Oxygen Orbitals Research Articles

Copper Versatility in Hydroxyapatite: Valence States, Clusters, and Optical Absorption Properties.

The solid-state reaction between a stoichiometric hydroxyapatite (HA) and CuO at temperatures above 1100 °C produces pure Cux-HA phases for x ≤ 0.7 with the general formula Ca10CuIx(PO4)6(OH)2-xOx. The Cu atoms are located at the center of the hexagonal tunnels between two hydroxyl ligands, as determined by Fourier analysis based on XRD data. During heat treatment, the reduction of Cu2+ ions into Cu+ is concomitant with the stabilization of copper in HA in the hexagonal tunnel. The incorporation of monovalent copper within the apatite, as revealed by XANES spectroscopy, explains the violet color of the samples. The incorporation of Cu+ ions, by substitution of a hydrogen atom by copper(I), results in the formation of linear O-Cu-O chains where the majority of which are isolated for x ≤ 0.3. In addition, the EXAFS investigation showed, thanks to the linear geometry of these clusters that results in multiple diffusion effects, the existence of [CuO]n chains with n ≥ 2, which only appear clearly for higher copper contents x ≥ 0.5. The strong covalency of the Cu-O bond in such a dumbbell configuration would lead to strong hybridization between the 3d and 4s orbitals of copper and the 2p orbitals of oxygen, as illustrated by ESR signals. In the case of Cu-doped HA prepared by coprecipitation and annealed at a lower temperature (T ≤ 600 °C), copper substitutes calcium according to the theoretical formula Ca10-xCux(PO4)6(OH)2, mainly at the Ca(2) site. This local environment is in line with the Jahn-Teller distortion induced by the Cu2+ ion (as evidenced by UV-vis-NIR, XPS, and XANES-EXAFS spectroscopy analyses) and also allows copper-copper interactions from one site to another, as observed by ESR spectroscopy. This versatility of copper in HA gives it optical properties that change from a violet color with near-IR absorption to a blue hue. In all cases, Cu-O-Cu interactions persist whatever the valence state, and heat treatment induces a redox phenomenon, with copper exchanging between two sites close to each other.

Study of the magnetic and structural properties of the simple perovskite YFeO3

In the present work, an experimental and theoretical study of the synthesis, magnetic, and structural properties of simple perovskite compounds YFeO3 has been investigated based on their potential applications, such as: solid oxide fuel cells, electrochemical sensors, sensor materials, magneto-optical materials, etc. Experimentally, the synthesis was carried out by the sol-gel method. X-rays and Rietveld refinements confirmed the orthorhombic structure. Scanning electron microscopy observations confirmed agglomerates of simple perovskite particles YFeO3. The study of the magnetic phase of the simple perovskite YFeO3 showed that it behaves like a weak ferromagnetic, this property delimits and directs magnetic fields into well-defined trajectories. On the other hand, the structural results, from a theoretical point of view, are in good agreement with the experimental ones. Through energy comparison, a possible competition was identified between the FM and AFM states, where the electronic properties are associated with the directional nature of the hybridization between the p orbitals of oxygen and the t2g states, where the superexchange mechanism prevails with e oxygen as mediator.

First-principles calculations of self-defects on the structural, electronic, and optical properties of BaXO3 (X = Ti, Zr, and Hf) perovskite oxide materials

This study utilizes computational simulations to examine the properties of BaXO3 (X = Ti, Zr, and Hf) perovskite oxide materials with self-defects, including vacancy and interstitial. The investigation focuses on their crystal structure, electronic characteristics, and optical behavior. The results show that the pristine BaTiO3, BaZrO3, and BaHfO3 exhibit p-type semiconductor behavior with varying band gaps. The electronic properties are primarily governed by the orbitals of oxygen and transition metal elements, suggesting potential for precise control of electron transport. Introduction of self-defects enhances the magnetization, making them suitable for magnetic applications. Moreover, these materials demonstrate promising optical properties for energy conversion and storage in the visible light and ultraviolet regions. We expected that these findings offer valuable insights for further research on perovskite oxide materials and their applications, particularly regarding the influence of self-defects during the fabrication process.

