Ni 3d Electrons Research Articles

Synthesis, structural phase transition and weak itinerant magnetism in NixNbSe2

The article presents the synthesis, structural and magnetic properties of NixNbSe2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25). The intercalation limit of Ni in NbSe2 is found to be 25 mol%, above which NiSe appears as a binary impurity phase. Single crystal X-ray diffraction studies show that all the compounds crystallize in the hexagonal P63/mmc space group. However, there is a distinct phase transition beyond x = 0.20 with Ni intercalation in NixNbSe2. The NixNbSe2 compounds crystallized in the parent 2H phase with x value between 0 ≤ x ≤ 0.2. Subsequently, the phase transforms into the 2H(I) phase with an order superstructure of 2 ao × 2 ao × 1 co for 0.20 ≤ x ≤ 0.25 in NixNbSe2. The superstructure formation is attributed to the occupation of Ni at the alternate octahedral voids present in the van der Waals gap of the NbSe2. The XPS results and theoretical analysis clearly show that the intercalation of Ni results in a more metallic character in NixNbSe2. Magnetic measurements on NixNbSe2 (x = 0.05, 0.1, 0.2, 0.25) shows very small magnetic moment at low temperature for all the compounds. Theoretical calculation on Ni0.25NbSe2 shows that the Ni-3d electrons are delocalized and are very weakly spin-polarized, which may be the reason for the experimentally observed weak magnetism in these compounds.

Strain-bandgap quasi-linear correlation applied to modulation of NiBr2 monolayer optical properties

Recently, two-dimensional transition metal dihalides have shown diverse applications in spintronics, quantum devices, and optoelectronic devices. In this work, density functional theory (DFT) is adopted to investigate the effect of biaxial strain upon the electronic structure and optical properties of NiBr2 monolayer. The results show that NiBr2 is an indirect semiconductor with band gap 2.62 eV (DFT + U) and 3.18 eV (HSE06), which could vary quasi-linearly from 2.73 eV to 1.71 eV modulated by biaxial strain (6 % to −10 %). Combined with the analysis of the density of states (DOS), the primary reason for the alteration in the band gap is that strain significantly affects the behavior of Ni 3d electrons. When the strain changes from positive to negative, the DOS of Ni 3d shifts towards the Fermi level, resulting in a reduction in the band gap. Within this strain range, all of the optical properties exhibit a quasi-linear tendency similar to the band gap variation. The optical peaks of NiBr2 under compressive strain are not only red-shifted into the visible light (VL) region but also significantly enhanced in the VL and ultraviolet (UV) regions. Applying strain can greatly boost the electrical and optical properties of NiBr2 monolayer, making it a promising candidate in the field of ultraviolet detectors.

Investigation on the physical properties of Ni doped SrTiO3 by first-principle calculations

In present work, the magneto-electronic and optical features of Sr1-xNixTiO3 (x = 12.5%, 25%, 50% and 75%) compounds are calculated using full potential linearized augmented plane wave (FP-LAPW) scheme within density functional theory (DFT) as employed in WIEN2k software. The electronic band structures (BS) and density of states (DOS) interpret the induced half metallic ferromagnetism mainly originating from highly spin polarized Ni-d states. The computed value of total magnetic moment of Sr1-xNixTiO3 is 1.99998, 1.99991, 2.00003 and 2.00005 µB at 12.5%, 25%, 50% and 75% concentration respectively, which emerge primarily due to Ni-3d electrons. Furthermore, the optical features (refraction, dielectric function, absorption, and reflectivity) have also been computed within energy range of 0-10 eV. Sr1-xNixTiO3 is optically active in visible to ultraviolet (UV) region owing to low reflectivity and high absorption. Results portray that the studied compound is a potential contender for its usage in the development of spintronic and optoelectronic devices.

Photoinduced Phase Transition in Infinite-Layer Nickelates.

