Fe 3d States Research Articles

Predicting Hydration Reactivity of Cu-Doped Clinker Crystals by Capturing Electronic Structure Modification

Chemical doping is a promising scheme for sustainable development of green and economical cement manufacturing related to global energy and environmental concerns. Understanding and controlling doping effect on reactive cement components is the prerequisite. This study aims to propose effective theoretical methods to reveal the correlation between hydration reactivity of doped clinker crystals and their electronic structures. Four dominant Portland cement clinker phases with copper doping are comparatively investigated by the state-of-the-art ab initio calculation. It finds that Cu ions doped in silicate and aluminate phases shift the nucleophilic reactive sites from Ca-centered regions to defect sites by decreasing the effective charges of Ca ions, which accounts for the hydration retardation of these phases. Exceptionally, the reactive sites in ferrite phase are scarcely changed by Cu doping due to the intense band-edge localization of Fe 3d state, which implies the relatively lesser influence on hydrat...

Effect of Fe Doping and Point Defects (VO and VSn) on the Magnetic Properties of SnO2

We investigated the effects of Fe doping and point defects (oxygen and tin vacancies, VO and VSn, respectively) on the ferromagnetism of SnO2 through first-principles calculations by using generalized gradient approximation + U under density functional theory. Calculation results show that Fe-doped SnO2 with VO or VSn exhibit long-range ordered ferromagnetism and Curie temperature above room temperature. The system has different magnetic properties with different doping modes, which are beneficial to the magnetic adjustment of dilute magnetic semiconductor (DMS). The double-exchange effects among the electrons of O–2p and Fe–3d states are the origin of ferromagnetism in Sn15FeO31. Similarly, ferromagnetism in Sn14FeO32 is derived from the double-exchange effects among the electrons of O–2p and Fe–3d states. These effects are produced by hole carriers after complexes form by Fe doping and VO or VSn. Simultaneously, Sn14FeO32 shows a half-metallic behavior, thus resulting in 100% conduction hole polarizability, which is highly beneficial to DMSs as a hole-injection source. Moreover, results also show that the decreased distance between Fe and vacancies (VO or VSn) lowers formation energy and thus increases the system stability.

Atomistic Insights into Hydration Reactivity of Clinker Crystals with Chemical Impurity

In this work, multiple theoretical methods including Fukui functions, effective charges, localization index, and density of states are employed to clarify the influence of Zn impurity on hydration reactivity of clinker phases at atomic level. Zn ions are found to decrease the number of reactive sites and effective charges of tricalcium silicate and tricalcium aluminate dramatically, which could account for the significant retarding effect in these two phases. For tetracalcium aluminoferrite, Zn doping causes no influence on the number of reactive sites but an increase of effective charges and total localization index, which implies somewhat reactivity promotion. PDOS analysis suggests this particular effect of tetracalcium aluminoferrite is due to the strong localization capacity of Fe-3d states. Our calculated results is in good agreement with previous experimental observations.

Survey of electronic properties and local structures around Fe in selected multinary chalcogenides

Paper presents detailed studies of local and electronic structure around Fe in Cd0.97Fe0.03Te, Cd0.98Fe0.02Te0.97Se0.03 and Cd0.99Fe0.01Te0.91S0.09 multinary chalcogenides by means of X–ray absorption fine structure (XAFS), X–ray magnetic circular dichroism (XMCD) and electron paramagnetic resonance (EPR) measurements. In addition, electronic consequences of Fe incorporation into CdTe semiconductor host were studied by means of first principles calculations. In order to improve accuracy of the calculated total energies, the band gaps and the band edge positions, special attention is paid to the treatment of exchange–correlation interaction and the description of highly localized Fe 3d–states. Also, the Bader theory of the topological properties of the electron charge density is used to access details of the nature, strength and distribution of the (next) nearest neighbour bonds. Local and electronic structure around Fe in Cd0.97Fe0.03Te and Cd0.98Fe0.02Te0.97Se0.03 systems have been found to exhibit similar characteristics, since the first coordination sphere around Fe comprises four Te atoms located at approximately the same distance. In Cd0.99Fe0.01Te0.91S0.09 system, however, local bimodal distribution of distances has been revealed, with one FeTe bond replaced with much shorter FeS bond, resulting in much stronger crystal–field. Along with the crystal field effect, the spin–orbit interaction has proven to play decisive role in determining the nature of Fe doped CdTe systems. While the systems with higher Fe concentrations (25 at.%) are intrinsic insulators, in systems with only 3.125 at.% Fe one spin channel contributes to the density of states at the Fermi level, which makes them suitable for spin selective electronic transport applications.

