3d T2g Research Articles

X-ray absorption spectroscopy study of the electronic and structural properties of hexavalent chromium adsorbed by volcanic ashes

X-ray absorption spectroscopy (XAS) was used to characterize the volcanic ash (VA) materials as a low-cost and natural environmental adsorbent for the sorption of Cr (VI). The batch adsorption method was employed to synthesize the sample of chromium (VI) sorbed by VA. The results showed that the Cr-VA material is in the chromium oxidation state (III) and can be described as a composition of Cr2O3 (47.9%), FeCr2O4 (45.2%), and Na2Cr2O7 (6.9%). The promotion of the 1s Cr core electron implies the electronic transitions 1s to 3d(t2g) and 1s to 3d(eg). The first and second shells in the Cr-VA material were coordinated with the oxygen and chromium atoms in a distorted geometrical arrangement. The adsorption mechanism was associated with the chemical reduction of Cr (VI) to Cr (III) via an ion-exchange process. The XAS technique significantly contributes to the characterization of environmental VA, which can be proposed as an economical and alternative adsorbent for the sorption of Cr from contaminated wastewater.

Detailed Fe 3d electronic structures of Prussian blue analogs studied by X-ray absorption spectroscopy

Herein, we studied the detailed electronic structures of Prussian blue analogs with hexacyanoferrate around Fe L- and K absorption edges using room temperature X-ray absorption spectroscopy (XAS). Fe is trivalent for all the synthesized samples. By L3-edge XAS measurements, the excitation from Fe 2p3/2 core level to the 3d (t2g↓), 3d (eg↓), 3d (eg↑) states, and the π*, and σ* states of the Fe–CN molecular orbitals are identified with 3d (t2g↑)3 3d (t2g↓)2 configuration. The same excitation from the 1 s core level is identified using Fe K-edge XAS spectra. The relationship between lattice constants and the energy difference between 3d (t2g↓) to π* and σ* transitions from the Fe 2p3/2 core level suggests that the change in the lattice constant influences the electronic structure near the Fe atom.

Atomical Reconstruction and Cationic Reordering for Nickel‐Rich Layered Cathodes

AbstractCapacity fading and safety concerns accompanied other deep‐rooted challenges have severely hindered commercial development of Ni‐rich layered cathodes. Herein, a robust Sr‐doped Ni‐rich cathode is structurally designed by the reconstruction of the crystal lattice and electronic distribution. Notably, the orbital hybridization between Ni 3d (t2g) and O 2p is remarkably reinforced owing to the shortened NiO bond enabled by the electrostatic interaction between Ni and Sr atoms, giving rise to the enhanced crystal structure. Theoretically, the formation energy of oxygen vacancies is greatly increased due to the intensified electronic polarization between Ni and O states evoked by the weak electronegativity of Sr, resulting in the alleviation of lattice oxygen loss. More impressively, the distances of LiO bonds and the OLiO slab are also extended on account of the electrochemically inactive Sr ion functioning as a pillar, further promoting the transmission of lithium ions. Therefore, the as‐designed Sr‐modified NCM delivers an ultrahigh capacity retention of 98.5% after 150 cycles. This work provides a powerful mechanistic incentive to increase the stability of the crystal phase and electrochemical performance for Ni‐rich layered cathodes through appropriate chemical and mechanical engineering, facilitating the practical applications of Ni‐rich cathodes in high‐performance electric vehicles.

Probing reversal of orbital symmetry in CaCu3-xTi4-xFe2xO12 (x = 0.0–0.7) by X-ray absorption spectroscopy

The electronic properties studied using the X-ray absorption near-edge structure (XANES) spectroscopy performed in the vicinity of O K-, Ca L3,2-, Fe L3,2-, and Cu L3,2-edges demonstrate the reversal of orbital symmetry in CaCu3Ti4O12 on Fe3+ions substitution. The study of Cu L3,2- and Fe L3,2-edges confirms that the valence states to be +2 and +3 for Cu and Fe ions, respectively. The O K-edge XANES spectra show a very systematic change leading to a reversal of relative intensities of the spectral features assigned to 3d(t2g) and 3d(eg) peaks. This implies that Fe substitution induces the tilted octahedral to the square planar symmetry transformation. This is well supported by the cation distribution and also indicates that there is a strong hybridization of Cu-Fe 3d and O-2p orbitals and a weakening of the hybridization of Ti 3d, Fe 3d, and O-2p orbitals.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V–V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Strain-induced structural transition of rutile type ReO2 epitaxial thin films

