure to able the dopant concentration, x = 0.250 at the Ba-site. The calculated bandgap of pure YBa2Cu3O7-δ (Eg = 3.67 eV) shows 8.25% difference to the referred theoretical value using the GGA-PBE+U functional. Upon Ag-dopant, the bandgap was calculated to be Eg = 3.85 eV. The energy of the Ba 6p orbital on the valence band has shifted from − 10.49 eV to − 10.67 eV, away from the Fermi energy level, and the density of state has increased significantly by + 57.52%. The Ag dopant enhanced the electron density of Cu and O atoms along the CuO chain and CuO2 plane. The study enables the prediction of impurity dopant concentration prior to experimental work, hence promoting green research.

Oxygen Vacancies in Zirconia and Their Migration: The Role of Hubbard-U Parameters in Density Functional Theory

Cubic zirconia (c-ZrO2) is studied using Density Functional Theory with Hubbard-U corrections (DFT+U). It is shown that the determination of the U-parameters from first principles leads to values for U(Zr-4d) and U(O-2p) which are very different from standard choices. The calculated band gap with these values for U closely matches the experimental gap. Oxygen vacancies are studied using this approach, and it is found that it is possible to closely reproduce the vacancy migration energies calculated with a hybrid functional. The oxygen vacancy is associated with two excess electrons which localize in the vacancy’s cavity. In the presence of these excess electrons, the barrier for vacancy migration is very high. If instead, a charged vacancy VO2+ is considered, its mobility increases considerably—an effect that is attributed to the absence of space charges localized in the cavity.

Metal-to-Insulating Transition in the Perovskite System YSr2Cu2FeO8-δ (0 < δ < 1) Modeled by DFT Methods.

Progress in the design of functional perovskite oxides relies on advances in density functional theory (DFT) methods to efficiently and effectively model complex systems composed of several transition-metal ions. This work reports the application of DFT methods to investigate the electronic structure of the YSr2Cu2FeO8-δ (0 < δ < 1) family in which the insulating, metal, or superconducting behaviors and even anion conductivity can be tuned by modifying the oxygen content. In particular, we assess the performance of the generalized gradient approximation (GGA), its Hubbard-U correction (GGA + U), and the strongly constrained and appropriately normed (SCAN) to model the metallic (idealized YSr2Cu2FeO8) and insulating (idealized YSr2Cu2FeO7) phases of the system. The analysis of the DFT results is supported by DC resistivity measurements that denote the metal character of the synthesized YSr2Cu2FeO7.86 and the semiconducting character of YSr2Cu2FeO7.08 prepared under reducing conditions. In addition, the band gap of YSr2Cu2FeO7.08, in the range of 0.73-1.2 eV, has been extracted from diffuse reflectance spectroscopy (DRS). While the three methodologies (GGA, GGA + U, SCAN) permit the reproduction of the crystal structures of the synthetized oxides (determined here in the case of YSr2Cu2FeO7.08 by neutron powder diffraction (NPD)), the SCAN emerges as the only one capable to predict the basic electronic and magnetic properties across the YSr2Cu2FeO8-δ (0 < δ < 1) series. The picture that emerges for the metal (δ = 0) to insulating (δ = 1) transition is the one in which oxygen vacancies contribute electrons to the filling of the Cu/Fe-3dx2-y2 states of the conduction band. These results validate the SCAN functional for future DFT investigations of complex functional oxides that combine several transition metals.

Determining surface-specific Hubbard-U corrections and identifying key adsorbates on nickel and cobalt oxide catalyst surfaces.