Study of band structure of iron nitrides by angle resolved photoemission spectroscopy

Abstract The band structure of Fe-N film is studied by angle resolved photoemission spectroscopy. It is found that the 3d band of Fe-N film disappears due to the hybridization of Fe 3d states and N 2p states. The result can account for the less ferromagnetism and conductivity of Fe-N film when compared with those of Fe film. This finding indicates that the high content of nitrogen can significantly change the band structure of iron nitrides, which exceeds the conventional theoretical prediction. The present work may evoke the re-recognization to the effect of coupling of Fe and N, and promote the further research of band structure of iron nitrides.

First-principles calculations of electronic structures and ferromagnetism of Fe3Si(001)//MgO(001) films

The first-principles calculations based on density functional theory (DFT) were carried out in investigating electronic structures and ferromagnetism of Fe3Si films epitaxial on MgO(001). Firstly, the various geometric structures of Fe3Si(001)//MgO(001) constructed near lattice constant c = 3.995 Å were optimized to gain the most steady equilibrium state at c = 3.980 Å. Then, the calculated cohesive energy and negative heat of formation indicate that Fe3Si(001)//MgO(001) formed in this manner obtain high structural stability. The calculated results of spin-polarized energy band structures and density of states show that Fe3Si(001)//MgO(001) exhibit the metallic feature whose bonding orbitals are constituted by covalent bond and metallic bond. Two peaks located in both the sides of the Fermi level and the total density of states (TDOS) in this energy range are all due to the Fe 3d states, which implies that the pseudo energy gap exists in the Fermi level and covalent electron orbit hybridization takes place. Ferromagnetism of Fe3Si(001)//MgO(001) are determined by the 3d states of Fe atoms. There are two occupied sites for Fe atoms with different local magnetic moments, which is 1.34 [Formula: see text]/atom for Fe[A, C] atoms and a value of 2.68 [Formula: see text]/atom for Fe[B] atoms, likewise indicating Fe3Si films epitaxial on MgO(001) are ferromagnetic.

CPA simulation of electronic properties of La1-xSrxFeO3-x/2 at high-temperature

The coherent potential approximation is applied for studying the electronic spectrum features in cubic perovskite-like ferrite La1-xSrxFeO3-δ with ionic compensation of strontium acceptors at 0 ≤ x ≤ 0.75. The energy characteristics and magnetic moments calculated with accounting for electron correlations agree with the experimental data. The concentration dependent trends in electrical properties are shown to reflect combined effects of formation of vacancy bands, hybridization and splitting of Fe 3d states in the vicinity of the Fermi energy.

First principles investigations of the structural, electrical and optical properties of iron-doped zinc oxide (0 0 0 1) surfaces

