Substitutional Transition Metal Impurities Research Articles

Thermoelectric Properties of Graphene-Boron-Nitride Nanoribbons with Transition Metal Impurities

The thermoelectric properties of graphene-hBN nanoribbons with substitutional transition metal impurities (Cr, Mn, Fe, Co, Ni) are investigated in the framework of constrained spin density functional theory calculations. The focus on atomic-sized thermoelectric devices is not only supported by the benefits of scaling but also by the enhanced thermopower, which arises from typically sharp variations of the device conductance. Some of the investigated structures present a change in the sign of the Seebeck coefficient by raising the temperature, as a direct consequence of resonant electron transmission. Different transition metal substitutions on boron and nitrogen atoms are investigated comparatively and an overall enhancement of the thermopower and figure of merit is found.

Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the ${[{\mathrm{Mn}}^{2+}]}^{0}$ plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity $d$ electrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic ${[{\mathrm{Fe}}^{3+}]}^{0}$ and the ionized ${[{\mathrm{Fe}}^{2+}]}^{\ensuremath{-}}$ states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of $d$ orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state ${[{\mathrm{Fe}}^{2+},h]}^{0}$, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of ${[{\mathrm{Mn}}^{2+},h]}^{0}$. In contrast, for ${[{\mathrm{Fe}}^{3+}]}^{0}$ electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.

Ti, Fe, and Ni in Si and their interactions with the vacancy and theAcenter: A theoretical study

Transition-metal (TM) impurities from the $3d$ series have been a source of concern in Si technology for nearly 60 years. Surprisingly, numerous issues remain unresolved. In this first-principles theoretical study, we examine the properties of Ti and Ni at a similar level as that used in a recent study of Fe, except that larger supercells are used and that potential-energy surfaces are obtained using the nudged elastic band method (some Fe results have been updated). The equilibrium sites, spin and charge state(s), activation energies for diffusion, and gap levels of the isolated interstitial TMs (${\text{TM}}_{i}$'s) are calculated and match the measured ones when data are available. The interaction of a ${\text{TM}}_{i}$ with a pre-existing vacancy $(V)$ shows that the reaction ${\text{TM}}_{i}+V\ensuremath{\rightarrow}{\text{TM}}_{s}$ (substitutional TM) occurs with a large energy gain, yet smaller than the formation energy of the vacancy. The electrical properties of interstitial and substitutional TM impurities are opposite to each other. In particular, vacancies passivate or partially passivate ${\text{Ti}}_{i}$ and ${\text{Fe}}_{i}$ and thus may play unrecognized but beneficial roles in some processes commonly used by industry. A population analysis of the ${\text{TM}}_{s}$'s shows that the $3d$ shell is not full, even in the case of Ni. The interaction of a TM with the $A$ center (${\text{O},V}$ pair) results in two nearly energetically degenerate configurations, the ${{\text{TM}}_{i},\text{O},V}$ and the ${{\text{TM}}_{s},{\text{O}}_{i}}$ complexes.

Magnetic states of transition metal impurities in silicon carbide

Electronic properties of 3C silicon carbide with substitutional transition metalimpurities are calculated using an ab initio full-potential linearized augmented planewave technique. It is shown that transition metal atoms in an SiC host may existin both magnetic and nonmagnetic states. For different impurity species, thetransition from the nonmagnetic to the magnetic state and the correspondingchange of the atomic magnetic moments may take place either gradually as thevolume of the unit cell changes or when the energy gap between the two states isovercome. The magnetic and nonmagnetic solutions are characterized by significantlydifferent unit cell geometries, which are analysed in detail. Magnetic ordering isstudied for a number of transition metal impurities in SiC and mean-field estimatesof the Curie temperature are given. Calculated densities of states are used toanalyse the nature of the exchange interaction between the impurities. Someproperties of transition metal impurities are compared for 3C and 4H silicon carbidesubstitution.

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.

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.

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.

The structural properties of transition metal hydrogen complexes in silicon

A first-principles spin-polarised local density functional cluster method is used to explore the structural and vibrational properties of several substitutional transition-metal impurities complexed with hydrogen.

Electric-field gradient in dilute V-based alloys.

Electric-field gradients (EFG's), due to substitutional transition-metal impurities in vanadium have been evaluated using a discrete-lattice static method. The valence and size EFG's are generated simultaneously with the help of perfect and imperfect crystal potentials. The EFG's at the first- and second-nearest neighbors (1NN, 2NN) of the impurity are calculated for the dilute alloys VNb, VMo, VTa, and VW. The d-band effects of both the host and impurity are included through the crystal potential. The calculated results are in agreement with the experimental values. The valence EFG is larger than the size EFG at the 1NN while the size EFG starts dominating at the 2NN sites.

