Ni 3d Band Research Articles

Site preference of Re in NiAl and valence band structure of NiAl containing Re: First-principles study and photoelectron spectrum

The site preference of Re in NiAl was studied using first-principles calculations. The calculation of formation energies of the NiAl alloys indicated the site preference of Re on the Ni sites. The valence band structures of the NiAl alloys were investigated by photoelectron spectroscopy. The valence band spectra of the NiAl with Re shifted away from the Fermi energy level so that the Ni d-band centroid moved to a higher energy by 0.25 eV as Re was added. Such a shift could be attributed to the Ni-Re interaction, which was supported by the photoelectron spectroscopy measurement.

X-ray photoelectron spectroscopy and magnetism of Mn1−x Alx alloys

Abstract X-ray photoelectron spectroscopy (XPS), magnetization and magnetic susceptibility of Mn1−x Alx (x = 0.0, 0.2, 0.4, 0.5, 0.6, 0.7) alloys are reported. X-ray diffraction measurements showed that all investigated samples have the same crystallographic structure as the parent compound (AuCu3-structure type). The alloys are disordered for x ≤ 0.5, but become almost crystallographically ordered for higher Al concentration. This change in the crystallographic order is reflected both in the magnetization and Curie temperature values. The exchange interaction is ferromagnetic between the pairs of the near-neighbour Mn-Ni and Ni-Ni magnetic moments and antiferromagnetic for Mn-Mn pairs. The last one is present only in the disordered alloys, which leads to smaller values of the magnetization of these alloys in comparison with the ordered ones. The Mn magnetic moment has the fully ordered value of 3.2 μB in all investigated alloys. The decrease of the Ni magnetic moment as the Al concentration increases may be explained by the hybridization of the Ni 3d and Al 3sp states, which leads to a partial filling of the Ni 3d band. The magnetic susceptibility measurements pointed out the existence of spin fluctuations on Ni sites.

Theoretical study of methane adsorption on perfect and defective Ni(1 1 1) surfaces

We employ periodic density functional theory (DFT-GGA-PW91) calculations to study the adsorption of CH 4, on a perfect and defective (1 1 1) face of nickel, at a coverage of 0.25 monolayer (ML). As a surface defect, we consider a Ni adatom. We investigate systematically the site preference for CH 4, for various molecular orientations with 1, 2 or 3 H pointing toward the surface. Whatever the CH 4 adsorption site could be, the most stable configurations are obtained when 2 H atoms are directed to the surface. CH 4 stabilises weakly on the flat Ni surface, the adsorption energies being at best in the 50 meV range. However, beside a dominating physical interaction, some features are indicative of a chemical interaction through the Ni d-band. In presence of a Ni adatom, the chemical nature of the interaction manifests plainly, with sizeable adsorption energies up to 0.37 eV. The molecular restructuring and the mechanism of the interaction are examined.

Electronic structure of the LaNi 5-xGa x intermetallic compounds

The equilibrium structures and electronic structure of LaNi5-xGax (x=0, 0.5, 1.0) compounds have been investigated by all-electron calculations. Based on the full geometry optimization, the densities of states and electron densities of LaNi5-xGax are plotted and analyzed. It is clear that the substitution of Ga at the Ni site leads to a progressive filling of the Ni-d bands, the ionic interaction between Ni and Ni, with Ga plays a dominant role in the stability of LaNi5-xGax compounds. The smaller the shift of E_F toward higher energy region, the more stable the compounds will be. The increased contribution of the Ni-d-Ga-d interactions near E_F and the low energy metal-gallium bonding bands indicate that the compounds become more stable. The results are compared with experimental data and discussed in light of previous studies.