Atomistic modulating of structural, elastic, and optoelectronic behavior of pure TiO2 and Fe/TiO2 for photoelectrochemical water splitting application

Titanium dioxide (TiO2) has attracted much attention because of their desirable physicochemical properties especially in the water splitting process. In this work pure and Fe-doped TiO2 compounds are studies theoretically with the help of Generalized Gradient Approximation with the revised Pardew–Burke–Ernzerh (RPBE) exchange–correlation scheme. Total Density of States (TDOS) and Partial Density of States (DOS) were analyzed in detail which show that iron (Fe) and oxygen (O) orbitals hybridize, especially in the region of the doping system conduction band minima for both modes. Additionally, this interaction produces an energy level that effectively reduces the bandgap of the adsorbed system. Optical properties were elucidated which shows that Fe-doped TiO2 system results in high absorption and photoconductivity. Moreover, the results demonstrate low bandgap energy which is suitable for the reduction in water splitting without the need for external energy. Magnetic properties demonstrated that Fe-doped TiO2 systems show very low diamagnetic responses. The calculated elastic properties of Fe-doped TiO2 indicate ductile nature of the material with a strong average bond strength. Fe-doped TiO2 exhibited less microcracks with a mechanically stable composition.

Enhancing the stability of Li-Rich Mn-based oxide cathodes through surface high-entropy strategy

Li-rich Mn-based layered oxides (LMR) hold promise as cathode materials for lithium-ion batteries (LIBs) due to their high reversible capacity. However, issues such as oxygen release and structural degradation associated with oxygen redox cause voltage decay and capacity decline during cycling. In this study, a monodisperse Li1.2Ni0.13Co0.13Mn0.54O2 cathode material with surface high-entropy architecture (MZCN-LMR) was synthesized to enhance the overlap between the non-bonding orbitals of oxygen (|O2p) and the (TM-O)* occupied band. This enhancement enables regulation of the depth of oxygen oxidation, improving reversible oxygen redox and creating highly flexible structures, with mitigated anisotropic modulus and lattice strain evolution during (de)intercalation. Consequently, the developed MZCN-LMR cathode exhibits impressive capacity retention of 80.5 % after 400 cycles and minimal voltage decay of 0.7 mV per cycle with the voltage range of 2.1–4.6 V. This surface high-entropy strategy offers a universal approach to enhancing the redox stability of anions, contributing to the application of LMR.

Enhancement of Magnetic Stability in Antiferromagnetic CoO Films by Adsorption of Organic Molecules.

Antiferromagnets are a class of magnetic materials of great interest in spintronic devices because of their stability and ultrafast dynamics. When interfaced with an organic molecular layer, antiferromagnetic (AF) films are expected to form a spinterface that can allow fine control of specific AF properties. In this paper, we investigate spinterface effects on CoO, an AF oxide. To access the magnetic state of the antiferromagnet, we couple it to a ferromagnetic Co film via an exchange bias (EB) effect. In this way, the formation of a spinterface is detected through changes induced on the CoO/Co EB system. We demonstrate that C60 and Gaq3 adsorption on CoO shifts its blocking temperature; in turn, an increase in both the EB fields and the coercivities is observed on the EB-coupled Co layer. Ab initio calculations for the CoO/C60 interface indicate that the molecular adsorption is responsible for a charge redistribution on the CoO layer that alters the occupation of the d orbitals of Co atoms and, to a smaller extent, the p orbitals of oxygen. As a result, the AF coupling between Co atoms in the CoO is enhanced. Considering the granular nature of CoO, a larger AF stability upon molecular adsorption is then associated with a larger number of AF grains that are stable upon reversal of the Co layer.

Structure of small yttrium monoxide clusters, chemical bonding, and photoionization: threshold photoionization and density functional theory investigations.