A first principles density function investigation of the structural, electrical and optical properties of iron-doped ZnO (0 0 0 1) surfaces is conducted by considering the substitutional sites in the three relaxed ZnsbndO bilayers and the interstitial site in the centre of the octahedron surrounded by zinc atoms. Calculations are performed with the GGA+U approach which can accurately estimate the energy of strong correlation semiconductors. The calculated results show that the iron atom energetically prefers to occupy the zinc site on the topmost ZnsbndO bilayer of ZnO (0 0 0 1) surface. Compared with the pristine ZnO (0 0 0 1) surface, the Fe-3d states will generate several impurity levels in the forbidden band of the material system doped by the iron atom in the three substitutional sites, whereas they will shift the conduction band to a-low energy region and reduce the bandgap to 1.434 eV in the interstitial site. Electron density difference calculation confirms the bonding situation among iron atom and the neighboring oxygen and zinc atoms. Comparison of the absorption coefficients of the pristine and iron-doped ZnO (0 0 0 1) surface systems shows that light absorption is significantly enhanced from 0 to 3.5 eV. The absorption peak of the substitutional sites shifts to lower energy while the absorption peak of the interstitial site shifts to 2.23 eV. This work is beneficial for the development of ZnO photocatalyst.

Investigation of the Preferential Doping Site and Regulating on the Visible Light Response and Redox Performance for Fe- and/or La-Doped InNbO4.

The preferential doping site, visible light response, and redox potential of Fe- and/or La-doped InNbO4 (INO) were investigated using first-principles density functional theory. Eight designed doping models, including Fe and/or La doping at In or/and Nb sites of INO are constructed, respectively. It was found that Fe-doping and Fe,La-codoping to substitute In into an INO cell are energetically favorable, confirming that the steric hindrance plays a vital role for the selective doping site than the charge of the dopants. Fe doping always formed two impurity bands between the conduction and valence bands, originated from Fe 3d state, inferring the well visible light response. Furthermore, the presence of La has a specific regulation effects for Fe doping although the energy levels of the single La-doped models were completely similar to those of the undoped INO. The electron exchange between La and Fe dopants results in the significant interaction for codoping INO. Importantly, by doping La into INO cell, the redox potentials of Fe-doped INO could be well-regulated. The band potential moved to the more positive energy level of the models Fe-doped at Nb sites, while it shifted to the more negative level if Fe was doped at In site of La-INO. The present investigation may provide the guidance for the designative dopants to construct the photocatalyst with stable, visible response, and good redox performance.

Spin-flip effect enhanced photocatalytic activity in Fe and single-electron-trapped oxygen vacancy co-doped TiO2

On the basis of density functional theory calculations and experimental results, we propose a spin-flip effect in a novel single-electron-trapped oxygen vacancy (SETOV; Vo) and Fe co-doped TiO2 photocatalyst which improves significantly the propylene photo-oxidation under visible light illumination through the two spin channels photo-absorption and one spin channel recombination congestion. The presence of SETOV can make strongly localized Fe 3d states within the band gap get more delocalized, which not only increases the carrier mobility but also has an effect on restraining carrier recombination, thus resulting in the remarkably improved photocatalytic activity. Here the proposed photocatalytic process referring to spin-flip effect might afford a way for us about how to design a material owning the expected photoactivity in terms of spin selection rules about interband transition, opening the way to ground breaking applications in energy conversion.

Electronic structure and fermiology of LaFe2Ge2

The Density functional theory has been used to investigate the electronic structure properties of layered ternary compound LaFe2Ge2. The main trends in band structure, Fermi level, DOS, and charge density for the Fe2Ge2-layer with nonmagnetic La atom have been analyzed. The electron charge density has shown a hybridization between Fe-3d and Ge-4p causing the splitting of Fe-3d holes and electrons states which increases the Fe magnetic moment. The DOS of Fe-3d and Ge-4p orbitals has indicated that only a small set of 3d and 4p electronic states has been hybridized just under Fermi energy. Additionally, most of Fe-3d and Ge-4p states have been hybridized in the energy range from −5 eV to −2 eV. The magnetic moment of LaFe2Ge2 is 4.96 μB per formula unit where a localized magnetic moment of 2.54 μB is associated with the Fe muffin-tin sphere. The Fe-4s and Ge-4s, 4p bands crossing Fermi level have formed a complicated Fermi surface for majority channel, whereas the spin down Fermi surface consists of simple narrow tubes.