NiO is a popular transition metal oxide (TMO) with high thermal and chemical stability and Co3O4 is a relatively more reducible TMO due to weaker metal-oxygen bonds. Both are often used as catalysts in a variety of chemical transformations. Density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) are used to investigate catalysis on TMO surfaces, yet both techniques have their own limitations. The accuracy of DFT highly depends on the choice of Hubbard U correction. The bulk-property optimized U value of 5.3 eV for NiO and different U values for Co3O4, without any consensus, are often used in the literature to simulate surface catalysis. However, U values optimized using bulk properties often fail to reproduce surface-adsorbate interactions on TMOs. Similarly, there exists arbitrariness in assigning observed XPS shifts to different surface species on these metal oxides. Hence, a synergistic application of XPS and DFT+U is implemented to determine the surface specific U values for NiO and Co3O4, and to identify adsorbed surface moieties corresponding to experimentally observed XPS shifts. For the NiO (100) surface, the U value of ∼2 eV is able to reproduce the experimentally observed XPS O1s core level binding energy shifts correctly, instead of the bulk property optimized and commonly used U value of 5.3 eV. Using this surface specific U value of 2 eV, the experimentally observed XPS shifts are assigned. Similarly, for Co3O4 (100) surface, ∼3 eV of U value could successfully predict the experimentally observed XPS shifts and corresponding adsorbates. The surface adsorbates and configurations suggested in this work will help analyze experimental XPS data and the surface specific U values will ensure accurate predictions of adsorption and reaction energetics on these catalysts.

Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs

The ion insertion redox chemistry of manganese dioxide has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and nonprotonic cations in interstitial sites. Despite many reports on individual ion-polymorph couples, much less is known about the selectivity of electrochemical ion insertion in MnO2. In this work, we use density functional theory to holistically compare the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+) in aqueous and nonaqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that for nonprotonic cations, ion selectivity depends on the oxygen coordination environments inside a polymorph. When H+ is present, however, the driving force to form hydroxyl bonds is usually stronger. In aqueous electrolytes, only three ion-polymorph pairs are thermodynamically stable within water's voltage stability window (Na+ and K+ in α-MnO2, and Li+ in λ-MnO2), with all other ion insertion being metastable. We find Al3+ may insert into the δ, R, and λ polymorphs across the full 2-electron redox of MnO2 at high voltage; however, electrolytes for multivalent ions must be designed to impede the formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions coinsert with water in α-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in δ-MnO2. Taken together, these findings explain reports of mixed ion insertion mechanisms in aqueous electrolytes and highlight promising design strategies for safe, high energy density electrochemical energy storage, desalination batteries, and electrocatalysts.

Mean Value Ensemble Hubbard-U Correction for Spin-Crossover Molecules.

High-throughput searches for spin-crossover molecules require Hubbard-U corrections to common density functional exchange-correlation (XC) approximations. However, the Ueff values obtained from linear response or based on previous studies overcorrect the spin-crossover energies. We demonstrate that employing a linearly mixed ensemble average spin state as the reference configuration for the linear response calculation of Ueff resolves this issue. Validation on a commonly used set of spin-crossover complexes shows that these ensemble Ueff values consistently are smaller than those calculated directly on a pure spin state, irrespective of whether that be low- or high-spin. Adiabatic crossover energies using this methodology for a generalized gradient approximation XC functional are closer to the expected target energy range than with conventional Ueff values. Based on the observation that the Ueff correction is similar for different complexes that share transition metals with the same oxidation state, we devise a set of recommended averaged Ueff values for high-throughput calculations.

Density Functional Studies on the Atomistic Structure and Properties of Iron Oxides: A Parametric Study.

With the aim to find the best simulation routine to accurately predict the ground-state structures and properties of iron oxides (hematite, magnetite, and wustite) using density functional theory (DFT) with Hubbard-U correction, a significant amount of DFT calculations were conducted to investigate the influence of various simulation parameters (energy cutoff, K-point, U value, magnetization setting, smearing value, etc.) and pseudopotentials on the structures and properties of iron oxides. With optimized simulation parameters, the obtained equation of state, lattice constant, bulk moduli, and band gap is much closer to the experimental values compared with previous studies. Due to the strong coupling between the 2p orbital of O and the 3d orbital of Fe, it was found that Hubbard-U correction obviously improved the results for all three kinds of iron oxides including magnetite which has not yet been tested with U correction before, but the U value should be different for different oxides (3 ev, 4 ev, 4 ev for hematite, magnetite, and wustite, respectively). Two kinds of spin magnetism settings for FeO are considered, which should be chosen according to different calculation purposes. The detailed relationship between the parameter settings and the atomic structures and properties were analyzed, and the general principles for future DFT calculation of iron oxides were provided.