Density functional theory study on LaNi 4.5 Al 0.5 hydride phase: electronic properties and sites occupation

In this paper the crystal structure, electronic structure and hydrogen site occupation of LaNi4.5Al0.5Hy hydride phase (y = 5.0, 6.0) have been investigated by using full-potential linearized augmented plane wave method. The hydrogen atoms were found to prefer the 6m, 12o and 12n sites, while no 4h sites were occupied. A narrowed Ni-d band is found due to the lattice expansion, the total density of states at EF increases with y, which indicates that the compounds become less stable. The interaction between Al and Ni, H plays a dominant role in the stability of LaNi4.5Al0.5Hy hydride phase. The smaller the shift of EF towards the higher energy region, the more stable the compounds will be. The obtained results are compared with experimental data and discussed in the light of previous works.

Energy-loss near-edge structure (ELNES) and first-principles calculation of electronic structure of nickel silicide systems

The electronic structures of nanometre-sized nickel silicide systems, Ni 2Si and NiSi, have been studied by energy-loss near-edge structure (ELNES) and first-principles band structure calculations. Experimental ELNES of Ni L 3- and Si L 2,3-edges could be explained well using theoretical spectra calculated for the ground state without the core hole, suggesting metallic properties for both silicides. It was shown that a slight difference in ELNES spectra of Ni 2Si and NiSi comes from the coupling among the Ni d and Si p, d states in the unoccupied bands. The density of states and the contour plots of all the valence electron densities for Ni 2Si, NiSi together with NiSi 2 show that Ni 2Si has the bond with the strongest covalent character between Ni and Si atoms and the most transition metal-like character of the Ni 3d band among the three silicides.

Electronic structure and site occupation for the intermediate phase of LaNi 4.5Al 0.5H y

The crystal structure, electronic structure and hydrogen site occupancy of LaNi 4.5Al 0.5H y intermediate phase ( y = 2.0 , 3.0, 4.0) have been investigated using the full-potential linearized augmented plane wave (FLAPW) method. For the first time we analyzed the interstitial site occupation of hydrogen atoms. The H atoms were found to prefer the 6m, 3f and 12n sites, while no 4h sites were occupied. A narrowed Ni-d band is found due to the lattice expansion: the total DOS at E F increases with y , which indicates that the compounds become less stable. The interaction between Al and Ni, with H plays a dominant role in the stability of LaNi 4.5Al 0.5H y intermediate phase. The smaller the shift of E F towards the higher energy region, the more stable the compounds will be. Our results are compared with experimental data and discussed in the light of previous works.

Effect of phase formation on valence band photoemission and photoresonance study of Ti/Ni multilayers using synchrotron radiation

This paper presents investigation of Ti–Ni alloy phase formation and its effect on valence band (VB) photoemission and photoresonance study of as-deposited as well as annealed Ti/Ni multilayers (MLs) up to 600 °C using synchrotron radiation. For this purpose [Ti (50 Å)/Ni (50 Å)]X 10 ML structures were deposited by using electron-beam evaporation technique under ultra-high vacuum (UHV) conditions. Formation of different phases of Ti–Ni alloy due to annealing treatment has been confirmed by the X-ray diffraction (XRD) technique. The XRD pattern corresponding as-deposited ML sample shows crystalline nature of both Ti and Ni deposited layers, whereas 300 °C annealed ML sample show solid-state reaction (SSR) leading to amorphization and subsequent recrystallisation at higher temperatures of annealing (≥400 °C) with the formation of TiNi, TiNi 3 and Ti 2Ni alloy phases. The survey scans corresponding to 400, 500 and 600 °C annealed ML sample shows interdiffusion and intermixing of Ni atoms into Ti layers leading to chemical Ti–Ni alloys phase formation at interface. The corresponding recorded VB spectra using synchrotron radiation at 134 eV on as-deposited ML sample with successive sputtering shows alternately photoemission bands due to Ti 3d and Ni 3d, respectively, indicating there is no mixing of the consequent layers and any phase formation at the interface during deposition. However, ML samples annealed at higher temperatures of annealing, particularly at 400, 500 and 600 °C show a clear shift in Ni 3d band and its satellite peak position to higher BE side indicates Ti–Ni alloy phase formation. In addition to this, reduction of satellite peak intensity and Ni 3d density of states (DOS) near Fermi level is also observed due to Ti–Ni phase formation with higher annealing temperatures. The variable photon energy VB measurements on as-deposited and ML samples annealed at 400 °C confirms existence and BE position of observed Ni 3d satellite structure. The observed changes and modifications in the VB photoemission are discussed and interpreted in terms of structural changes at the interface due to annealing treatment.

