Ni 3d States Research Articles

The electronic and optical properties of Ni-doped Bi4O5I2: First-principles calculations

The electronic and optical properties of Ni-doped Bi4O5I2 systems were investigated via first-principle calculations. For the pristine Bi4O5I2, the band gap was calculated to be 2.17 eV, which was close to the experimental report. When the concentration of Ni increased, the band gap obviously decreased, which was ascribed to the introduction of Ni 3d doping states in the forbidden band gap. Based on the dielectric function analysis, the variations of the optical properties mainly took place in the visible light range, and the additional peak in the visible light range originated from the electron transition between the Ni 3d and Bi 6p states. Accordingly, the light absorption in the visible light range obviously increased, which was favorable for photocatalytic applications.

Chemical interaction and electronic structure in a compositionally complex alloy: A case study by means of X-ray absorption and X-ray photoelectron spectroscopy

Chemical interaction and changes in local electronic structure of Cr, Fe, Co, Ni and Cu transition metals (TMs) upon formation of an Al8Co17Cr17Cu8Fe17Ni33 compositionally complex alloy (CCA) have been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. It was found that upon CCA formation, occupancy of the Cr, Co and Ni 3d states changes and the maximum of the occupied and empty Ni 3d states density shifts away from Fermi level (Ef) by 0.5 and 0.6 eV, respectively, whereas the Cr 3d empty states maximum shifts towards Ef by 0.3 eV, compared to the corresponding pure metals. The absence of significant charge transfer between the elements was established, pointing to the balancing of the 3d states occupancy change by involvement of delocalized 4s and 4p states into the charge redistribution. Despite the expected formation of strong Al–TMs covalent bonds, the Al role in the transformation of the TMs 3d electronic states is negligible. The work demonstrates a decisive role of Cr in the Ni local electronic structure transformation and suggests formation of directional Ni–Cr bonds with covalent character. These findings can be helpful for tuning deformation properties and phase stability of the CCA.

Vacancy defects induced changes in the electronic and optical properties of NiO studied by spectroscopic ellipsometry and first-principles calculations

Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO, oxygen-rich NiO with Ni vacancies (NiO:VNi), and Ni-rich NiO with O vacancies (NiO:VO). The optical properties were obtained by spectroscopic ellipsometry, while valence band spectra were probed by high-resolution x-ray photoelectron spectroscopy. The experimental results are directly compared to first-principles density functional theory + U calculations. Computational results confirm that gap states are present in both NiO systems with vacancies. Gap states in NiO:Vo are predominantly Ni 3d states, while those in NiO:VNi are composed of both Ni 3d and O 2p states. The absorption spectra of the NiO:VNi sample show significant defect-induced features below 3.0 eV compared to NiO and NiO:VO samples. The increase in sub-gap absorptions in NiO:VNi can be attributed to gap states observed in the electronic density of states. The relation between native vacancy defects and electronic and optical properties of NiO are demonstrated, showing that at similar vacancy concentration, the optical constants of NiO:VNi deviate significantly from those of NiO:VO. Our experimental and computational results reveal that although VNi are effective acceptors in NiO, they also degrade the visible transparency of the material. Hence, for transparent optoelectronic device applications, an optimization of native VNi defects with extrinsic doping is required to simultaneously enhance p-type conductivity and transparency.

First-principles study on structural, mechanical, and electronic properties of disordered Pt1-xNix alloys

Pt-Ni alloys have gained widespread attention due to the excellent properties and wide applications. However, the research on the structural, electronic and mechanical properties of disordered Pt1-xNix solid solutions remain unclear and not enough. In this work, the first-principle calculation is used to investigate the structural, mechanical properties, and electronic structure of Pt1-xNix solid solutions, which will be compared to the reference of PtmNin intermetallic compounds. It is found that the atomic interaction of Pt1-xNix solid solutions being weakened as the increase of Ni content, due to the stronger metallic characteristics. The Pt1-xNix solid solutions show stable structure and high solid solubility. The ductile nature of Pt1-xNix solid solutions is reduced, in compared to pure Pt. It is found the ductile nature of Pt1-xNix solid solutions rebounds with a little degree in the range of Ni content (x) from 0.85 to 1.0. On the other hand, the stiffness is enhanced in the whole x range, in compared to pure Pt. The catalytic ability of Pt1-xNix solid solutions is predicted to be enhanced in the range of 0 <x < 0.8. The investigation on electronic structure of Pt1-xNix solid solutions indicates that the variation of above properties is related to the interaction behavior between Pt-5d states and Ni-3d states. By using the first-principle calculations, the underlying mechanism that causes the variation of properties is clarified in this work. This provides theoretical guidance for the application of Pt1-xNix solid solution as structural materials and catalysts.

