3d Impurities Research Articles

Strain field due to transition-metal impurities in Fe

The Kanzaki lattice static method is used to calculate the strain field due to substitutional transition-metal impurities in Fe. The effective ion–ion interaction potential due to Wills and Harrison is used to calculate the dynamical matrix and the impurity-induced force up to second nearest neighbor of impurity. The atomic displacements due to 3d, 4d, and 5d substitutional transition-metal impurities (Cr, Mn, Ni, Cu, Nb, Mo, W, and Pt ) are calculated up to 24 nearest neighbors. The displacements are minimum for 3d impurities Cr and Mn and maximum for 4d impurity Nb. A similar trend is found in the calculated relaxation energy also. The calculated values are in qualitative agreement with the available experimental data.

Electronic Structure of ZnS:Co Semiconductors: X-ray and Optical Spectroscopy Studies

Experimental studies of X-ray photoelectron and Co L α X-ray emission spectra of the ZnS:Co semiconductor were carried out. It was established that Co ions are in a Co 2 + configuration and that the Co 3d impurity states are localized above the top of the valence band by 1.0 ′ 0.2 eV.

Investigation of interatomic bonding in 3C‐SiC:M by nonempirical quantum chemistry methods

AbstractIn this work, the presence of 3d metal (MTi, V, Cr and Ni) impurity atoms in the cubic silicon carbide (3C‐SiC) was simulated theoretically. Electronic structure, parameters of chemical bonding, and binding energies were calculated by the cluster density functional theory DFT approach for M substitutions in silicon, carbon, and interstitial sites. The full‐potential FLMTO technique was employed to calculate the cohesive energy for the M → Si substitutions and the crystal lattice relaxation effects around the impurity atoms. We found that for stoichiometric substitutions all 3d impurities occupy Si positions but for nonstoichiometric SiC the Ti, V(Ni), and Cr atoms may also occupy the interstitial, Si, and C sites, respectively. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

ATOMIC DISPLACEMENTS DUE TO POINT DEFECTS IN Ni DILUTE ALLOYS

The Embedded atom method has been used to investigate the strain field due to substitutional transition metal impurities in Ni. The calculations are carried out in the discrete lattice model of the metal using Kanzaki lattice static method. The results for atomic displacements due to 3d, 4d and 5d impurities (Cu, Pd, Pt and Au) in Ni are given up to 20 NN's of impurity and are compared with the earlier calculations and with the available experimental data. The maximum displacements of 3.6% of 1NN distance are found for NiAu, while the minimum displacements of 0.78% of 1NN distance are found for NiCu alloy respectively. The relaxation energy for Cu are found less than those for Pd, Au and Pt impurities in the Ni host.

Strain field due to transition metal impurities in Ni and Pd

The strain field due to body centered substitutional transition metal impurities in Ni and Pd metals are investigated. The calculations are carried out in the discrete lattice model of the metal using Kanzaki lattice static method. The effective ion-ion interaction potential due to Wills and Harrison is used to evaluate dynamical matrix and the impurity-induced forces. The results for atomic displacements due to 3d, 4d and 5d impurities (Fe, Co, Cu, Nb, Mo, Pd, Pt and Au) in Ni and (Fe, Co, Cu, Ni, Nb, Mo, Pt and Au) impurities in Pd are given up to 25 NN’s of impurity and these are compared with the available experimental data. The maximum displacements of 4.6% and 3.8% of 1NN distance are found for NiNb and PdNb alloys respectively, while the minimum displacements of 0.63% and 0.23% of 1NN distance are found for NiFe and PdFe alloys respectively. Except for Cu, the atomic displacements are found to be proportional to the core radii and d state radius. The relaxation energies for 3d impurities are found less than those for 4d and 5d impurities in Ni and Pd metals. Therefore, 3d impurities may easily be solvable in these metals.