Structure, transport and magnetic properties of MgNi 3B 2

The structural and physical characterization of the ternary magnesium nickel boride MgNi 3B 2 is reported. The crystal structure, studied by single-crystal and powder X-ray diffraction techniques, is hexagonal, hP18- P6 422 (no. 181), lattice constants a = 4.880(1) Å, c = 8.787(1) Å, with complete ordering of the three atomic species over different crystallographic sites. The present results remove the uncertainty as regards the true composition of the ternary compound reported previously in the literature as ‘Mg 2Ni 5B 4’, ‘MgNiB’ or ‘MgNi 3B 2’, whose relevance is due to the observation that it is found as a parasitic phase in the preparation of superconducting MgB 2 wires, when using Ni or Ni-alloy tubes. The electrical resistivity of MgNi 3B 2 shows metallic behaviour with no evidence of superconductivity down to at least 1.8 K. High values of the Debye temperature θ D (>600 K) are obtained both from the resistivity and heat capacity data, indicating a relatively rigid lattice. The susceptibility is diamagnetic at room temperature, which shows that the Ni-3d bands are completely filled in this compound.

Photoemission studies and electronic structure ofU2T2In (T = Ni,Rh, Pt) compounds

The electronic structure of the tetragonalU2T2In (T = Ni,Rh, Pt) compounds in the paramagnetic phase were studied by x-ray photoelectronspectroscopy (XPS). Both valence band and core level spectra were analysed.The experimental data are compared with the calculations of the densityof states using the tight-binding linear muffin-tin orbital method (TB-LMTO)and full-potential local-orbital full-relativistic method (FPLO). The calculateddata reveal a dominant U 5f electron character for the states near the Fermi levelEF with a small contribution from U 5d, Ni 3d, Rh 4d, Pt 5d and In 5p states. The XPS valence bandsof these compounds are characterized by a sharp peak of the U 5f states near the Fermi level(EF) and broad peaks of the Ni 3d, Rh 4d and Pt 5d states at about 2.6, 3.2 and 4.0 eV belowEF, respectively. The small change in the position of the U 5f peak with respect toEF is−0.35 eV forT = Ni and−0.15 eV forT = Rh and Pt. A satellitebetween the Ni 2p1/2 and Ni 2p3/2 peaks is visible, suggesting that the Ni 3d band is not completely filled, and theexistence of a small induced magnetic moment on the Ni atoms cannot be ruledout.

First principles study of electronic structure of CeNi 4Cu

The results of the electronic structure investigation of CeNi 4Cu with the hexagonal CaCu 5-type structure have been presented. The results show domination of the 3d electronic states of Ni below the Fermi energy as well as the participation of 3d electronic states of Cu in the density of states in the energy region from −5 eV to −1 eV. The itinerant electrons of Ni and Cu are found to play a significant role in increase of the degree of hybridisation of Ce 4f with Ni 3d bands at the Fermi level. The hybridisation effects in valence band of CeNi 4Cu are discussed. The calculated value of electronic specific-heat coefficient does not suggest a heavy-fermion behaviour in the studied system.