Electronic structure and optical properties of K2Ti6O13 doped with transition metal Fe or Ag

Based on the experimental study of the optical properties of K2Ti6O13 doped with Fe or Ag, their electronic structures and optical properties are studied by the first-principles method based on the density functional theory (DFT). The calculated optical properties are consistent with the experiment results. K2Ti6O13 doped with substitutional Fe or Ag has isolated impurity bands mainly stemming from the hybridization by the Fe 3d states or Ag 4d states with Ti 3d states and O 2p states and the band gap becomes narrower, the absorption edge of K2Ti6O13 thus has a clear red shift and the absorption of visible light can be realized after doping. For Fe-doped K2Ti6O13, the impurity bands are in the middle of the band gap, suggesting that they can be used as a bridge for valence band electrons transition to the conduction band. For Ag-doped K2Ti6O13, the impurity bands form a shallow acceptor above the valence band and can reduce the recombination rate of photoexcited carriers. The experimental and calculated results are significant for the development of K2Ti6O13 materials that have absorption under visible light.

Exceptional properties of the magnetic interface state of Pd on Fe(1 1 0)

The magnetic characteristics of Pd thin films on Fe(110) were investigated by means of Magnetic Circular Dichroism in the Angular Distribution of photoelectrons (MCDAD). For uncovered Fe it was found that significant photoelectron intensity differences occur for electrons emitted from the 3d band near the Fermi level in the situation of a fixed helicity of the incoming circularly polarized radiation and opposite magnetization of the magnetic layer. The corresponding MCDAD asymmetry shows a strong dependence on the excitation energy in the VUV range and exhibits values up to about 10%.After deposition of Pd in the submonolayer regime up to a thickness of a few layers, the new features which are present in the photoemission spectra exhibit asymmetries, too. A strong hybridization takes place at the interface which is due to the intermixing of Pd 4d and Fe 3d states. This interface state shows no dispersion and no change of the asymmetry value for different photon energies. Additionally, the sign of the asymmetry is opposite to that of the Fe substrate. The reason is the strong hybridization which results not in a rigid band shift as found for less-interacting adsorbates but in new states being different in the majority and minority spin band. As a consequence, the interface layer exhibits an induced magnetic polarization. The structure near the Fermi energy exhibit the same sign of the asymmetry but with lower values. For a thickness of 3 monolayers the spectrum is comparable to bulk measurements and shows no MCDAD asymmetries in the entire binding energy region.

Manipulation of perpendicular magnetic anisotropy of single Fe atom adsorbed graphene via MgO(1 1 1) substrate

Perpendicular magnetic anisotropy is significantly important for realizing a long-term retention of information for spintronics devices. Inspired by 2D graphene with its high charge carrier mobility and long spin diffusion length, we report a first-principles design framework on perpendicular magnetic anisotropy engineering of a Fe atom adsorbed graphene by employing a O-terminated MgO (1 1 1) substrate. Determined by the adsorption sites of the Fe atom, a tunable magnetic anisotropy is realized in Fe/graphene/MgO (1 1 1) structure, with the magnetic anisotropy energy of −0.48 meV and 0.23 meV, respectively, corresponding to the in-plane and out of plane easy magnetizations. Total density of states suggest a half-metallicity with a 100% spin polarization in the system. Decomposed densities of Fe-3d states reveal the orbital contributions to the magnetic anisotropy for different Fe adsorption sites. Bonding interaction and charge redistribution regulated by MgO substrate are found responsible for the novel perpendicular magnetic anisotropy engineering in the system. The effective manipulation of perpendicular magnetic anisotropy in present work offers some references for the design and construction of 2D spintronics devices.