Rutile-related (tetragonal rutile and monoclinic distorted rutile) ReO2 epitaxial thin films were grown on rutile TiO2 single crystalline substrates by reactive pulsed laser deposition of the Re metal target under a N2O atmosphere. The ReO2 film underwent structural phase transition from the monoclinic to tetragonal phase with the decreasing film thickness, driven by compressive lattice strain from the substrate. X-ray photoelectron spectroscopy and electrical transport measurements revealed that both phases had metallic properties above 25 K. The distinct difference in valence band spectra between the two phases might be due to reconstruction of the band structure dominated by Re 5d t2g.

Structural characterizations and dielectric, magnetic, and optical properties of Ba2FeVO6 double perovskite ceramics

Here, we present the structural characterization and physical properties of Ba2FeVO6 (BFVO) ceramics prepared by the solid-state ceramic method, which is regarded as potential Mott multiferroics. Structural studies demonstrate that the BFVO ceramic samples crystallize in a hexagonal structure with R3c symmetry. SEM images show a spherical morphology of the ceramic grains with an average composition of Bi:Fe:V close to 2.0:1.0:1.0 determined by x-ray energy dispersive spectroscopy. Infrared spectra revealed the vibrational modes of [FeO6] and [VO6] octahedrons in the ceramics, and XPS spectra confirmed the dual chemical valence states of iron (Fe2+ and Fe3+) and vanadium (V3+ and V4+) and two states of oxygen species in the BFVO ceramics, while barium was present as Ba2+. The BFVO ceramics exhibit relaxor-like dielectric behavior with an activation energy of 0.62 eV. The antiferromagnetic (AFM) nature was confirmed in the BFVO ceramics, whereas a weak magnetism was observed at 2 K. The temperature dependence of magnetizations measured with (and without) field cooling across 2 K–300 K under external magnetic fields indicates the spin glass characteristics in the present system. A strong variation of the dielectric behavior observed close to the AFM transition temperature was due to the induced dielectric polarization via the magnetostrictive effect driven by AFM transition. The bandgaps (Eg) of the investigated ceramic samples were deduced from their UV–visible spectra. The Eg values were about 1.15 eV and 1.50 eV, which were ascribed to the d–d electronic transitions of Fe 3d t2g → Fe 3d eg and 6A1g → 4T1g, respectively.

Emergence of quasi-two-dimensional electron gas at the interface of LaAlO3/Sr2AlNbO6(001) heterostructures

Quasi-two-dimensional electron gas (q-2DEG) at the interface of oxide heterostructures, as an alternative to semiconductor electronics, is limited by its low carrier mobility. This is largely due to the nature of Ti 3d t2g conduction electrons of the SrTiO3 substrate. Here, we explore the interface conductivity between LaAlO3/Sr2AlNbO6(001) using density functional theory based first principles calculations. These heterostructures show an insulator–metal transition with increasing overlayer thickness and the carriers being Nb 4dxy in origin. The relatively higher dispersion of these carriers yields a higher carrier mobility and, hence, is significant to electronics applications based on q-2DEG.

Artificial quantum confinement in LaAlO3/SrTiO3 heterostructures

Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands.

Effect of crystallite size on electrical and magnetic properties of CuCr2S4 nanoparticles obtained by mechanical alloying from sulphides

The CuCr2S4 nanoparticles obtained by high-energy ball milling of CuS and Cr2S3 sulphides were about 3.3 nm in size. SEM images showed formation of smaller or larger agglomerates and DFT based ab-initio calculations exhibited metallic ground state with band gap of 0.8 eV width just above (~0.5 eV) the Fermi level and magnetic moments located mostly at Cr sites. Measurements of magnetic susceptibility, magnetic hysteresis, electrical conductivity and thermoelectric power showed antiferromagnetic order with the Néel temperature of 40 K, superparamagnetic-like behaviour with the blocking temperature of 90 K, n-type conductivity, and a metal-insulator transition at 375 K and below this temperature the variable-range hopping in the temperature range of 108 K < T < 256 K. These results are due to the reduction of crystallite sizes to the nanometre scale, which causes narrowing of the 3d t2g mixed-valence band of chromium ions.