The photoionization (PI) spectra of small gas-phase yttrium monoxide clusters, YnO (n = 1-8), are investigated, and the adiabatic ionization energies are determined. The stable structures are obtained from density functional theory (DFT) calculations. The ground state structures are further confirmed by the CCSD(T) method. The PI spectra are calculated for these stable structures and are compared with the experimental PI spectra. The ground-state structures of the neutral and cation clusters are experimentally assigned with confidence on the basis of a favourable agreement between the experimental and calculated PI spectra. New structures are proposed for Y2O, Y6O, and Y8O compared to the previous literature. Y2O is a linear molecule in the ground state that was previously proposed as a C2v bent molecule. The YnO clusters become 3-dimensional from n ≥ 3. The O atom stays outside, bridging a triangular face of yttrium clusters. Chemical bonding between the yttrium and oxygen atoms is mostly ionic. The excess charge on the oxygen atom is around 1.4e-, transferred from the yttrium atoms bonded with it. Yttrium atoms are mostly covalently bonded. However, for the bigger clusters, free charges of both polarities appear on yttrium atoms that are not bonded with oxygen, indicating ionic interactions. Frontier orbitals consist of mainly delocalized 4d electrons with some 5s contributions, forming Y-Y bonding interactions, but with little contribution and zero contribution from the oxygen orbitals, regardless of the cluster size. The lost electron of YnO+ mostly comes from the 5s orbitals of all Y atoms in the cluster up to size n = 4, and then from 4d-5s hybrid orbitals from n ≥ 5, with the d contribution increasing with size. This is contrary to the previous view in the literature that photoionization occurs from a localized 4d orbital.

Experimental research progress of charge order of nickelate based superconductors

Ever since the discovery, nickelate superconductors have attracted great attention, declaring a “nickel age” of superconductivity. Currently, there are two types of nickelate superconductors: low-valence nickelate superconductors RE<sub><i>n</i>+1</sub>Ni<sub><i>n</i></sub>O<sub>2<i>n</i>+2</sub> (RE, rare earth; <i>n</i>, number of adjacent NiO<sub>2</sub> layers) and high-pressure nickelate superconductors La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> and La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>. Charge order plays a crucial role in studying the strongly correlated systems, especially the cuprate superconductors, in which potential correlation between charge order and superconductivity has been indicated. Thus, great efforts have been made to explore the charge order in nickelate superconductors. In the infinite-layer nickelate RENiO<sub>2</sub>, the evidence of charge order with in-plane wavevector of <i> <b>Q</b> </i><sub>//</sub> ≈ (1/3, 0) has been found in the undoped and underdoped regime but not in the superconducting samples. However, subsequent studies have indicated that this is not the true charge order inherent in the NiO<sub>2</sub> plane,which carries unconventional superconductivity, but rather originates from the ordered excess apical oxygen in the partially reduced impurity phases. On the other hand, the overdoped low-valence nickelate La<sub>4</sub>Ni<sub>3</sub>O<sub>8</sub> shows well-defined intertwined charge and magnetic order, with an in-plane wavevector of <i> <b>Q</b> </i><sub>//</sub> = (1/3, 1/3). Resonant X-ray scattering study has found that nickel orbitals play the most important role in the multi-orbital contribution of charge order formation in this material, which is significantly different from the cuprates with oxygen orbitals dominating the charge modulation. Although the spin order in La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> has been well established, there is still controversy over its spin structure and the existence of coexisting charge order. In La<sub>4</sub>Ni<sub>3</sub>O<sub>10</sub>, intertwined charge and spin density waves have been reported, the origin and characteristics of which remain unknown. Owing to the research on the nickelate superconductors just starting, many questions have not yet been answered, and the exploration of charge order in nickelate superconductors will still be the center of superconductor research.

Modulating Fe spin state in FeNC catalysts by adjacent Fe atomic clusters to facilitate oxygen reduction reaction in proton exchange membrane fuel cell

Iron,nitrogen-codoped carbon (FeNC) has emerged as promising alternatives to precious metals for oxygen reduction reaction (ORR). Herein, we demonstrate that the ORR activity of FeNC can be markedly enhanced by the incorporation of adjacent Fe few-atom clusters (FeAC), where the octahedral field of FeAC boosts the splitting of the parallelogram field of FeN4, and facilitates the transition from high-spin (t2g3eg2) Fe(III)N4 to medium-spin (t2g5eg1) Fe(II)N4 and hence the interaction with the π* antibonding orbitals of oxygen. This leads to a remarkable ORR performance due to optimized desorption of the OH* intermediate on FeN4, with a half-wave potential of +0.80 in 0.1 M HClO4, in comparison to that with only FeN4 single-atom moieties. In H2-O2 fuel cell tests, a high peak power density of 0.80 W cm−2 is obtained. Results from this work highlight the significance of spin engineering in the manipulation and optimization of the ORR activity of single-atom catalysts.