Theoretical study of magnetism and superconductivity in three-dimensionaltransition-metal–MgB2 alloys

We have studied the electronic structure of three-dimensionaltransition-metal–MgB2 alloys, Mg0.97TM0.03B2, (TM = Sc,Ti, V, Cr Mn, Fe, Co, Ni, Cu, Zn) using the Korringa–Kohn–Rostokercoherent-potential approximation method in the atomic-sphere approximation.For unpolarized calculations, our results for Mg0.97TM0.03B2 alloys are similar tothat of 3d impurities in other s and s–p metals. In particular, the local densities ofstates (DOS) associated with the 3d impurities are similar to our earlierwork on 3d impurities in bulk Al (Singh P P 1991 Phys. Rev. B 43 3975;Singh P P 1991 J. Phys.: Condens. Matter 3 3285). For spin-polarizedcalculations, we find only the alloys of V, Cr, Mn, Fe and Co with MgB2to be magnetic of all the 3d elements. We also find that Cr and Mn inMgB2 have a relatively large local magnetic moment of 2.43 and 2.87μB,respectively. We have used the unpolarized, self-consistent potentialsof Mg0.97TM0.03B2 alloys, obtained within the coherent-potentialapproximation, to calculate the electron–phonon coupling constant λ usingthe Gaspari–Gyorffy formalism and the superconducting transition temperatureTcusing the Allen–Dynes equation. We find that the calculatedTcis lowest for Mg0.97Cr0.03B2 and highest for Mg0.97Zn0.03B2, inqualitative agreement with experiment. The calculated trend in variation ofTc fromMn to Zn is also similar to the available experimental data. Our analysis of the variationin Tc,in terms of the DOS and the spectral function along the Γ to Adirection, shows the variation to be an interplay between the total DOS atthe Fermi energy and the creation/removal of states along the Γ to Adirection (Singh P P 2002 Preprint cond-mat/0201093).

Microscopic Hamiltonian for Zn- or Ni-substituted high-temperature cuprate superconductors

We have derived the effective low energy Hamiltonian for Zn or Ni substituted high-T_c cuprates from microscopic three-band models consisting of the most relevant Cu or impurity 3d and O 2p orbitals. We find that both scattering potential and hopping integral induced by impurities have a finite range but decay very fast with distance from the impurity. The Zn scattering potential is very strong and attractive for electrons. The Ni scattering potential is much weaker than the Zn case, resulting from the hybridization between Ni ions and O holes. This profound difference is due to neither the electric charge nor d-level location, but rather because of the interplay between the valence state of the impurity and the strong correlation background. It gives a natural account for the unusual effect of Ni and Zn on the reduction of superconducting transition temperature. The interlayer hopping of electrons is highly anisotropic and nonlocal, determined by the in-plane electronic structure. This leads to a quantum interference of states from different sites and affects strongly the scanning tunneling spectrum perpendicular to CuO_2 planes.

Atomic displacements in dilute alloys of Cr, Nb and Mo

Kanzaki lattice static method is used to calculate the atomic displacements due to substitutional impurities in 3d (Cr) and 4d (Nb, Mo) metals. Wills and Harrison interatomic potential is used to calculate dynamical matrix and the impurity-induced forces up to second nearest neighbors. The calculated atomic displacements for 3d, 4d and 5d impurities in Cr (V, Mn, Fe, Ni, Nb, Mo, Ta and W), Nb (V, Cr, Mn, Fe, Zr, Mo, Ta and W) and Mo (V, Cr, Mn, Fe, Zr, Nb, Ta and W) are tabulated up to 10 NN’s. The strain field due to 3d impurities is least in Cr metal while it is larger in Nb and Mo metals. For 4d and 5d impurities the strain is larger in Cr metal than in Nb and Mo hosts. Similar trend is found for relaxation energies also.