Bandgap engineering in V-substituted α-Fe2O3 photoelectrodes

We demonstrated bandgap engineering of iron oxide (α-Fe2O3), which is one of the most promising photocatalysts for solar water splitting. It was found that V substitution and thermal annealing in air following film deposition efficiently narrow the bandgap of α-Fe2O3. The experimental results suggested that the observed decrease in the bandgap energy is due to the effective hybridization of the 3d t2g orbitals of V5+ and Fe3+ at the conduction band minimum of V-substituted α-Fe2O3. Consequently, a significant enhancement in the photocurrent is observed using the Fe1.67V0.33O3-based photoelectrochemical cell over the wavelength range of 500–800 nm.

First principles investigation into the phase stability and enhanced hardness of TiN-ScN and TiN-YN alloys

We study the phase stability, mechanical properties, and electronic structure of two quasi-binary ceramic systems, Ti1-xScxN and Ti1-xYxN (0 ≤ x ≤ 1), using first principles methods based on density functional theory, cluster expansion formalism, and Monte Carlo techniques. Owing to the similarity in ionic radii and electronegativities of their respective transition metals, strong exothermic mixing of TiN and ScN is predicted, with four ordered intermetallic phases lying on the convex hull: TiScN2, TiSc8N9, TiSc9N10, and Ti3Sc2N5. These structures form layered rocksalt-type configurations to minimize strain energy while maximizing occupation of bonding states. The fully-detailed phase diagram including these predicted ground states and known end members is constructed, revealing an upper consolute temperature of 660 K. In contrast to Ti1-xScxN, the mismatched properties of TiN and YN lead to large structural distortions and positive strain energies. As a result, endothermic mixing with significant upward bowing in the formation energy is observed at intermediate concentrations, with the consolute temperature of 7225 K being predicted from the phase diagram of Ti1-xYxN. TiN, ScN, and YN are found to display hardness values of 23.4, 25.1, and 20.6 GPa respectively, in good agreement with experimental data. The intermetallic phase Ti3Sc2N5 is predicted to exhibit an exceptionally high hardness of 27.3 GPa. From analysis of projected electronic density of states and Crystal Orbital Hamiltonian Populations, we attribute enhanced hardness to strong nitrogen p and metal d hybridization, being related to 3d eg occupation, and decreased tendency towards shearing, being related to minimal 3d t2g occupation. These features extend to the case of random solutions, which we model using special quasirandom structures, showing a maximum hardness at a valence electron concentration of 8.4. Based on these findings, we suggest Ti1-xScxN alloys for implementation in hard coating applications.

Elucidating the origin of magnetic ordering in ferroelectric BaTiO3-d thin film via electronic structure modification

With the motive of unraveling the origin of native vacancy induced magnetization in ferroelectric perovskite oxide systems, here we explore the consequences of electronic structure modification in magnetic ordering of oxygen deficient epitaxial BaTiO thin films. Our adapted methodology employs state-of-the-art experimental approaches viz. photo-emission, photo-absorption spectroscopies, magnetometric measurements duly combined with first principles based theoretical methods within the frame work of density functional theory (DFT and DFT+U) calculations. Oxygen vacancy (O) is observed leading partial population of Ti 3d (t2g), which induces defect state in electronic structure near the Fermi level and reduces the band gap. The oxygen deficient BaTiO2.75 film reveals Mott–Hubbard insulator characteristic, in contrast to the band gap insulating nature of the stoichiometric BaTiO3. The observed magnetic ordering is attributed to the asymmetric distribution of spin polarized charge density in the vicinity of O site, which originates unequal magnetic moment values at first and second nearest neighboring Ti sites, respectively. Hereby, we present an exclusive method for maneuvering the band gap and on-site electron correlation energy with consequences on magnetic properties of BaTiO system, which can open a gateway for designing novel single phase multiferroic system.