Magnetic Field-Enhanced Oxygen Evolution Reaction via the Tuneability of Spin Polarization in a Half-Metal Catalyst.

The magnetic field response of an electrochemistry process, such as the oxygen evolution reaction (OER), provides not only a strategy for enhanced catalytic activity by applying an external field but also a platform for revealing the functionality of the multiple degrees of freedom of the catalyst. However, the mechanism of the magnetic field tuneable OER is controversial. The strong correlation between the d and p orbitals of transition metal and oxygen still puzzles the dominant role of spin in an OER process. Here in this study, we have employed the manganite La0.7Sr0.2Ca0.1MnO3 as the ferromagnetic OER catalyst, which has a ferromagnetic/paramagnetic transition (TC) around the room temperature. It is found that the overpotential can be reduced by ∼18% after applying a 5 kOe magnetic field. Furthermore, this magnetic field can trigger a further improvement of the OER performance, and it demonstrates a strong temperature dependence which is incongruent with its magnetoresistive behavior. So our experiments suggest that the observed magnetic response originates dominantly from the triplet state of the O2, where the spin-polarized d and oxygen p orbitals lower the Gibbs free energy for every reaction step in OER. This study offers experimental evidence on comprehending the spin degree in the OER process, meanwhile benefiting the further design and engineering of the promising magnetic electrochemistry catalysts.

Electronic interaction Upp on oxygen p orbitals in oxides: Role of correlated orbitals on the example of UO2 and TiO2

We carry out a detailed study of the role of electronic interaction on $p$ oxygen orbitals in a Mott insulator oxide (${\mathrm{UO}}_{2}$) and a charge transfer oxide (${\mathrm{TiO}}_{2}$). First, we calculate values of effective interactions ${U}_{ff}, {U}_{pp}$, and ${U}_{fp}$ in ${\mathrm{UO}}_{2}$ and ${U}_{dd}, {U}_{pp}$, and ${U}_{dp}$ in ${\mathrm{TiO}}_{2}$. Second, we analyze the role of electronic interactions ${U}_{pp}$ on $p$ orbitals of oxygen in spectral and structural properties. Finally, we show that this role depends strongly on the definition of correlated orbitals and that using Wannier functions leads to more physical results for spectral and structural properties.

Robust ferromagnetism in a cubic perovskite oxide with Curie temperature above 600 K

Robust ferromagnetism with high Curie temperatures (TC), especially above room temperature, in perovskite oxides is extremely rare because the oxygen-mediated antiferromagnetic superexchange generally dominates in these nearly cubic perovskite lattices. Fe4+ ions show potential to promote magnetism responses due to more apparent hybridization between Fe4+ and oxygen orbitals with Jahn-Teller distortion, while this distortion also sacrifices the stability of Fe4+-based oxides. Here, by engineering oxygen vacancies, we report robust ferromagnetism with TC above 600 K in cubic perovskite BaFeO3-δ films epitaxially grown on KTaO3 substrates. A mixture of Fe4+ and Fe3+ cations, mediated by the oxygen vacancies, was identified to be responsible for the robust magnetism as revealed by atomic-scale studies and element-selective circularly polarized X-ray absorption spectra. This study indicates that spin-state ordering could be manipulated in a simple perovskite oxide by engineering oxygen stoichiometry and would be of significance toward future technological applications.

Particle-hole asymmetry in the dynamical spin and charge responses of corner-shared 1D cuprates

Although many experiments imply that oxygen orbitals play an essential role in the high-temperature superconducting cuprates, their precise role in collective spin and charge excitations and superconductivity is not yet fully understood. Here, we study the doping-dependent dynamical spin and charge structure factors of single and multi-orbital (pd) models for doped one-dimensional corner-shared spin-chain cuprates using several numerically exact methods. In doing so, we determine the orbital composition of the collective spin and charge excitations of cuprates, with important implications for our understanding of these materials. For example, we observe a particle-hole asymmetry in the orbital-resolved charge excitations, which is directly relevant to resonant inelastic x-ray scattering experiments and not captured by the single-band Hubbard model. Our results imply that one must explicitly include the oxygen degrees of freedom in order to fully understand some experimental observations on cuprate materials.