Investigation of martensitic transformation behavior in Ni-Mn-In Heusler alloy from a first-principles study

Composition dependence of martensitic transformations as well as the magnetic properties for the Ni2Mn1+xIn1-x (0.25 ≤ x ≤ 0.58) alloys were investigated by using the first-principles calculations. Key results demonstrate that the stability of parent austenite (A) decreases gradually with increasing Mn content whilst it is opposite for the martensitic phase. This causes the total energy difference between the austenite and martensite phases increscent with increasing Mn contents. When x = 0.33, the martensite transformation during cooling is PA→FA→NM. When x ≥ 0.42, an intermartensitic transformation occurs from modulated 6 M martensite to non-modulated (NM) martensite with the martensite transformation sequence of PA→FA→6M→NM. The martensitic transformation from austenite to martensite accompanies the transition from ferromagnetic to ferrimagnetic state. This is a typical magneto-structural coupling transformation. The analysis of the density of states demonstrates that the Ni 3d state plays an important role in the phase stability.

Intermediate valence of CeNi2Al3 compound and its evidences: Theoretical and experimental approach

We present measurements of magnetic, transport and electronic properties obtained for polycrystalline CeNi2Al3 intermetallic compound. Magnetic susceptibility χ(T) was investigated in the range from 2 to 700 K, and its behavior is characteristic of a compound with unstable valence, varying between Ce3+ and Ce4+. In the temperature range down to 2 K there was no trace of magnetic order, no anomalies in the temperature dependence of the specific heat were found. The Sommerfeld coefficient extracted from the linear term of the heat capacity takes a value of γ = 21 mJ/(mol K2). The dependence of S(T) is linear up to about 25 K, which is symptomatic of a thermopower in the Fermi's liquid regime.The structure of satellites in the Ce(3d) electron spectrum obtained by the X-ray photoelectron spectroscopy (XPS) method indicates that the states of Ce(4f) are of mixed valence character. Analysis of Ce(3d) states based on Gunnarsson-Schönhammer theory shows that the energy of hybridization of Ce(4f) states with a conduction band is about 78 meV. For more detailed information about electronic states the fully relativistic band structure was calculated within the density functional theory (DFT) for the first time. Below Fermi's energy, the density of states is mainly formed by Ni(3d) states hybridized with Ce(4f) ones.

First principles study of electrocatalytic behavior of olivine phosphates with mixed alkali and mixed transition metal atoms.

Lithium transition metal olivine phosphates are well known Li-ion battery cathode materials, but these materials can also be used as electrocatalyst. Recent experimental studies showed that olivine phosphates with mixed alkali metals (Li and Na) and mixed transition metals (Ni and Fe) provide better electrocatalytic activity compared to single alkali and transition metal alternatives. In the current work, we analyzed the role of alkali metals, transition metals and vacancies on the reactivity of a series of olivine phosphates with different stoichiometries using first principles calculations. To this end, we investigated the adsorption of water at the surface of these materials. We found that water binds preferably at Ni surface sites for materials devoid of alkali ion vacancies. We further found correlation between the calculated adsorption energy with experimentally measured overpotentials for a series of olivine phosphates. Additionally, we found correlation between the adsorption energy of the systems with the total charge polarization of surface and adsorbate. To explain the computed trends, we analyzed the occupancies of the partial density of states of the Ni and Fe 3d states and Bader atomic charges.

Observations of the Nickel Layer in the Mesopause Region at Mid-Latitudes

Observations of the mesospheric Ni layer have been performed by lidar in January-March 2018 at Kuehlungsborn/Germany (54°N, 12°E). These soundings provide only the second Ni data set after initial observations by Collins et al. at Chatanika/Alaska (65°N, 147°W)[1]. We utilized for the first time a transition from the low-lying excited Ni(3D) state at 341 nm. For all soundings, nightly mean peak densities varied between ~280 cm−3 and 450 cm3, which is a factor of ~40 less than previously reported for Chatanika [1]. The observed Ni abundance is especially important if compared with the abundance of other metals like Fe, and with their respective abundances in evaporating meteoroids, which form the source of the metal layer in the upper mesosphere. Here, we present exemplarily a sounding from January 8, 2018. Beside the Ni raw data and density profiles we show a temperature profile as measured simultaneously be the co-located RMR lidar and the temperature variation due to gravity waves and tides.