Hybridization of Nanodiamond and CuFe-LDH as Heterogeneous Photoactivator for Visible-Light Driven Photo-Fenton Reaction: Photocatalytic Activity and Mechanism

Establishing a heterojunction for two kinds of semiconductor catalysts is a promising way to enhance photocatalytic activity. In this study, nanodiamond (ND) and CuFe-layered double hydroxide (LDH) were hybridized by a simple coprecipitation method as a novel heterojunction to photoactivate H2O2. The ND/LDH possessed a hydrotalcite-like structure, large specific surface area (SBET = 99.16 m2/g), strong absorption of visible-light and low band gap (Eg = 0.94 eV). Under the conditions of ND/LDH dosage 0.0667 g/L, H2O2 concentration 19.6 mmol/L, and without initial pH adjustment, 93.5% of 10 mg/L methylene blue (MB) was degraded within 120 min, while only 78.3% of MB was degraded in the presence of LDH instead of ND/LDH. The ND/LDH exhibited excellent stability and maintained relatively high activity, sufficient to photoactivate H2O2 even after five recycles. The mechanism study revealed that in the heterojunction of ND/LDH, the photoelectrons transferred from the valence band of LDH (Cu/Fe 3d t2g) to the conduction band of LDH (Cu/Fe 3d eg) could spontaneously migrate onto the conduction band of ND, promoting the separation of photo-induced charges. Thus, the photoelectrons had sufficient time to accelerate the redox cycles of Cu3+/Cu2+ and Fe3+/Fe2+ to photoactivate H2O2 to produce hydroxyl radicals, resulting in excellent photo-Fenton efficiency on MB degradation.

Transition-metal solvated-electron precursors: diffuse and 3d electrons in V(NH3).

Ground and excited electronic states of V(NH3)0,±6 complexes, investigated with ab initio electronic structure theory, consist of a V(NH3)62+ core with up to three electrons distributed over its periphery. This result extends the concept of super-atomic, solvated-electron precursors from alkali and alkaline-earth complexes to a transition metal. In the approximately octahedral ground state of V(NH3)6, three unpaired electrons occupy 3dxz, 3dyz and 3dxy (t2g) orbitals of vanadium and two electrons occupy a diffuse 1s outer orbital. The lowest excitations involve promotion of diffuse 1s electrons to 1p or 1d diffuse orbitals, followed by a 3d (t2g → eg) transition. V(NH3)6+ is produced by removing a diffuse 1s electron, whereas the additional electron in V(NH3)6- populates a 1p diffuse orbital. The adiabatic ionization energy and electron affinity of V(NH3)6 equal 3.50 and 0.48 eV, respectively.

Electronic structure and spin dynamics of ACo2As2(A=Ba,Sr,Ca)

The electronic structures, charge and spin dynamics of the cobalt pnictide compounds ACo2As2 (A=Ba, Sr, Ca) in the paramagnetic state are investigated by using density functional theory combined with dynamical mean-field theory. In contrast to their iron counterparts, these cobalt pnictide compounds have three-dimensional electronic structures and strong ferromagnetic low-energy spin excitations. The Co 3d eg orbitals dominate the electronic states around the Fermi level and have stronger electronic correlation strength than the Co 3d t2g orbitals. The overall electronic correlation strength is much weaker than that in the iron arsenides, however, the most strongly correlated Co 3d x2-y2 orbital, especially in CaCo2As2, has electronic correlation strength comparable to Fe 3d t2g orbitals in iron arsenides. ACo2As2 (A=Ba, Sr, Ca) shows similar electronic structures where a conduction band of primarily Co 3d x2-y2 orbital character is close to a Van Hove singularity around the Brillouin-zone corner, which promotes ferromagnetic low-energy spin excitations. Originated from its increased nearest-neighbor Co-Co distance and significantly reduced As height from the Co plane, the strong electronic correlation strength and close proximity to the Van Hove singularity of the Co 3d x2-y2 orbital in CaCo2As2 is responsible for its unique A-type antiferromagnetic order observed in experiments. In comparison, despite substantial ferromagnetic low-energy spin excitations, BaCo2As2 and SrCo2As2 remain paramagnetic down to very low temperature because the Co 3d x2-y2 orbital has weaker electronic correlation strength and is further away from the Van Hove singularity.