Absence of Luther-Emery superconducting phase in the three-band model for cuprate ladders

Correlated-electron theories of superconductivity in layered cuprates often start from the premise of a gapped spin-liquid phase proximate to the superconducting state. This assumption is justified based on analytical and numerical demonstrations of a superconducting Luther-Emery phase in the doped 2-leg one-band Hubbard ladder, and the perceived analogy between coupled ladders and the two-dimensional ${\mathrm{CuO}}_{2}$ layer. We demonstrate from accurate density matrix renormalization group studies the absence of the superconducting Luther-Emery phase in the doped 2-leg three-band ladder consisting of both copper and oxygen, even as the spin gap is large in the undoped three-band ladder. For realistic oxygen-oxygen hopping and Hubbard repulsion on the oxygen atoms, density-density rather than pairing correlations are dominant at long range. This result is equally valid whether or not the oxygens outside the ladder proper, over and above the rung and leg oxygens, are included in the computation. These results demonstrate the critical importance of oxygen orbitals, and raise disturbing questions about the applicability of many of the existing correlated-electron theories of superconductivity.

Polaron hopping conduction mechanism and magnetic properties of Pb‐doped LaMnO3

AbstractIn the present work, we have synthesized La1‐xPbxMno3 (0 ≤ x ≤ 0.20) perovskites through a conventional solid‐state reaction route. The powder x‐ray diffraction data analyzed through the Rietveld refinement program confirm monophase rhombohedral distorted structure (R‐3c space group). Fourier transform infrared spectra exhibit strong and well‐defined absorption bands of a typical perovskite structure. The x‐ray photoelectron spectroscopy studies confirm the elemental composition and oxidation state of various elements in the samples. Low temperature resistivity measurements do not reveal metal–semiconductor transition (MST) in the pristine sample while Pb‐doped samples show MST. The metallic region of the temperature‐dependent resistivity data is well fitted with ρ(T) = ρο + ρ2.5T2.5, which reveals the significance of electron–magnon interaction. Moreover, the semiconducting behavior of all the samples follows the Mott's variable range hopping conduction process. Additionally, the semiconducting part is also fitted with the small‐polaron hopping (SPH) model that establishes the non‐adiabatic SPH mechanism obeyed for the lower doping, whereas adiabatic mechanism for the higher Pb concentration. Magnetic studies clearly indicate a paramagnetic to weak ferromagnetic transition associated with the Pb doping that may be due to the double‐exchange mechanism between the manganese ions via oxygen orbitals. The outcomes are elucidated on the basis of substitution effect induced by Pb2+ ions.

Tomographic reconstruction of oxygen orbitals in lithium-rich battery materials.

The electrification of heavy-duty transport and aviation will require new strategies to increase the energy density of electrode materials1,2. The use of anionic redox represents one possible approach to meeting this ambitious target. However, questions remain regarding the validity of the O2-/O- oxygen redox paradigm, and alternative explanations for the origin of the anionic capacity have been proposed3, because the electronic orbitals associated with redox reactions cannot be measured by standard experiments. Here, using high-energy X-ray Compton measurements together with first-principles modelling, we show how the electronic orbital that lies at the heart of the reversible and stable anionic redox activity can be imaged and visualized, and its character and symmetry determined. We find that differential changes in the Compton profile with lithium-ion concentration are sensitive to the phase of the electronic wave function, and carry signatures of electrostatic and covalent bonding effects4. Our study not only provides a picture of the workings of a lithium-rich battery at the atomic scale, but also suggests pathways to improving existing battery materials and designing new ones.