The quantum phase transition caused by regulating the electronic correlation in strongly correlated quantum materials has been a research hotspot in condensed matter science. Herein, a photon-induced quantum phase transition from the Kondo-Mott insulating state to the low temperature metallic one accompanying with the magnetoresistance changing from negative to positive in the infinite-layer NdNiO2 films is reported, where the antiferromagnetic coupling among the Ni1+ localized spins and the Kondo effect are effectively suppressed by manipulating the correlation of Ni-3d and Nd-5d electrons under the photoirradiation. Moreover, the critical temperature Tc of the superconducting-like transition exhibits a dome-shaped evolution with the maximum up to ≈42K, and the electrons dominate the transport process proved by the Hall effect measurements. These findings not only make the photoinduction a promising way to control the quantum phase transition by manipulating the electronic correlation in Mott-like insulators, but also shed some light on the possibility of the superconducting in electron-doped nickelates.

Dual Ni active sites mediated by In to separate ethane activation and oxidation for enhanced ethene production via chemical looping scheme

Chemical looping oxidative dehydrogenation (CL-ODH) opens a unique avenue to achieve highly efficient conversion of alkanes to value-added alkenes. The challenge lies in identical metal-oxo active sites responsible for both alkane activation and oxidation causing either insufficient activity or over-oxidation thus inferior ethene yield and productivity at relatively low temperature (< 650 °C). Herein, we designed a novel redox catalyst by the incorporation of main-group In (6NiInx/HY, x indicates the atomic ratio of In/Ni) which achieved more than 92 % ethene selectivity at 40 % ethane conversion and long-term stability at 600 °C with 0.22 mmol g−1 ethene productivity at 650 °C. This was because Ni2+ Lewis acid sites conducted catalytic dehydrogenation of ethane to ethene and hydrogen, and the Ni-O-In moiety outside HY framework enabled selective oxidation of hydrogen rather than ethene due to well-regulated Ni 3d electron densities leading to the reduction of adsorption for ethene thus decreasing over-oxidation.

First Principles Calculations of the Optical Response of LiNiO2

We discuss optical properties of layered Lithium Nickel oxide (LiNiO2), which is an attractive cathode material for realizing cobalt-free lithium-ion batteries, within the first-principles density functional theory (DFT) framework. Exchange correlation effects are treated using the generalized gradient approximation (GGA) and the strongly-constrained-and-appropriately-normed (SCAN) meta-GGA schemes. A Hubbard parameter (U) is used to model Coulomb correlation effects on Ni 3d electrons. The GGA+U is shown to correctly predict an indirect (system wide) band gap of 0.46 eV in LiNiO2, while the GGA yields a bandgap of only 0.08 eV. The calculated refractive index and its energy dependence is found to be in good agreement with the corresponding experimental results. Finally, our computed optical energy loss function yields insight into the results of recent RIXS experiments on LiNiO2.

Structural, electronic and optical studies of Sr2NiTeO6 double perovskite by first-principle DFT–LDA + U calculation

The structural, electronic and optical properties of monoclinic Sr2NiTeO6 double perovskite have been studied using corrected density functional theory–local density approximation with applying of Hubbard U corrected energy (DFT–LDA + U) method in plane-wave pseudopotential basis. The influence of on-site Coulomb interaction on structural, electronic and optical properties of Sr2NiTeO6 double perovskite compound, which consists of strongly localized Ni 3d electrons, were investigated. The highly Coulomb repulsion between electrons was corrected using Hubbard U parameter, varying from 0 to 8 eV for Ni 3d orbitals. The calculated results demonstrated that Sr2NiTeO6 double perovskite was sensitive to the change in U values. The optimized structural properties give a good agreement with experiment and other calculation data with lattice parameters a = 5.673 Å, b = 5.565 Å, c = 7.847 Å, α = 89.999°, β = 90.257° and γ = 90.000°. The DFT–LDA + U predicted the calculated electronic band structure of Sr2NiTeO6 was showed metallic behaviour (0 and 2 eV) and insulator behaviour (4, 6 and 8 eV). The density of state (DOS) shows that there was a significant effect on hybridization of Ni 3d and O 2p states at the conduction and valence band, respectively. Moreover, the results of optical studies such as dielectric function, absorption and reflectivity were found significant to the variation of U values applied indicates that the U values give a better description on the electronic localization of Ni 3d states.