First-principles calculations on the superconductivity and magnetism of doping MgCNi3

We use the first-principles method to calculate the electronic band structures of MgCNi3. The calculated results show that the hybridization between C 2p and Ni 3d electron orbits leads to the planar characteristic of Ni 3d band. The Fermi level is located on the right slope of the van Hove singularity (vHs) peak. The high density of states (DOS) at the Fermi level with vHs and large magnetic fluctuations near the ferromagnetic phase transition point are the important factors for the superconductivity of MgCNi3. We study the variance of superconductivity and magnetism for three types of substitute doping in MgCNi3, and find that the electron doping moves the Fermi level to the lower DOS side, and transforms MgCNi3 to paramagneticm phase without superconductivity. The inter-metal compounds with the same valence electrons doping change the shape of Fermi surface, resulting from the hybridization between atoms. The decrease of the DOS decreases their superconductivity. The hole doping enhances the DOS at the Fermi level on the peak of vHs, but the large magnetic exchange interactions induce the ferromagnetic order, and the superconductivity disappears abruptly.

Comparison of hybrid density functional, Hartree–Fock, and GW calculations on NiO

AbstractThe electronic band structure of NiO in the ferromagnetic state is calculated using the B3LYP hybrid density functional, the Hartree–Fock (HF) method, and the GW approximation (GWA) with dielectric functions constructed using either B3LYP or HF wave functions and energy eigenvalues. The band structure from the GWA calculation based on B3LYP wave functions is quite similar to the parent B3LYP band structure; the main differences are in valence bandwidth and in the upward shift of the empty minority spin Ni 3d bands relative to the same bands in the B3LYP calculation. The band structure from the GWA calculation based on HF wave functions differs in that there are large upward shifts in valence band positions in the GWA calculation relative to the HF calculation, which result from screening of the bare exchange term in the Fock operator. Matrix elements of the HF exchange operator are obtained using wave functions from self‐consistent HF or B3LYP calculations. Magnitudes of these matrix elements for several states at the Γ point of the Brillouin zone are compared, and it is found that the only major difference occurs in the empty minority spin bands derived from Ni 3d states of e symmetry. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Revelation of Ni magnetic moment in GdNi single crystal by soft X-ray magnetic circular dichroism

Magnetic moment of Ni in GdNi single crystal was studied through the soft X-ray absorption spectra (XAS) and the magnetic circular dichroism (MCD) at the Ni L 2,3-edges and the Gd M 4,5-edges. Our experiment revealed for the first time that the Ni 3d band is not filled completely even at the content of 50 at.% of Gd and the Ni does retain a total magnetic moment coupling antiparallel to that of Gd. This result implied that the Ni in GdNi holds an intrinsic magnetic moment even at 50 at.% of Gd and contradicts the well-known charge transfer model. Further by employing the magneto-optical sum rule, the spin and orbital angular magnetic moment were evaluated and discussed.

First-principles study on the crystal, electronic structure and stability of LaNi 5− xAl x ( x = 0, 0.25, 0.5, 0.75 and 1)

The crystal, electronic structures and stability of LaNi 5− x Al x ( x = 0, 0.25, 0.5, 0.75 and 1) have been investigated using a plane-wave pseudo-potential method. It shows that aluminum atoms tend not to be close to each other and prefer to be set dispersedly in all planes B (the plane where 3g sites lie) rather than in partial ones. The formation and cohesive energy of LaNi 5− x Al x alloy is calculated and analyzed. Aluminum induces Ni-d bands to be narrowed and the interaction of Ni-d with Al-sp and La-d to be weakened. The covalence and ionicity analysis shows that the interactions of Ni with Al and La are mainly covalent and ionic, respectively. The relationship of the formation energy of LaNi 5− x Al x to its electronic structures shows that the total electronic densities of states at E F changes with x with the almost same trend to the formation energy of LaNi 5− x Al x . The interaction between Ni and Al plays a dominant role in the stability of LaNi 5− x Al x .

Photoemission spectroscopy and x-ray absorption spectroscopy study of delafossite AgTO2 (T=Fe,Co,Ni)

Electronic structures of delafossite AgTO2 (T=Fe,Co,Ni) have been investigated by using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The T 2p XAS spectra of AgTO2 show that Fe and Co ions are in the trivalent states but that Ni ions are mainly in the divalent states, suggesting that the metallic nature of AgNiO2 arises from hole carriers due to divalent Ni ions. The valence-band PES study of AgTO2 (T=Co,Ni) reveals that T 3d states are located close to EF and that the Ni 3d band is more metalliclike than the Co 3d band.