Electronic Structure of FeSe Monolayer Superconductors: Shallow Bands and Correlations

Electronic spectra of typical single FeSe layer superconductors obtained from ARPES data reveal several puzzles: what is the origin of shallow and the so called "replica" bands near M-point and why the hole-like Fermi surfaces near $\Gamma$-point are absent. Our extensive LDA+DMFT calculations show that correlation effects on Fe-3d states can almost quantitatively reproduce rather complicated band structure, which is observed in ARPES, in close vicinity of the Fermi level for FeSe/STO and K$_x$Fe$_{2-y}$Se$_{2}$. Rather unusual shallow electron-like bands around the M(X)-point in the Brillouin zone are well reproduced. However, in FeSe/STO correlation effects are apparently insufficient to eliminate the hole-like Fermi surfaces around the $\Gamma$-point, which are not observed in most ARPES experiments. Detailed analysis of the theoretical and experimental quasiparticle bands with respect to their origin and orbital composition is performed. It is shown that for FeSe/STO system the LDA calculated Fe-3d$_{xy}$ band, renormalized by electronic correlations within DMFT gives the quasiparticle band almost exactly in the energy region of the experimentally observed "replica" quasiparticle band at the M-point. For the case of K$_x$Fe$_{2-y}$Se$_{2}$ most bands observed in ARPES can also be understood as correlation renormalized Fe-3d LDA calculated bands, with overall semi-quantitative agreement with our LDA+DMFT calculations. Thus the shallow bands near the M-point are common feature for FeSe-based systems, not just FeSe/STO. We also present some simple estimates of "forward scattering" electron-optical phonon interaction at FeSe/STO interface, showing that it is apparently irrelevant for the formation of "replica" band in this system and significant increase of superconducting $T_c$.

Opposite Surface and Bulk Solvatochromic Effects in a Molecular Spin-Crossover Compound Revealed by Ambient Pressure X-ray Absorption Spectroscopy.

We investigate the solvatochromic effect of a Fe-based spin-crossover (SCO) compound via ambient pressure soft X-ray absorption spectroscopy (AP-XAS) and atomic force microscopy (AFM). AP-XAS provides the direct evidence of the spin configuration for the Fe(II) 3d states of the SCO material upon in situ exposure to specific gas or vapor mixtures; concurrent changes in nanoscale topography and mechanical characteristics are revealed via AFM imaging and AFM-based force spectroscopy, respectively. We find that exposing the SCO material to gaseous helium promotes an effective decrease of the transition temperature of its surface layers, while the exposure to methanol vapor causes opposite surfacial and bulk solvatochromic effects. Surfacial solvatochromism is accompanied by a dramatic reduction of the surface layers stiffness. We propose a rationalization of the observed effects based on interfacial dehydration and solvation phenomena.

Electronic structures and magnetic properties of Fe3Si films epitaxial on Si(001)

The electronic structures and magnetic properties of Fe3Si films epitaxial on Si(001) were systematically investigated by using the first-principle calculations on plane-wave pseudo-potential theory. The calculated results show that Fe3Si films epitaxial on Si(001) have the most stable equilibrium state at the lattice constant c = 5.63 Å. The negative heat of formation and cohesive energy of Fe3Si(001)//Si(001) imply that Fe3Si films epitaxial on Si(001) formed in this manner have high structural stability. The calculated spin polarized energy band structures and density of states indicate that Fe3Si films epitaxial on Si(001) have characteristic of metal, whose bonding modes are covalent bond and metallic bond. The band through Fermi level is mainly due to the Fe 3d states and the Si 3p states. Ferromagnetic properties of Fe3Si(001)//Si(001) are attributed to 3d states of the Fe atoms. The atomic magnetic moments of Fe[A,C] and Fe[B] are different from each other, likewise implying Fe3Si films epitaxial on Si(001) is ferromagnetic.