Electronic Structures and f-d Orbital Hybridization of a Heavy Fermion YbNiGa

The electromagnetic and thermal properties of a heavy fermion YbNiGa are investigated using first-principle methods with local density approximation (LDA) and LDA+U approaches. The Yb f-bands are located near the Fermi energy and hybridized with the Ni-3d states. This hybridization plays important roles to make characteristics of this material. Band calculation with the LDA framework does not match with that of experimental result because of the strong correlation interaction between f orbitals. The calculation shows that the specific heat coefficient underestimates the experiment value by a factor of 38. The discrepancy is attributed to the formation of quasiparticle. The exchanging interaction between the local f electron and the conduction d electrons will result in the formation of quasiparticle. The exchange interaction between the f and the conduction electrons is large, which makes enhancement the specific heat coefficient.

Strong Jahn-Teller effect at NiO4 tetrahedron in NiCo2O4 spinel

The local structural distortion and electronic structure of the NiCo2O4 spinel are studied from first-principles calculations. It is found that very strong Jahn-Teller (J-T) distortion occurs to the NiO4 tetrahedron and a large energy gain is obtained, leading to a substantial elongation of the lattice and a cubic to tetragonal phase transition. Further analysis shows that the Ni-3d states are split by not only the broken Td symmetry of the NiO4, chemical bonding interactions also exist between half-occupied dyz and dxz orbitals, which further lower the energy levels of the occupied dyz and dxz states in the spin-up channel. As a result, the energy gains from the J-T distortion are obtained to be 0.53 eV, 0.84 eV, and 0.83 eV per NiCo2O4 formula unit for using LDA, PBE and PW91 functionals, respectively.

Electronic and optical properties of HoNi5 and HoNi4Si: An LSDA + U study

We investigated the effect of Si substitution on structural, electronic and optical properties of [Formula: see text]-type [Formula: see text] alloys. The optimized lattice constants and internal cell parameters are in agreement with the available data. We found that the valence band is mainly dominated by Ni-3d states in the energy range 0–4 eV below the [Formula: see text], whereas conduction band is contributed by spin-down Ho-[Formula: see text] states and lies about 2 eV above the [Formula: see text]. Substitution of Si atoms for Ni decreases the total magnetic moment from 6.308 [Formula: see text]/f.u. [Formula: see text] to 4.052 [Formula: see text]/f.u. [Formula: see text], whereas magnetic moments on [Formula: see text]-ions increase from 3.92 [Formula: see text]/atom [Formula: see text] to 4 [Formula: see text]/atom [Formula: see text]. On the other hand, induced moment on Ni[Formula: see text]-ions decrease rapidly to a negligibly small value. By the use of charge density estimates, we found that Ho-5d, Ni-3d and Si-3p orbitals are mainly involved in bonding and there is a weak hybridization between Ni-3d and Si-3p orbitals which varies with substitution of Si for Ni. Furthermore, the complex role played by Ho-[Formula: see text] electrons is also investigated, they are found to be partly involved in metallic bondings as well as in the intra-atomic charge transfer from [Formula: see text] to [Formula: see text] states in [Formula: see text]-ions. The calculated interband optical conductivity spectra reproduce the main features of experimental spectra well and also reveal their metallic nature.

Fabrication and Study on Magnetic-Optical Properties of Ni-Doped ZnO Nanorod Arrays.

Zn1-xNixO nanorod arrays were prepared on Si substrates by magnetron sputtering and hydrothermal methods at 100 °C. We studied the effects of doped concentration and hydrothermal growth conditions on the crystal structure, morphology, photoluminescence, and magnetic properties of Zn1-xNixO nanorod arrays. The research results show that the Zn1-xNixO nanorod have the hexagonal wurtzite structure without the appearance of the second phase, and all samples have a highly preferred orientation of a (002) crystal face. The Zn1-xNixO nanorod arrays exhibit obvious room temperature ferromagnetism with saturation magnetization at 4.2 × 10−4 emu/g, the residual magnetization is 1.3 × 10−4 emu/g and the coercive field is 502 Oe, and also excellent luminescent properties with seven times greater luminous intensity than that of ZnO nanorod arrays. The redshift of the ultraviolet emission peak was found by Ni2+ doping. We further explained the source and essence of the magnetic properties of Zn1-xNixO nanorod arrays and deemed that the magnetic moment mainly comes from the hybrid electron exchange of O 2p and Ni 3d state.