Strain field due to substitutional transition-metal impurities in bcc metals: Application to dilute vanadium alloys.

The formalism for the generalized Kanzaki lattice static method is developed for a metallic crystal with bcc symmetry. The interactions up to second-nearest neighbors are included in the derivation of the dynamical matrix, impurity-induced force, and the atomic displacements. The formalism is applied to calculate the strain field due to 3d, 4d, and 5d substitutional transition-metal impurities (Ti, Cr, Mn, Fe, Nb, Mo, Ta, and W) in the vanadium metal, the only bcc systems in which the study of electric field gradients has been performed. The effective ion-ion interaction potential for the transition metals, proposed by Wills and Harrison, which includes properly the d-band effects, is used in the numerical calculations. In all the systems the atomic displacements are calculated up to 21 nearest neighbors and these show oscillatory behavior. The maximum displacement (or strain field) is caused by a Ta impurity and is about 3.5% of the first-nearest-neighbor distance. The strain field is found to depend, both in strength and range, on the excess ion-ion interaction potential due to the impurity. The atomic displacements exhibit the same trend, as shown by x-ray studies of the fractional change in the lattice parameter.

Computer Modeling of the Optical Properties of Transition-Metal Ions in Solids

ABSTRACTComputational methods for modeling the optical properties of substitutional transition-metal impurities in insulating solids, potentially applicable to some scintillator and phosphor materials, are reviewed. Methods considered include crystal-field and semi-empirical ligand-field models; SCF-Xα-SW, SCF-RHF-LCAO, SCF-UHF-LCAO and CI ab initio methods: and ICECAP and HADESR embedded cluster methods with lattice relaxation. A detailed example of the application of the HADESR method to crystal-field spectra of Cr3+ in halide elpasolites is described. In this method, ab initio molecular-orbital calculations with effective core potentials are performed for selected ionic configurations. Simultaneous relaxation of the cluster and surrounding lattice, with mutual pair. potential interactions, is accomplished by a modified lattice statics program. Calculated properties include pressure-dependent optical transition energies, vibration frequencies and radiationless transition rates.

Systematic Aspects of the Electronic Structure of 3d Transition-Metal Compounds

Systematic changes in the electronic structure of 3d transition-metal compounds with varying chemical compositions are studied by photoemission spectroscopy and configuration-interaction cluster-model analysis. The changes consist of (i) the smooth variation of model parameters such as the charge-transfer energy Δ and the d-d Coulomb repulsion energy U with atomic number and valence and (ii) the apparently irregular multiplet corrections to Δ and U, which are functions of the nominal electron number. These systematics are reflected upon the band gaps, covalency and character of doped carriers in transition-metal oxides and chalcogenides, and upon the optical absorption spectra and the ionization energies of substitutional transition-metal impurities in semiconductors.

Electronic structures and magnetic moments of Fe3+ySi1-y and Fe3-xVxSi alloys with DO3-derived structure.

We present ab initio calculations of magnetic moments and local electronic densities of states in binary ${\mathrm{Fe}}_{3+\mathit{y}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ and ternary ${\mathrm{Fe}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$Si random alloys with ${\mathit{DO}}_{3}$-derived structures. The local-spin-density approximation, the linear muffin-tin orbital method, and the coherent-potential approximation (LMTO-CPA) were used. In ${\mathrm{Fe}}_{3+\mathit{y}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ the excess (as compared to ${\mathrm{Fe}}_{3}$Si) Fe atoms occupy the Si[D] sites of the ${\mathrm{Fe}}_{3}$Si host and create a strong d scattering. A realistic-alloy calculation thus requires the application of the CPA. In both types of alloys the moments of Fe atoms occupying the B sites remain essentially unchanged when the concentrations (y in ${\mathrm{Fe}}_{3+\mathit{y}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$, x in ${\mathrm{Fe}}_{3\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$Si) are varied. The Fe[A,C] moments vary almost linearly with y and x, in agreement with experimental observations. The sublattice density-of-states functions provide insight into the site preference for substitutional transition-metal impurities in ${\mathrm{Fe}}_{3}$Si.

Dependence of transition-metal impurity levels on host composition in III-V semiconductors.

Model calculations of substitutional transition-metal impurities in III-V semiconductors have been performed with the purpose of investigating how the defect energy levels vary as the host composition is changed. The calculations are based on the theory of Haldane and Anderson and include electron-electron interactions in a self-consistent manner. The band structure has been described in terms of an ${\mathrm{sp}}^{3}$ second-nearest-neighbor tight-binding approximation. We find that the transition-metal acceptor energy levels have fixed positions relative to the average vacancy level or the vacuum level independent of the chemical composition of the host material. This stability is a consequence of self-consistent competition between the strongly localized Coulomb interaction at the impurity site and the hybridization with the nearest neighbors.