Exploring the impact of the nitrogen layer on a Cu(001) substrate on the spin crossover properties of [Fe(SalEen-I)2]Br: A DFT study

This work investigates the impact of the Cu:N ratio in the nitrogen layer of a Cu(001) substrate coated with [Fe(SalEen-I)2]Br, an Fe(III) spin crossover (SCO) molecule. Specifically, we probe Cu/Cu(001), Cu3N/Cu(001), Cu2N/Cu(001), and CuN/Cu(001) substrates. To explore the molecule-substrate interaction, density functional theory calculations with Hubbard-U correction (DFT + U) and van der Waals interactions were conducted. From the analysis of the electronic structure, we propose that Cu2N/Cu(001) is the best candidate due to its weak chemical bonding to the adsorbed molecule and preservation of SCO bistability. Based on the total-energy calculations of various adsorption geometries and binding sites, the iodine atoms of the molecule adsorbed diagonally on copper atoms of the Cu2N(001) surface in the low-spin state is the most stable configuration. The energy difference between the high- and low-spin states of this configuration is 8.48 kJ/mol, which is significantly decreased compared to that of the free molecule of 29.14 kJ/mol.

Vacancy defects transform zinc tellurite glass from insulator to semiconductor: A first-principles prediction

The effect of vacancy defects on the zinc tellurite glass (ZnTeO3) was investigated by performing density functional theory using GGA-PBESOL functional and norm-conserving pseudopotential. The effect of the inclusion of Hubbard-U correction for only Ud, Zn state or Up, O state or both states was investigated to obtain the experimental band gap. The lattice parameters, band structure, and density of states of ZnTeO3 were predicted. The value of Up, O has a significant impact on the band gap (Egap) value of ZnTeO3. Our calculated Egap exactly agrees with the experimental value (4.1 eV). The Egap values of the vacancies were significantly decreased as compared to the perfect structure. The Egap values of the Zn-vacancy and Te-vacancy were 2.5 eV and 2.0 eV, respectively. Three kinds of O vacancies were studied and their Egap values ranged from 2.2 eV to 2.5 eV. The vacancy defects changed the insulating character of ZnTeO3 to semiconductor behavior, which may have promising applications. The vacancies stability follows the order of perfect crystal > O-vacancy > Zn-vacancy > Te-vacancy, i.e., based on their cohesive energies. The formation energies were calculated and indicated that the spontaneous formation of the vacancies is impossible; however, the formation of the O-vacancy was determined to be the most probable, followed by Zn-vacancy and then Te-vacancy under specific conditions. The optical properties (refractive index, reflectivity, dielectric function, optical conductivity, and loss function) and thermodynamic properties of zinc ZnTeO3 and its vacancy defects were predicted.

Site selective 5f electronic correlations in β-uranium

We investigate the electronic structure of β-uranium, which has five nonequivalent atomic sites in its unit cell, by means of the density functional theory plus Hubbard-U correction with U from linear response calculation. It is found that the 5f electronic correlations in β-uranium are moderate. More interestingly, their strengths are site selective, depending on the local atomic environment of the present uranium atom. As a consequence, the occupation matrices and partial 5f density of states of β-uranium manifest site dependence. In addition, the complicate experimental structure of β-uranium could be well reproduced within this theoretical framework.

Electronic structure, sodium diffusion and redox potentials of low-symmetry NaMFe(MoO4 )3 (M=Mg, Ni).

Sodium polyanionic compounds with transition metals are of considerable research interest for the search of new cathode materials for sodium-ion batteries. In this work, we employed ab initio calculations to evaluate three key battery properties: phase stability, diffusion barriers and redox voltage of low-symmetry molybdates NaMFe(MoO4 )3 (M=Mg, Ni) with α- and β-NaFe2 (MoO4 )3 structures. First, we showed that the Hubbard Ucorrection within the GGA + Uapproach is necessary to correctly refer these molybdates to semiconductors. The sodium diffusion along various pathways was thoroughly examined, which allowed us to establish the most probable pathways with the lowest migration barriers. These compounds have different directions of sodium diffusion with twice different barriers that was associated to the peculiarities of their crystal structure. The high potentials and structural stability during sodium extraction, along with low-diffusion barriers predicted for NaMFe(MoO4 )3 (M=Mg, Ni) indicate that these molybdates may be promising high-voltage cathode materials for sodium-ion batteries.