Reduced electronic correlation effects in half substituted Ba(Fe1−xCox)2As2

We report a comprehensive study of the tridimensional nature and orbital character of the low-energy electronic structure in 50% Cobalt doped Ba(Fe1−xCox)2As2 (d6.5), by using polarization- and photon energy-dependent angle-resolved photoemission spectroscopy. An extra electron-like Fermi surface is observed around the Brillouin zone boundary compared with isoelectronic KyFe2−xSe2 (d6.5). The bands near the Fermi level (EF) are mainly derived from Fe/Co 3d t2g orbitals, revealing visible dispersions along the kz direction. In combination with the local density approximation and the dynamical mean-field theory calculations, we find that the As 4p bands are non-renormalized and the whole 3d band needs to be renormalized by a “single” factor of ∼1.6, indicating moderate electronic correlation effects. The “single” factor description of the correlation strength among the different 3d orbitals is also in sharp contrast to orbital-dependent correlation effects in BaFe2As2. Our findings indicate a remarkable reduction of correlation effects with little difference among 3d orbitals in BaFeCoAs2, due to the increased filling of the electronic 3d shell in the presence of significant Hund's coupling. The results support that the electronic correlation effects and multiple orbital physics play an important role in the superconductivity of the 122 system and in other ferropnictides.

Plasmon-Induced Visible-Light Photocatalytic Activity of Au Nanoparticle-Decorated Hollow Mesoporous TiO2: A View by X-ray Spectroscopy

Plasmonic photocatalyst of Au nanoparticle-decorated hollow mesoporous TiO2 with 0, 0.1, 0.25, 0.5, and 1% Au content was successfully synthesized by a hydrothermal method. Controlling the particle size of Au coated on TiO2 hollow microspheres (AuTHMSs) is expected to improve the photocatalytic ability. Our results of X-ray absorption spectroscopy (XAS) indicated that the coated Au ions are nulvalent and cause a lattice distortion as well as a variation in Ti 3d orbital orientation. It is also inferred that TiO6 octahedral symmetry is significantly affected by the Au incorporation, giving rise to an increase in the Ti 3d t2g unoccupied state. UV–visible absorption spectra and I–V measurements were performed to examine localized surface plasmon resonance (LSPR) effect and photoelectrocatalytic (PEC) ability. We present the first in situ XAS measurements on AuTHMS system, which enabled us to correlate the electronic structure and photocatalytic property of the material. An analysis of the results showed an ...

Effective theory of exotic superconductivity in LaAlO3/SrTiO3 interfaces

Motivated by experimental and theoretical works about superconductivity at the oxide interfaces, we provide a simple model for possible unconventional pairings inside the exotic two-dimensional electron gas formed in heterostructures of SrTiO3 and LaAlO3. At the low energy limit, the electron gas at the interfaces is usually modeled with an effective three band model considering of 3d t2g orbitals which are slightly coupled by atomic spin-orbit couplings (SOC). Considering direct superconducting pairing in two higher delocalized bands and by exploiting a perturbative scheme based on canonical transformation, we derive the effective pairing amplitudes with possibly exotic nature inside the localized dxy band as well as various inter-band pairing components. In particular we show that equal-spin triplet pairings are possible between the band dxy and any of other dxz and dyz bands. In addition weaker effective pairings take place inside the localized band itself and between delocalized dxz and dyz bands with singlet and opposite-spin triplet characters. These unconventional effective pairings are indeed mediated by SOC-induced higher order virtual transitions between the bands and particularly into the localized band. Our model suggest that unconventional effective superconductivity is possible at oxide interfaces, simply, due to the special band structure and important role of atomic SOC and perhaps other magnetic effects present at these heterostructures.

Composition-dependent properties and phase stability of Fe-Pd ferromagnetic shape memory alloys: A first-principles study

The composition-dependent properties and their correlation with the phase stability of Fe75+xPd25−x (−10.0≤x≤10.0) alloys are systematically investigated by using first-principles exact muffin-tin orbitals (EMTO)-coherent potential approximation (CPA) calculations. It is shown that the martensitic transformation (MT) from L12 to body-centered-tetragonal (bct) occurs in the ordered alloys with about −5.0≤x≤10.0. In both the L12 and bct phases, the evaluated a and c/a agree well with the available experimental data; the average magnetic moment per atom increases whereas the local magnetic moments of Fe atoms, dependent on both their positions and the structure of the alloy, decrease with increasing x. The tetragonal shear elastic constant of the L12 phase (C′) decreases whereas that of the bct phase (Cs) increases with x. The tetragonality of the martensite (|1−c/a|) increases whereas its energy relative to the austenite with a negative value decreases with Fe addition. All these effects account for the increase of MT temperature (TM) with x. The MT from L12 to bct is finally confirmed originating from the splitting of Fe 3d Eg and T2g bands upon tetragonal distortion due to the Jahn-Teller effect.