Experimental and theoretical studies of lattice distortions caused by charged 3d impurities in II–IV semiconductors

The results of studies of photoinduced lattice vibration modes associated with charged 3d impurities (in particular, Ni) in II–IV semiconductors (ZnSe, ZnO) and solid solutions ZnSSe and ZnCdSe are presented. Experimental optical exciton-vibration spectroscopy studies revealed that the local vibration modes are coupled, which is due to strong anharmonicity of the local vibrations. Analysis of phonon replicas of the zerophonon line in the spectra allows one to elucidate the nature of the anharmonicity. Model calculations performed for ZnSe: Ni and ZnO: Ni crystals revealed that the nearest neighbor environment of a charged Ni impurity is highly distorted. A change in the charged state of Ni leads to shifts in lines of calculated and experimentally measured x-ray emission spectra and, therefore, should be taken into account.

Hyperfine fields at 3d impurities in ZrFe2 intermetallic compound: A theoretical study

In this article we study the systematics of the local magnetic moments and hyperfine fields of 3d impurities diluted in ZrFe2 intermetallic compound. In order to describe the magnetic properties of this intermetallic, we use a two coupled sublattice Hubbard Hamiltonian, treated in the mean field approximation. We include in the hopping the next neighbor perturbation, due to the transnational invariance breaking introduced by the 3d impurity. After determining the local charge potential by using an extended Friedel sum rule, we obtain the perturbed density of states and the local magnetic moments at the impurity site. The corresponding hyperfine fields are in good agreement with experimental data.

Steric effect on o– p conversion of a H 2 interacting with a 3d impurity sitting on a metal oxide surface

Steric effect on the o– p conversion of a H 2 interacting with a 3d impurity sitting on a metal oxide surface is investigated by comparing the o– p conversion yield/rate for a H 2 doing cartwheel-like rotation (i.e., the rotational axis of H 2 is parallel to the surface) with that for a H 2 doing helicopter-like rotation (i.e., the rotational axis of H 2 is perpendicular to the surface). The yield of o– p H 2 conversion is calculated by using the perturbation theory, where Fermi's contact interaction (intra-molecular hyperfine contact interaction) and the H 2-surface electron exchange interaction (Coulomb interaction) represent the perturbation terms. On the basis of the calculated results, for the transition from the lowest rotational state of an ortho H 2 (with rotational quantum number j=1) to the lowest rotational state of a para H 2 ( j=0), the o– p H 2 conversion yield shows that for a H 2 doing cartwheel-like rotation is higher than that for a H 2 doing helicopter-like rotation.

Magnetic properties of 3d impurities substituted inGaAs

We have calculated the magnetic properties of substituted 3d impurities (Cr-Ni) in a GaAs host by means of first-principles electronic structure calculations. We provide anovel model explaining the ferromagnetic long-range order ofIII-V dilute magnetic semiconductors. The origin of theferromagnetism is shown to be delocalizedspin-uncompensated As dangling-bond electrons. Besides thequantitative prediction of the magnetic moments, our model provides an understanding of the half-metallicity, and the riseof the critical temperature with the impurity concentration.

Strain field due to point defects in Ni dilute alloys

We report the results for strain field due to substitutional transition metal impurities in Ni. The Kanzaki lattice static method has been used to calculate the strain field, the effective ion-ion interaction potential due to Wills and Harrison is used to calculate dynamical matrix with first nearest neighbour and the impurity-induced force with second nearest neighbour. The results for 3d, 4d and 5d impurities (Fe, Co, Cu, Nb, Mo, Pd, Pt and Au) are compared with the experimental data, which are found in agreement.

Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca2GeO4: Cr4+ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca2GeO4: Cr4+ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr4+ ions substituted for Ge4+ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr4+ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr4+ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.