Structure, mechanical and thermo-physical properties of lanthanide fission product doped UO2 in U(V) state: A density functional study

During the nuclear fission of UO2 fuel, many fission products are generated which are trapped in the fuel matrix. These fission products can amend the mechanical and thermo-physical properties of UO2 fuel, which are a prime concern for nuclear safety. In this regard, we investigate the effect of different lanthanide (Ln) fission products on structural, mechanical, electronic and thermal properties of UO2. The substitution of U4+ ions by Ln3+ ions creates a charge imbalance in the lattice which will be neutralized by either oxidation of U atoms from +4 to +5 state (U2Ln2O8) or creation of oxygen vacancies (U2Ln2O7). Here, we study the former case, where the U(IV) ions are oxidized to U(V) state in the Ln substituted UO2 (Ln- UO2) fuel using density functional theory with Hubbard-U corrections. The volume of different Ln-UO2 structures is calculated and compared with undoped UO2. The mechanical properties such as bulk modulus and Young's modulus are calculated from the optimized structures of Ln-doped UO2 and show deviation from undoped UO2 and Ln-doped UO2 with oxygen vacancies. The total density of states and projected density of states indicate that the presence of Ln fission products alters the electronic properties of UO2. The thermal property such as specific heat capacity is evaluated at different temperatures using quasi-harmonic approximation applied to phonon frequencies obtained from density functional perturbation theory. We do observe an excellent agreement of the calculated specific heat capacity with the experimental values for the pure UO2. The counter balancing effect of heat capacity values of U2Ln2O8 and U2Ln2O7 systems may control the net heat capacity of the UO2 fuel matrix.

Barriers to predictive high-throughput screening for spin-crossover

Current spin-crossover (SCO) energy calculations depend on nearly artisanal skill in picking quantum mechanical approximations and computational methods. That is incompatible with automated (work-flow-driven) screening. An acceptable methodology must be quantum mechanically sound, yield both basic structure and property values, and accurate ΔEHL without steering or tuning. Cost vs. accuracy causes focus on density functional theory (DFT). We show by a near-exhaustive study of schemes for calculating the basic molecular high-low spin energy difference, ΔEHL, that presently there is no combination of a constraint-based, non-empirical density functional approximation (DFA) and a set of well-defined semi-empirical corrections to it adequate for such a protocol. Somewhat successful hybrid DFA calculations of ΔEHL are too costly for high-throughput screening of condensed phases. Lower-cost alternatives combine a generalized gradient approximation (GGA) DFA with a Hubbard-U correction (DFT + U). But we show that neither U=0 nor any currently available unsteered U calculation gives a decent ΔEHL value for [Mn(taa)] without also degrading molecular structure or property predictions. Moving to the SCAN meta-GGA does not solve the problem. The revised-restored SCAN (r2SCAN) meta-GGA together with its deorbitalized version r2SCAN-L give improved but not wholly satisfactory results. We also document and diagnose several non-obvious technical and procedural sensitivities and inter-code differences. In addition to being a formidable challenge to DFA development, the lack we delineate is a major impediment to progress in the development of quantum materials and spintronics.

Structural, Electronic, Elastic and Dynamical Properties of MCO3 (M: Mn, Co, Ni) Precursor Materials for Li-Ion Batteries: A First-Principles Study

This communication reports first-principles calculations on the structural, thermodynamic, electronic, elastic and dynamic properties of transition metal carbonates, MCO3 (M=Mn, Co, Ni) at 0 K. These materials are projected to be excellent candidates as precursors for advanced cathode materials in rechargeable lithium-ion batteries which employ an NMC chemistry. We have employed the plane-wave pseudopotential method framed within the density functional theory (DFT) as embedded in the VASP code. The exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE) was used. Moreover, the Hubbard U-correction in the rotationally invariant form was applied to improve the description of the strongly correlated 3d electrons of the transition metals. The structural cell parameters were calculated to 96% agreement with the experimental data, warranting the robustness of the approach employed. All MCO3 crystal systems have negative enthalpies of formation, indicating thermodynamic stability leading to good cycling performance and safety. All the elastic constants for the considered transition metal carbonates satisfied the necessary stability conditions, indicating mechanical stability. Moreover, negative vibrations are not observed in the high symmetry directions of the Brillouin zone, suggesting dynamical stability.