Soft X-ray absorption spectroscopy and magnetic circular dichroism under pulsed high magnetic field of Ni-Co-Mn-In metamagnetic shape memory alloy

In this study, X-ray absorption spectroscopy (XAS) experiments for Ni45Co5Mn36.7In13.3 metamagnetic shape memory alloy were performed under high magnetic fields up to 12 T using a pulsed magnet. Field–induced reverse transformation from martensite phase to austenite phase caused considerable changes in the magnetic circular dichroism (MCD) signals and the magnetic moments of the Mn, Ni, and Co with ferromagnetic coupling were determined. The spin magnetic moment, Mspin, and orbital magnetic moment, Morb, of Mn atom in the induced austenite ferromagnetic phase, estimated based on the sum-rule analysis, were 3.19 and 0.08 μB, respectively, resulting in an Morb / Mspin ratio of 0.03. In the element-specific magnetization curves obtained at 150 K, metamagnetic behavior associated with the field–induced reverse transformation was clearly observed and the transformation magnetic fields could be detected. There was almost no change in the magnetically averaged XAS spectrum for Mn-L2,3 edges between in the martensite and in the magnetic field-induced austenite phases, however, it was visible for Ni. This implies that Ni 3d-electrons mainly contribute to martensitic transformation.

A Ni nanoparticles encapsulated in N-doped carbon catalyst for efficient electroreduction CO2: Identification of active sites for adsorption and activation of CO2 molecules

Electrochemical CO2 reduction (ECR) offers a promising strategy to achieve global carbon balance and mitigate the global climate. However, it is still a challenge to fabricate excellent catalyst and illustrate the active site for adsorption and activation of CO2 molecules. Herein, we develop a reduced graphene oxide-supported N-doped carbon-encapsulated nickel (Ni@N-C/rGO) catalyst for ECR to CO. The optimal Ni@N-C/rGO(4,4′-bipy) catalyst exhibits high catalytic activity, selectivity and stability with a current density of 20 mA cm−2 and Faradaic efficiency of 88% toward CO at −0.97 V vs. RHE for 10 h. The well-designed poisoning and control experiments deduce that the catalytic activity originates from the N-doped carbon layer coated on Ni particles. Density functional theory calculations suggest that pyrrolic-N is the optimal active site for the adsorption and activation of CO2 molecules, and the spontaneous charge transfer from Ni 3d electrons to the π orbitals of the N-C skeleton promotes the easy desorption of *CO from the active sites, thereby improving the ECR activity.

Effect of Ni doping on optical, structural, and morphological properties of ZnO thin films synthesized by MSILAR: Experimental and DFT study

In this study the thin films of Zinc Oxide (ZnO) and Nickel (Ni) doped ZnO were successfully synthesized by the simple modified successive ion layer adsorption and reaction method, with 30 cycles of dipping a substrate directly in cationic and anionic precursors without rinsing in water. The effect of nickel doping with this method was investigated. The X-ray diffraction (XRD) analysis confirms the wurtzite structure of ZnO in the thin films, without any secondary phases. A decrease in grain size and cell parameters was observed. Scanning electron microscopy (SEM) with energy dispersive X-Ray (EDX) analysis showed that Ni significantly affects the morphology of thin films. UV-visible spectra are obtained in the wavelength range between 300 nm to 1100 nm. A decrease in band gap from 3.23 eV to 3.11 eV was observed. First principal calculation was used to reveal the effect of Ni on electronic and magnetic properties. It showed that ZnO and Ni doped ZnO exhibits a direct band gap semiconductor and Ni-3d electrons are the primary source of magnetic momentum in ZnO matrix.