Effects of Ni and Cu Antisite Substitution on the Phase Stability of CuGa2 from Liquid Ga/Cu–Ni Interfacial Reaction

Ga and Ga-based alloys have received significant attention for applications in the liquid state and also for their potential as a bonding material in microelectronic assemblies. This study investigates the phase stability of the CuGa2 phase as a product of the interfacial reaction between liquid Ga and Cu-10Ni substrates at room temperature. In the binary Ga-Cu system, CuGa2 is decomposed into liquid Ga and Cu9Ga4 as the temperature increases to around 260 °C, which prevents the widespread application of this alloy. In contrast to CuGa2 grown from a pure Cu substrate, CuGa2 from the Cu-10Ni substrate shows an increase in the decomposition temperature during heating from 25 to 300 °C. According to our first-principle calculations, there is only a minor difference in the total free energy between Ni solute at the Cu sublattice and the Ga sublattice in the tetragonal CuGa2 crystal structure. This result indicates that both of the sublattices can accommodate the dilute Ni solute with comparable probability. Regardless of the sublattice where the Ni impurities are located, the presence of diluted Ni in the matrix stabilizes the CuGa2 system by inducing some localized Ni 3d states at energy levels near the Fermi level. It is also shown that the formation of Cu antisite defects, which also stabilizes CuGa2, is preferable if the CuGa2 matrix is grown on a Ni-containing substrate.

Electronic and magnetic properties of Ni2MnGa: Monte-Carlo and ab-initio calculations

This paper describes computational calculations based on the density functional theory (DFT), in order to investigate the electronic and magnetic properties of Ni2MnGa alloys. The local density approximation (LDA) and generalized gradient approximation (GGA) methods were used, which are implemented in the MACHIKANEYAMA package designed and made by Akai [1]. As result, we found that the alloys possess a ferromagnetic behaviour with magnetic moment of 4,07μB/cell. The analysis of the density of states (DOS) shows that the alloys have a half-metallic behaviour due to the Mn-d and Ni-d states crossing the Fermi level. Moreover, the Monte-Carlo simulation method in the framework of the Ising model gives the magnetization and the susceptibility values, respectively.

Intermediate Modulation on Noble Metal Hybridized to 2D Metal-Organic Framework for Accelerated Water Electrocatalysis

Summary The rational modulation of reaction intermediates is critical to achieving high-performance heterogeneous catalysis. However, practical realization remains challenging. Using water electrocatalysis as a model reaction, we report an interfacial-bond-induced intermediate modulation to accelerate the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by hybridizing a 2D nickel metal-organic framework (MOF) and Pt nanocrystals into a heterostructure. The design is initially verified via theoretical calculations, indicating charge relocation on newly formed Ni–O–Pt interfacial bonds for increased OH* adsorption energy together with reduced H* adsorption energy. Experimental findings confirm the formation of the Ni–O–Pt bonds, which electronically modify the heterostructure to increase electron density for Pt and the high-energy Ni 3d state for the MOF, optimizing the adsorption for H* and OH*, respectively. This hybridized material delivered HER activity of 7.92 mA μg−1 Pt, which is among the best reported for alkaline electrocatalysts, and an improved OER activity.

Europium incorporation dynamics within NiO films deposited by sol-gel spin coating: Experimental and theoretical studies

Europium doped NiO thin films were grown onto glass substrates via sol-gel spin coating. The evolution of crystal structure, surface topography, and optical properties of NiO was investigated as a function of Eu doping concentration. AFM observations and XRD analysis revealed that NiO:Eu films composed of nano-sized grains exhibit a polycrystalline cubic bunsenite structure with (200) preferential orientation. Optical study showed that the optical band gap of doped NiO films initially increased with low Eu incorporation ratio then decreased with higher Eu content. However, the Urbach energy increased gradually with Eu doping level. First principle calculations were performed to elucidate the changes on electronic, magnetic and optical properties of NiO due to Eu doping. The incorporation of Eu within NiO host lattice induced a strong hybridization between Eu-f and Ni-d states resulting in important changes of bulk magnetization.

Hard X-ray spectroscopic methods using emitted X-ray to understand charge compensation in positive electrode materials for lithium-ion batteries

We examined the applicabilities of hard X-ray spectroscopic methods to understand the charge-compensation (CC) behaviors of Ni in a LiNiO2-based positive electrode (PE) material for lithium-ion batteries: Ni K pre-edge high energy resolution fluorescence detection X-ray absorption spectroscopy (XAS) and Ni 1s2p resonant X-ray emission spectroscopy (RXES). Both methods exhibited sequential changes in spectral features with the change in the state of charge of the PE material, similarly to Ni L2,3-edge XAS. This elucidates that these hard X-ray spectroscopic methods can become alternatives to soft X-ray L2,3-edge XAS with the additional ability to measure reactive samples kept in a sealed pouch. Moreover, Ni 1s2p RXES can separate the electronic transition to the unoccupied Ni orbital from those to hybridized orbitals with neighboring atoms by tuning the incident X-ray energy. In this sense, it can provide spectral information reflecting the unoccupied Ni 3d state, which could be a good oxidation-state descriptor. These methods can be done in a usual hard X-ray beamline having an appropriate crystal analyzer. The ability to offer information on the unoccupied 3d state with hard X-ray and the ubiquitous feature of these methods would prompt in situ/operando studies on CC in PE materials for rational materials development.