Insight into the Roles of Metal Loading on CO2 Photocatalytic Reduction Behaviors of TiO2.

The photocatalytic reduction of carbon dioxide (CO2) into value-added chemicals is considered to be a green and sustainable technology, and has recently gained considerable research interest. In this work, titanium dioxide (TiO2) supported Pt, Pd, Ni, and Cu catalysts were synthesized by photodeposition. The formation of various metal species on an anatase TiO2 surface, after ultraviolet (UV) light irradiation, was investigated insightfully by the X-ray absorption near edge structure (XANES) technique. CO2 reduction under UV-light irradiation at an ambient pressure was demonstrated. To gain an insight into the charge recombination rate during reduction, the catalysts were carefully investigated by the intensity modulated photocurrent spectroscopy (IMPS) and photoluminescence spectroscopy (PL). The catalytic behaviors of the catalysts were investigated by density functional theory using the self-consistent Hubbard U-correction (DFT+U) approach. In addition, Mott–Schottky measurement was employed to study the effect of energy band alignment of metal-semiconductor on CO2 photoreduction. Heterojunction formed at Pt-, Pd-, Ni-, and Cu-TiO2 interface has crucial roles on the charge recombination and the catalytic behaviors. Furthermore, it was found that Pt-TiO2 provides the highest methanol yield of 17.85 µmol/gcat/h, and CO as a minor product. According to the IMPS data, Pt-TiO2 has the best charge transfer ability, with the mean electron transit time of 4.513 µs. We believe that this extensive study on the junction between TiO2 could provide a profound understanding of catalytic behaviors, which will pave the way for rational designs of novel catalysts with improved photocatalytic performance for CO2 reduction.

Carbon encapsulation of magnetite nanoparticles enhances magnetism at room-temperature due to spin-polarized charge transfer

This study fabricated amorphous-carbon-encapsulated magnetite (Fe3O4@a-C) nanoparticles (NPs) and observed that the encapsulation significantly enhanced the room-temperature magnetization of the NPs by approximately 22.5%. Generalized gradient approximation with the Hubbard-U correction involved the ionic picture hypothesis of electron acceptors for Fe3O4, namely, e2↓ t23↓ → e2↓ t23↓ e↑, which link the spin-polarized charge transfer to Fe ions at the tetrahedral site FeA of Fe3O4. Fe K-edge X-ray absorption near-edge structure (XANES) spectra suggested a formal charge reduction in the FeA of the Fe3O4@a-C NPs. Moreover, Fe L3-edge x-ray magnetic circular dichroism yielded the same conclusion as that obtained using the XANES spectra and suggested the restoration of an orbital moment in the Fe3O4@a-C NPs. Synchrotron radiation spectroscopy confirmed the charge reduction of FeA, which accounted for the magnetic enhancement.

Effects of bipolarons on oxidation states, and the electronic and optical properties of W18O49.

W18O49 has been studied by means of ab initio techniques in the framework of the density functional theory using the onsite Hubbard-U correction applied to the W-d states as well as using the hybrid potential. The existence of bipolarons is found to be an intrinsic feature of this oxide resulting in the presence of different oxidation states of W atoms (W6+ and W5+) and in the co-existence of localized and delocalized electrons. We also discuss possible switching from the W6+ to W5+ and from the W5+ to W4+ oxidation states in the presence of an O vacancy. It appears that O vacancy formation does not cause any additional charge localization at W sites but solely contributes to delocalized electrons. The calculated absorption and reflection coefficients manifest a transparency window in the visible region. At the same time, sizable absorption, occurring due to the presence of free carriers, is detected in the far and mid infrared regions. Additionally, in the near infrared region we confirm and explain an experimentally observed shielding effect originating from transitions involving the localized bipolaronic states.