The structural, electronic, magnetic and optical properties of the new promising spintronic material Bi0.92Tb0.08FeO3: A first-principles approach

With a view to obtain high performance multiferroic devices for applications in data storage media and multiple storage memories, we have investigated the multiferroic properties of Bi0.92Tb0.08FeO3 for the first time based on density functional theory calculations. Our results on Bi0.92Tb0.08FeO3 reveal the presence of incommensurate band gaps in the up and down spin channels and thus confirm the existence of bipolar magnetic semiconductor, a rare and promising candidate among spintronic materials. The hybridization between Bi, Tb 6s states and O 2p states gives rise to cation off-centering and distortion and the partial density of states indicates the presence of exchange splitting of Fe 3d states. In addition, we have also obtained a ferrimagnetic arrangement of Fe atoms as a result of destruction of the antiferromagnetic spin structure. Accounting for optical properties, when electric field is perpendicular with respect to the crystallographic c axis (E⊥c) the maximum value of refractive index reached to 3.3 at 3.3 eV (375.7 nm, UV region) and 3.3 at 2.5 eV (496 nm, visible region). This is one of the largest values of refractive index known in the visible region of electromagnetic spectrum and thus Bi0.92Tb0.08FeO3 can become a promising candidate for optical communication devices. Also the dielectric constant reaches maximum to 10.6 at 2.5 eV (496 nm, visible region) for E⊥c and 10.5 at 3.3 eV (375.7 nm, UV region). A high polarization value 82.56 µC/cm2 was calculated for Bi0.92Tb0.08FeO3 and is in good agreement with the known experimental values.

Electronic Structure and Visible-Light Absorption of Transition Metals (TM=Cr, Mn, Fe, Co) and Zn-Codoped SrTiO3: a First-Principles Study**Supported by the National Natural Science Foundation of China under Grant No 51474011, the Postdoctoral Science Foundation of China under Grant No 2014M550337, and the Key Technologies R&D Program of Anhui Province of China under Grant No 1604a0802122.

First-principles calculations are performed on the influence of transition metal (TM=Cr, Mn, Fe, Co) as codopants on the electronic structure and visible-light absorption of Zn-doped SrTiO3. The calculated results show that (Zn,Mn)-codoped SrTiO3 requires the smallest formation energy in four codoping systems. The structures of the codoped systems display obvious lattice distortion, inducing a phase transition from cubic to rhombohedral after codoping. Some impurity Cr, Mn and Co 3d states appear below the bottom of conduction band and some Fe 3d states are located above the top of valence band, which leads to a significant narrowing of band gap after transition metal codoping. The enhancement of visible-light absorption are observed in transition metals (TM=Cr, Mn, Fe, Co) and Zn codoped SrTiO3 systems. The prediction calculations suggested that the (Zn,Mn)- and (Zn,Co)-codoped SrTiO3 could be the desirable visible-light photocatalysts.

The magnetic variation of hexagonal FeGe bulk depends on stress controlling

We report a system study of the stress regulates the electronic structure and magnetic property of FeGe. The density states of Fe was more than 85% contribution comes from Fe 3d, the density states of Ge mainly contributed from Ge 4p and Ge 4s, and the Fe 3d spin was induce the Ge 4p electron transfer. The induction effect increased the electron energy of germanium, weakened the Fe spin density states. The magnetic moment is 1.15μB per FeGe molecule with the stress of 0 GPa, and the magnetic moment is almost linearly decrease with the stress increasing from −30 GPa to 30 GPa. The magnetic of FeGe mainly comes from the unoccupied Fe 3d orbital, the Fe 3d is positive spin-polarization state and the spin-polarization strength is decreased, the Ge1 4p and Ge2 4p are negative spin-polarization state and the spin-polarization strength are increased. The main reason is that the charge of Fe are negative whereas the Ge1 and Ge2 atoms are positively charged, the Fe atom lost charge, and the charge transferred to the Ge atom, under the stress. But the total numbers of gain and lose charge is not equal, so the FeGe electron system may be exist hybridization between the Fe 3d state and Ge 4p state. Moreover, electrons-spin polarization is relevant to the structure parameters of the FeGe system, and control spin-polarized electronic behavior by means of adjusting ferromagnetic.