Internal exchange interaction of Tb+3 ions and influence of Si substitution on structural, electronic, magnetic and optical properties of YNi4Si -type TbNi5−xSix (x = 0, 1, 2) compounds

We report the FPLAPW + lo study on new YNi4Si-type TbNi5−xSix (x = 0, 1, 2) compounds. The YNi4Si structure is a new structure type, which is an orthorhombic derivative of CaCu5 structure. The optimized lattice constants and internal cell parameters are in agreement with the available data. The calculated band structures show the valence band (VB) is mainly dominated by Ni–3d states in the energy range 0–4 eV below the EF, whereas conduction band (CB) by Tb–4f states about 2 eV above the EF. With the substitution of Si atoms for Ni, total magnetic moments decreases from 8.308 μB/f.u. (x = 0) to 6.052 μB/f.u. (x = 1), whereas magnetic moments on Tb+3ions increase from 5.92 μB/atom (x = 0) to 6 μB/atom (x = 1, 2) and the induced moments on Ni+2(x = 1, 2) ions decrease rapidly to a negligibly small value. Charge density estimates show that Tb-5d, Ni-3d and Si-3porbitals are mainly involved in bondings and there is a weak hybridization between Ni-3d and Si-3porbitals which varies with Si substitution. Furthermore, the complex role played by the Tb-5d1 electrons is also investigated, and found to be partly involved in metallic bondings as well as in intra-atomic charge transfer from 5d1 to 4f8 states. The calculated interband optical conductivity spectra reproduce the main features of the experimental spectra well and also reveal their metallic nature. Frequency-dependent refractive index, η(ω), the reflectivity, R(ω) and the extinction coefficient, κ(ω) for (x = 0, 1, 2) are also calculated for the incident radiation up to 12 eV.

Understanding the magneto-structural coupling of Ni50Mn35.4In14.6 alloy from first-principles calculations

Abstract The magneto-structural coupling of non-stoichiometric Ni50Mn35.4In14.6 alloy is investigated by the first-principles calculations in view of phase stability and magnetic properties of the austenite, the six-layered modulated (6M) martensite, and the non-modulated (NM) martensite. The calculated martensitic phase transformation sequence of the Ni50Mn35.4In14.6 alloy is Paramagnetic Austenite (PA) → Ferromagnetic Austenite (FA) → Ferrimagnetic 6M (FIM 6M) → Ferrimagnetic NM (FIM NM) as a function of temperature. The Mn-Mn interatomic distance reduces significantly when the FA parent phase transforms to a 6M or NM martensitic phase. At the same time, magnetism of the alloy changes from ferromagnetic to ferrimagnetic; this is a typical magneto-structural coupling phenomenon. The Ni 3d state electrons play an important role in driving martensitic transformation for the Ni50Mn35.4In14.6 alloy.

The physical properties of ThCr2Si2- type nickel-based superconductors BaNi2T2 (T = P, As): An ab-initio study

Employing first-principles computations depending on the density functional theory (DFT), we have investigated the structural, mechanical, electronic, bonding, optical and thermodynamics properties of the newly discovered bulk superconductors BaNi2P2 (Tc ∼ 3 K) and BaNi2As2 (Tc ∼ 0.7 K). Our optimized lattice parameters are in good agreement with the experimental results. The positive elastic constants reveal that both the superconductors are stable in nature. The analysis of the mechanical properties insures that both the phases are ductile in nature and show anisotropic behavior. The analysis of the electronic band structures and density of states (TDOS and PDOS) reveals the metallic nature for both the compounds, and the major contribution comes from the Ni-3d states for both the phases. The calculations of the chemical bonding specify that a mixture of covalent, ionic and metallic bonds exist in bothsuperconductors. The negative value of the real part of the dielectric constant of BaNi2T2 (T = P, As) also ensures that these compounds exhibit metallic behavior. The large reflectivity in the low energy region indicates that both the compounds might be useful as coating materialsfor reducing solar heating. By using the elastic constants information, the calculated Debye temperatures of BaNi2P2 and BaNi2As2 are 323.70 K and 272.94 K, respectively, which are in good accord with the experimental values. Finally, we have calculated the thermal conductivity of these compounds, which is 0.56 for BaNi2P2 and 0.46 for BaNi2As2.