Magnetic and electronic properties of NdNi5−xCux compounds

The NdNi5−xCux compounds crystallize in a CaCu5-type structure for x⩽2. Band structure calculations show that the magnetic moments, at 0K, for Ni (2c) and (3g) sites decrease when increasing Ni content and are nil for x≅2. XPS measurements, at room temperature, show the presence of unfilled Ni3d band in the entire composition range, in agreement with paramagnetic studies. The magnetic behaviour of nickel is analysed in models which take into account the electron correlation effects in d bands.

Mixed-valence and Kondo-like Effect in CeNi4X (X=B, Al, Ga)

We have employed the X-ray photoemission spectroscopy method to study the core levels of CeNi4X. The analysis of the Ce(3d) peaks in the framework of the Gunnarsson and Schonhammer model provides information on the localization degree. The hybridization parameter Δ ≈ 85, 37 and 64 meV is found for CeNi4B, CeNi4Al and CeNi4Ga, respectively. These compounds are of special interest due to the nearly filled Ni(3d) band implying a negligible contribution of Ni atoms to the resultant magnetic moment. In the temperature dependence of electrical resistivity we have observed a shallow minimum for CeNi4X (X=Al, B, Ga) below 20 K. It has been ascribed to a Kondo-like behavior.

Magnetic behavior of the Al–Gd–Ni hexagonal compounds

Abstract X–ray photoelectron spectroscopy, magnetization and magnetic susceptibility of seven Al–Gd–Ni ternary hexagonal compounds are reported. Magnetic states of Ni atoms and Gd–Ni interactions strongly depend on the nearest neighborhood of Ni ions, as well as, on the distances Ni–Ni and Ni–Gd. The hybridization between the 3d Ni and 5d6s Gd states leads to a charge transfer from Gd to Ni and consequently to a partial or complete filling of the Ni 3d-band.

Electron energy loss spectroscopic investigation of Ni metal and NiO before and after surface reduction by Ar + bombardment

Electron energy loss spectra (ELS) have been obtained from Ni metal, NiO and NiO after surface reduction by 2 keV Ar + bombardment. Two predominant loss features in the ELS spectra obtained from Ni metal are assigned as the surface and bulk plasmon excitations, and numerous other features arising from single electron transitions from both the bulk and surface Ni 3d bands to higher-lying conduction bands are also present. The surface and bulk ELS features have been unambiguously identified in this study by varying the primary beam energy. The peak assignments are based on a multitude of reported data on the electronic structure of Ni and NiO obtained using several different techniques, ranging from theoretical calculations to optical measurements and previous experimental ELS data. The ELS spectra obtained from NiO are markedly different than the Ni metal spectra as expected. The fact that the NiO spectral features do not vary appreciably with primary beam energy E p indicates that the surface electronic structure is similar to that of the bulk solid. Although most of the loss features from NiO can be attributed to interband transitions originating from the O 2p and Ni 3d states, the major peak centered at a loss energy of about 22.0 eV may be due to a collective excitation of the valence electrons. As expected, the ELS spectra obtained from the sputtered NiO surface exhibit characteristic features from both metallic Ni and NiO, with the metallic features becoming more prominent with increasing sputtering time. The most distinct change in the ELS spectra following reduction is the growth of the surface plasmon peak at 6.0 eV due to the presence of metallic Ni in the near-surface region. The loss features characteristic of bulk Ni metal are not as pronounced in the spectra obtained from the reduced NiO. These observations indicate that the metallic Ni formed during Ar + bombardment is confined to the very near-surface region, and they also demonstrate that ELS is sensitive to differences in the properties of thin metallic films or clusters and bulk Ni metal. The present results demonstrate that ELS can be used to distinguish between metallic nickel and NiO on the sputtered NiO surface.