Theoretical study of the electronic states of substitutional transition-metal impurities in InP

The electronic structure of the transition-metal impurities Cr, Mn, Fe, Co, Ni and Cu and of a vacancy in InP have been studied theoretically using the multiple-scattering X alpha -SCF method. It is found that the dangling bond hybrid t2 level moves into the band gap. For Fe this is in the middle of the gap providing a clue to the semi-insulating effect of Fe in InP. The crystal-field resonance level e goes from the gap into the valence band as one goes from Cr to Cu. The corresponding t2 level goes successively deeper into the valence band from Cr to Cu. The population density study shows that the general trend is an increase of transfer of charge as one scans the latter part of the first transition-metal series.

Trend in the local-force variations due to substitutional transition-metal impurities in ZnS

Numerical calculations of the impurity-induced Raman scattering intensity in ZnS crystals are made on the basis of Green's function theory. The isoelectronic defects studied consist in a series of substitutional transition-metal impurities (Cr, Mn, Fe, Co, Ni) with a variable number of 3 d electrons. The calculated results provide very good correspondence with all the existing experimental data, by considering that the impurity-sulfur bonds weaken by about 10 %. A monotonic evolution of the local force variations is found when going through the series of impurities : from Chromium to Nickel, the weakening of the bonds is decreasing. This trend appears to be a specific property of the transition-metal impurities, and a tentative explanation for it is proposed.

Trends in the electronic properties of substitutional3dtransition-metal impurities in GaAs

The $X\ensuremath{\alpha}$ scattered-wave method, in its spin-restricted version, has been used to calculate the electronic states associated with an ideal gallium vacancy as well as with the following substitutional transition-metal impurities in gallium arsenide: chromium, manganese, iron, cobalt, nickel, and copper. The gallium vacancy is found to behave as a simple acceptor with an ionization energy of approximately 0.50 eV. In addition, a level of ${a}_{1}$ symmetry with considerable amplitude in the vacancy sphere is found at an energy of approximately 0.75 eV below the valence edge. The transition-metal impurities can be broken up into two categories, depending on whether or not the $3d$ states of the transition metal play an active role in determining the active electronic states of the defect. Those impurities to the left of (and including) cobalt are found to behave in the standard way, as the active impurity states in the gap have substantial $d$ character and there is clear evidence of bond formation involving the impurity and the neighboring host ligands. By contrast, nickel and copper appear to behave as simple acceptors in the sense that the active defect levels have little $d$ character and are rather more characteristic of the broken bonds at the impurity site. The $d$ states of this latter group appear as resonances in the host valence band and play little direct role in determining the electronic properties of the material. The paper concludes with a discussion of these results and possible factors that might affect their validity.

Self-consistent embedded-cluster model for magnetic impurities: Fe, Co, and Ni inβ′--NiAl

Substitutional transition-metal impurities occupying either Ni or Al sites in the intermetallic compound ${\ensuremath{\beta}}^{\ensuremath{'}}$-NiAl with CsCl structure have been treated by a self-consistent molecular-orbital method. The electronic energy levels and charge and spin densities of $M{X}_{8}$ clusters representing a central atom and its nearest neighbors and $M{X}_{8}{Y}_{6}$ clusters representing central, nearest-neighbor and second-nearest-neighbor atoms have been calculated in the Hartree-Fock-Slater spin-polarized model, using a numerical discrete variational method. Interaction of the impurity cluster with the crystal environment is represented by a pseudopotential derived from cluster calculations on the pure compound. Results for Fe, Ni, and Co impurities in ${\ensuremath{\beta}}^{\ensuremath{'}}$-NiAl are discussed in connection with experimental resistivity, M\ossbauer isomer shift, and NMR data.

Self-consistent embedded-cluster model for magnetic impurities: β′NiAl

Substitutional transition metal impurities occupying either A or B sites in AB intermetallic compounds with CsCl structure have been treated by a self-consistent molecular-orbital method. The electronic energy levels and charge and spin densities of MX8 clusters representing a central atom and its nearest neighbors have been calculated in the Hartree-Fock-Slater spin-polarized model, using a numerical discrete variational method. The crystal lattice is included through a self-consistent empirical pseudopotential scheme. Results for Fe, Ni, and Co impurities in β′-NiAl are discussed in connection with experimental resistivity, Mössbauer isomer shift, and NMR data.