Magnetic, optoelectronic and thermodynamic properties of the double perovskite Ba2NiWO6 in its stable antiferromagnetic phase

In this paper, we have investigated the structural, magnetic, electronic and thermodynamic properties of the double perovskite Ba2NiWO6 (BNW) using density function theory (DFT) implemented in Wien2k code. The full-potential linearized augmented plane wave (FP-LAPW) method based on the new potential approximation known as the Tran-Blaha modified Beckee-Johnson exchange potential approximation (mBJ) was applied. The double perovskite Ba2NiWO6 with cubic structure was more stable in the antiferromagnetic configuration compared to the other phases. The magnetic moments calculated in Ba2NiWO6 were found to come specifically from Ni-3d electron states. With mBJ approach, the energy band gap obtained was 2.48 eV. In addition, thermodynamic properties predicted in this work, seem to be important determinants for the synthesis of such materials.

Magnetic and electronic properties of layered Sr2NiO2Cl2 with square planar coordination

Local coordination geometry is crucial in determining the physical properties of transition metal compounds. In this work, we report a detailed density functional theory study on the magnetic and electronic properties of layered Sr2NiO2Cl2, which possesses an unusual square planar coordination. Due to the crystal field and spin splitting, Ni 3d electrons fully occupy the dxz, dyz, and dxy orbitals, while partially occupy the dz2 and dx2-y2 states, leading to the high spin state of Ni2+ ions and strong antiferromagnetic in-plane nearest-neighbor exchange interaction J1. Thus, Sr2NiO2Cl2 is predicted to exhibit a G-type magnetic ground state, which all nearest-neighbor magnetic moments are antiparallel to each other in the NiO2 plane. A band gap is opened irrespective of the specific magnetic state since the unique square planar crystal field splitting. Finally, large orbital moments and giant easy-plane magnetocrystalline anisotropy energy are obtained, which is again related to the unique coordination environment.

Dimensionality Control of Electrocatalytic Activity in Perovskite Nickelates.

The dimensionality of the crystal structure plays an important role in the electronic structures of materials. Ruddlesden-Popper perovskite oxides offer an attractive platform for studying this role due to dimensional flexibility. The effects of dimensionality on physical properties in those oxides have been widely reported. However, the study of dimensional dependence on the chemical properties is still lacking. Here, we synthesized a series of Ruddlesden-Popper perovskite nickelates LanSrNinO3n+1 (n = 1, 2, 3, and ∞) to explore the role of dimensionality on oxygen-evolution reaction (OER) performance. As the dimensionality increased with n, the nickelates exhibited an enhanced OER activity. We found that the weakening of electron correlations among Ni 3d electrons by increasing the dimensionality induced an insulator-to-metal transition and a strengthened Ni-O hybridization, both of which accelerated the OER kinetics. This work sets up a bridge between the dimensionality and electrocatalysis, which provides guidance for designing highly efficient oxygen-evolving catalysts.

Specific heat, thermal conductivity, electron transport and electronic band properties of CeNi9In2

We report the results of specific heat Cp(T), thermal conductivity κ(T), thermoelectric power S(T), magnetoresistance MR(H) and Hall coefficient RH(H,T) measurements on an intermediate valence (IV) CeNi9In2 compound in the temperature range 2–300 ​K and in magnetic fields up to 9 ​T. In addition, the data of DFT calculations using FP-LMTO ​+ ​LDA and FP-LAPW ​+ ​PBE-GGA methods are presented. The studied sample reveals an enhanced Sommerfeld coefficient, metallic behaviour in κ(T), S(T), MR(H) and RH(T) properties, but does not exhibit clear crossover phenomenon expected for IV compounds. The DFT studies enunciate a weak 4f-3d hybridization and a major input to density of states at the Fermi level coming from the Ni 3d-electrons. These findings indicate that the contribution of the Ni atoms on the physical properties prevails over that of CeNi9In2 and hence the characteristics of IV of the Ce atoms cannot be demonstrable in the measured physical quantities.