Molecular orientation dependence of o–p H2 conversion on a 3d impurity sitting on a metal oxide surface

We investigate the molecular orientation dependence of the (o)rtho–(p)ara conversion of H2 interacting with a solid surface. As a first step, we consider the interaction of a H2 and a 3d impurity on a metal oxide surface. Taking the H2-surface electron exchange interaction (Coulomb interaction) and Fermi's contact interaction (intra-molecular hyperfine contact interaction) as perturbations, our calculation results indicate that the o–p H2 conversion yield strongly depends on the H2-surface distance and the H–H bond orientation with respect to the surface normal.

Application of the modified electrostatic model to the impurity diffusion in cobalt

The modified electrostatic model (G. Neumann, V. Tölle, Phil. Mag. A 71 (1995) 231; G. Neumann, V. Tölle, Phil. Mag. A 71 (1995) 249) is applied to the impurity diffusion in cobalt, on which only a small number of experimental data has been published as yet. z 0=0.65 is used for the effective charge of the cobalt ion. The temperature dependencies of the diffusion coefficients are calculated for sp and 3d impurities, disregarding a diffusion anomaly near the magnetic order–disorder transition.

Pairs of 3d impurities in Au: local moments and exchange coupling from first-principles

We use a first-principles approach, implemented directly in real space, to obtain the electronic structure and magnetic behavior of a pair of adjacent 3d impurities, one of them Fe, in Au. We find that the inter-atomic exchange coupling J between the Fe and the 3d impurity in the pair is negative for Ti, V and Cr leading to an antiferromagnetic arrangement, while in all other cases J is positive and a ferromagnetic configuration is favored. The local moment of Fe is not significantly affected by the presence of the 3d neighbors. Therefore, the strongly modified values of hyperfine field observed in some of these systems cannot be explained by changes in the moment of the Fe probe.

Spin frustration in Cr alloys with magnetic impurities

Magnetic irreversibility in the form of thermal hysteresis of the magnetic susceptibility χ( T) between zero-field-cooled and field-cooled states has been observed in bulk SDW Cr alloys containing Fe and Co. The observed irreversibility effects are discussed from the point of view of frustration of Cr moments in the vicinity of 3d impurities similar to that which may occur at the interface in FM/AFM layered systems.

Accuracy of atomic-sphere approximation in electronic-structure calculations for transition-metal systems

The simple atomic-sphere approximation (ASA) is used very often for the first-principles calculations based on density-functional theory, in order to study the electronic structure of complex transition-metal systems. We clarify the accuracy of ASA, comparing with the full-potential (FP) calculation results. We show that the ASA calculations reproduce very well the bulk properties, obatined by the FP calculations, while its accuracy decreases significantly for the non-periodic transition-metal system properties, such as vacancy formation energies and solution energies of 3d impurities. It is discussed that the ASA error worsens together with the anisotropic part of FP and the charge transfer between defect and host atoms. It is also discussed that the ASA error may become larger with the generalized-gradient approximation than with the local-spin-density approximation.

Geometrical information from electronic properties: application to 3d impurities

A better insight into the properties due to an impurity in an insulating lattice requires the knowledge of the actual impurity–ligand distance R. To solve this important problem, a study of the R dependence of spectroscopic parameters can be of great help. In the present work the R dependence of the crystal-field parameter, 10 Dq, and the isotropic superhyperfine constant, A s, for Fe complexes embedded in fluorides is firstly explored. It is concluded that R variations (induced by a hydrostatic pressure) down to 0.04 pm can be detected through A s measured by ENDOR. Attention has to be paid, however, when two different kinds of lattices containing the same complex are to be compared. Although the core polarization contribution to the isotropic hyperfine constant, A, is found to be slightly dependent on R a different situation can occur when the 3d–4s hybridization is allowed. In MX 6 complexes (M:d 9 ion; X:ligand) displaying a compressed geometry it is found that A becomes extremely sensitive to changes of axial and equatorial metal–ligand distances. Experimental results on BaZnF 4:Cu, K 2ZnF 4:Cu and CuCl 4(NH 3) 2 2− complexes in NH 4Cl support this conclusion.