Interplay between Zhang-Rice singlets and high-spin states in a model for doped NiO2 planes

Superconductivity found in doped NdNiO$_2$ is puzzling as two local symmetries of doped NiO$_2$ layers compete, with presumably far-reaching implications for the involved mechanism: a cuprate-like regime with Zhang-Rice singlets {\cblue is replaced by local triplet states at realistic values of charge-transfer energy, which would suggest a rather different superconductivity scenario from high-$T_c$ cuprates}. We address this competition by investigating Ni$_4$O$_8$ clusters with periodic boundary conditions in the parameter range relevant for the superconducting nickelates. With increasing value of charge-transfer energy we observe upon hole doping the expected crossover from the cuprate regime dominated by Zhang-Rice singlets to the local triplet states. We find that smaller charge-transfer energy $\Delta$ is able to drive this change of the ground state character when realistic values for nickel-oxygen repulsion $U_{dp}$ are taken into account. For large values of the charge-transfer energy, oxygen orbitals are less important than in superconducting cuprates as their spectral weight is found only at rather high excitation energies. However, a second Ni($3d$) orbital can easily become relevant, with either the $xy$ or the $3z^2-r^2$ orbitals contributing in addition to the $x^2-y^2$ orbital {\cblue to the formation of triplet states. In addition,} our result that $U_{dp}$ (acting between Ni and O) favors onsite triplets implies that correlation effects beyond purely onsite interactions should be taken into account when obtaining effective two-band models.

Orbital structure of the effective pairing interaction in the high-temperature superconducting cuprates

The nature of the effective interaction responsible for pairing in the high-temperature superconducting cuprates remains unsettled. This question has been studied extensively using the simplified single-band Hubbard model, which does not explicitly consider the orbital degrees of freedom of the relevant CuO2 planes. Here, we use a dynamical cluster quantum Monte Carlo approximation to study the orbital structure of the pairing interaction in the three-band Hubbard model, which treats the orbital degrees of freedom explicitly. We find that the interaction predominately acts between neighboring copper orbitals, but with significant additional weight appearing on the surrounding bonding molecular oxygen orbitals. By explicitly comparing these results to those from the simpler single-band Hubbard model, our study provides strong support for the single-band framework for describing superconductivity in the cuprates.

Bypassing the multireference character of singlet molecular oxygen, part 1:1,4‐cyclo‐addition

AbstractModeling reactions involving singlet molecular oxygen (O2 [1Δg]) is challenging because the degeneracy of the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) orbitals of oxygen causes a significant multireference character. Within the limit that singlet‐singlet near‐degeneracy disappears in the transition state, it would be possible to bypass singlet oxygen's multireference character by simply adding the experimentally determined singlet/triplet splitting (22.5 kcal/mol) to the energy of the triplet ground state of molecular oxygen. This method is tested by calculating rate constants for the reactions of singlet molecular oxygen with furan, 2‐methylfuran, 2,5‐dimethylfuran, pyrrole, 2‐methylpyrrole, 2,5‐dimethylpyrrole, and cyclopentadiene using transition state theory. We find that the reaction rate coefficients are within a factor of 15 of experimentally determined rate constants, indicating an error in the barrier energy of roughly 3 kcal/mol. Furthermore, we find that energy refinement at the CCSD(T)‐F12 level of theory is crucial to achieving accurate results. We conclude that, based on a comparison with an experiment, this approximation is valid to some degree and can be used for other systems involving the 1,4‐cyclo‐addition of singlet oxygen.

Influence of BiFeO3 crystallographic phase on the enhanced interfacial magnetization in BiFeO3/La2/3Sr1/3MnO3 heterostructures

Large induced magnetization is observed in rhombohedral-like BiFeO3 (RBFO) when grown on a ferromagnetic La2/3Sr1/3MnO3 layer. This magnetization survives up to room temperature and is strongly influenced by the deposition conditions and crystallographic phase of BiFeO3. The magnetization in RBFO is induced by the Fe–O–Mn ferromagnetic (FM) superexchange (SE) interaction, after a strong hybridization between Fe and Mn orbitals mediate through oxygen orbitals, which enhances the spin canting in a BFO layer near the interface. The reduction in the magnetization of BFO due to change in its growth conditions is caused by the altered oxygen content in the interfacial region, which has a direct impact on the FM SE Fe–O–Mn coupling at the interface. In marked contrast, the induced magnetization in the super-tetragonal phase (TBFO) of BFO, in which the canting of Fe spins is forbidden by the symmetry, shows a comparatively very small moment, which vanishes below the room temperature (∼200 K). We propose that the ferromagnetic ordering induced in TBFO is limited to the adjoined section only, whereas it extended to a certain degree of bulk in RBFO. A spin-glass like state is also observed in BiFeO3/La2/3Sr1/3MnO3 heterostructure with irreversible temperature ∼100 K.