The spin-resolved electronic structure of doped crystals si < Ni > and Si < B, Ni>: theoretical and experimental aspects

The spin-resolved electronic structure of doped crystals Si < Ni > and Si < B, Ni > has been evaluated within the generalized gradient approximation (GGA) framework. The strong correlation effects of Ni 3d electrons are treated by means of hybrid exchange-correlation functional PBE0. In the first stage, we obtained an electron energy spectrum for a supercell Si15Ni1, in which the Si atom is replaced by the Ni atom. Next, we performed calculations for the supercell Si14B1Ni1, in which the silicon atoms are replaced by boron and nickel atoms. We found that these two materials are ferromagnetic semimetals. Then were considered two materials, presented by supercells Si15B1Ni1, in which the silicon atom has been substituted by boron atom and nickel atom was situated first in the tetrahedral, and then in the octahedral void. Both materials turned out to be paramagnetic metals. The total binding energy of the material with nickel in the octahedral void was only 0.0014 eV lower, compared with that for the material with nickel in the tetrahedral void.

Correlation between structure and glass-forming ability in Al86Ni14-xLax(x = 3,5,9) alloys: An ab initio molecular dynamics study

The correlation of atomic and electronic structures with glass-forming ability (GFA) in Al86Ni14-xLax (x = 3,5,9 at.%) alloys were systematically investigated via ab initio molecular dynamics simulations for understanding the mechanism of GFA in Al-based alloys. The obtained glass transition temperatures indicate that GFA is better in Al86Ni9La5 alloy, consistent with experimental measurements. In contrast to most other alloy systems where topological order and atomic packing efficiency are key to GFA, in Al-based alloys the chemical short-range order around Al atoms play more important role in GFA. It is revealed that chemically random distribution of Al atoms favors GFA. Meanwhile, Ni and La atoms are more chemically repulsed and preferred to Al atoms, respectively. Moreover, the electronic structure calculations demonstrate that the Al-3p and Ni-3d electrons are strongly hybridized, which is found to be critical in determining Fermi surfaces and GFA in Al86Ni14-xLax (x = 3,5,9) alloys. Taken the hybridization effect into account, the coincidence between good GFA and the local minimum of the electronic density of states at the Fermi level is obtained and verified for the first time by ab initio calculations. This finding provides new physical understanding for designing better Al-based bulk MGs.

First principle study on atomic structure of La65X35(X=Ni, Al) metallic glasses

The atomic-level structure of metallic glasses (MGs) is one of the most fundamental and challenging topics in condensed matter physics. Unlike crystalline metals or alloys, the MGs are lacking in a well-defined description of structure order, which is a major obstruction for relating its structure to physical properties. Obviously, it is vitally important to have an in-depth understanding of the atomic packing scheme in MGs. Due to the limitations of experimental characterization methods, it is hard to obtain the atomic packing scheme of MGs in experiment. Computational simulation on an atomic scale has become an important method of characterizing the atomic structure of MGs. The La-based LaNiAl glass forming system is well-known for its good glass-forming ability, distinctive relaxation peak that is well separated from relaxation, and liquid-liquid transition at a temperature around 1000 K. Many efforts have been made to investigate these novel properties. However, the atomic structure of this system is rarely studied. In this paper, the atomic structure evolution from liquids to glass states in La-based binary MGs La65Ni35 and La65Al35 are studied via ab initio molecular dynamics based on the density functional theory. The local structures are systematically analyzed by the radical distribution function, partial radical distribution function (PRDF), Voronoi tessellation method, and bond-type method in Honeycutt-Andersen. The results indicate that the PRDF of NiNi is much weaker than that of AlAl, which indicates the NiNi avoidance in La65Ni35. The major peaks of PRDFs are always smaller than the sum of efficient radius of the two kinds of atoms, especially for LaNi pairs. Atomic structure of the two systems are coincident with dense atomic packing scheme and the difference between major Voronoi polyhedron types (0, 3, 6, 0 for La65Ni35 and 0, 2, 8, 1, 0, 2, 8, 0 for La65Al35) in local structures is controlled by their ratio of solute to solvent atomic size. The high five-fold local symmetry structure gradually increases in both systems with the decrease of temperature, which validates its pivotal part in hindering crystallization. The electronic structure is studied with the partial density of states. It is found that the significant bond-shortening between La and Ni is due to the strong hybridization between Ni-3d and La-5d electrons and this result may play a key role in understanding composition related structure and properties in MGs.

Orbital-Specific observation of O2p and Ni3d electrons in LiNi0.5Mn0.5O2, a cathode material for lithium-ion batteries

Cathode materials for lithium-ion batteries containing Ni2+ have attracted much interest because of their high theoretical capacity. However, the precise electronic structures of these cathode materials have not yet been clearly observed, especially the energy positions of the O2p and Ni3d orbitals and the shape of the density of states. The aim of this study was to investigate the relative energy positions and shape of the density of states of O2p and Ni3d for LiNi0.5Mn0.5O2 experimentally. We cleaved a LiNi0.5Mn0.5O2 pellet in an Ar-filled glove box and performed synchrotron ultraviolet photoelectron spectroscopy for different photon energies, which enabled us to investigate the relative cross-section intensity of O2p and Ni3d. As a result, the valence-band structure was determined. We found that O2p electrons are itinerant and exist in the vicinity of the Fermi energy more than Ni3d electrons. Ni3d electrons are more localized and spread mainly from 1.2–1.5 eV below the Fermi energy. To validate the electronic structure, we measured the synchrotron O K-edge X-ray absorption fine structure of electrochemically lithium-extracted LiNi0.5Mn0.5O2. The electronic structure demonstrated that ligand holes in the oxygen atoms form below the Fermi level during the initial stage of Li extraction and that the formation rate of the holes decreases with Li extraction.

Element specific electronic states and spin-flip-like behavior of Ce in (Ce0.2Gd0.8)Ni composed of heavy fermion CeNi and ferri-magnet GdNi through XMCD method

The electronic states of the three constituent elements in the crystal mixed system between CeNi and GdNi, Ce0.2Gd0.8Ni, were investigated by soft X-ray magnetic circular dichroism (XMCD) with the aid of sum rule analysis. Not only Gd 4f but also Ni 3d and Ce 4f electrons were magnetic and both magnetic moments of Ni and Ce were coupled anti-parallel to the direction of the Gd magnetic moment, which is in accordance with the general rule in rare earth (RE)-transition metal (TM) systems. After saturation, Ce magnetic moment decreased over 2T and this behavior was explained by a spin-flip behavior of the Ce magnetic moment with keeping their electronic states unchanged. Furthermore, the magnetic field dependence on a part of the XAS at Ce M4,5 absorption edges, which had been observed in the Gd=0.5 disappeared in the present sample in Gd-rich content of 0.8 and this could be explained by the increase of molecular field from Gd on Ce 4f electrons. In addition, sum rule analysis has revealed that the magnetic moments of Gd 4f and Ni 3d electrons could retain small values of angular (orbital) magnetic moments µL and this was explained consistently.

Temperature-dependent electronic structure of EuNi2P2 revealed by angle-resolved photoemission spectroscopy

We studied the electronic structure of EuNi2P2, which exhibits both heavy-fermion and mixed-valence behaviors, using angle-resolved photoemission spectroscopy. Multiple Ni 3d bands were observed near the Fermi energy, and one of them forms a hole-like Fermi surface around the X point of the Brillouin zone. We also found that the spectral weight of the Ni 3d states is rapidly enhanced with decreasing temperature, which is consistent with the temperature dependence of the mean valence of Eu ions. Our results thus demonstrate hybridization between the Ni 3d and Eu 4f electrons in EuNi2P2.