Sp Impurities Research Articles

Electronic structure and magnetic properties of hexagonal and cubic forms of aluminum nitride doped with sp impurities (B, C, O)

A comparative study of the band structure and magnetic properties of the hexagonal and cubic modifications of aluminum nitride doped with boron, carbon, and oxygen in the nitrogen sublattice has been performed using the ab initio FLAPW-GGA method. Preliminary conclusions on the comparative chemical activity of these phases are drawn from estimates for the energies of substitution of nitrogen atoms by dopants. It has been shown that the doping with boron and nitrogen leads to transition of hexagonal AlN into a magnetic state with high spin polarization of near-Fermi electrons, but for cubic AlN, this effect is absent.

Full-Potential KKR calculations for Lattice Distortion around Impurities in Al-based dilute alloys, based on the Generalized-Gradient Approximation

The presence of a point defect in a crystal, such as a vacancy or an impurity atom, generally cause a displacement of the neighboring host atoms from their ideal lattice positions. For examples, the experimental results are shown in Refs.1–3. From the theoretical point of view the treatment of structural relaxation due to defects in crystals is a difficult task. The problem of the lattice distortion has been mostly dealt with on a phenomenological basis, e.g., by applying models of lattice statics or continuum theory. A reliable microscopic description of lattice relaxation effects based on first-principles electronic structure calculations requires very accurate total energies or forces and has mostly been attempted so far for simple metals and semiconductors on the basis of pseudopotential treatments. The difficulty arises mainly from the fact that energy differences due to local atomic displacements are quite small, of the order of 0.1 ev, compared with, e.g., the cohesive energy (~5eV) of the solid. At present we can calculate accurately the atomic volumes of impurities in transition metals, by using ab-initio all-electron calculations of the electronic structure of solids, based on the FPKKR Green’s function method and the density functional theory. Papanikolaou et al succeeded in calculating the atomic volumes of 3d, 4sp, 4d, and 5sp impurities in Cu [4]. We also succeeded in calculating the lattice relaxation energies of vacancies in Cu and Al [5].

Full-Potential KKR Calculations for Point Defect Energies in Fe-Based Dilute Alloys, Based on the Generalized-Gradient Approximation

We present systematically ab-initio calculations for defect energies of 3d and 4sp impurities (Sc­Ge) in Fe. The calculations are based on the Generalized-Gradient-Approximation in the density-functional formalism and the full-potential Korringa-Kohn-Rostoker (FPKKR) Green’s function method. First we examine the distance dependence, from the 1st- to the 10th-neighbors, of the impurity-impurity (I­I; I = Sc­Ge) interaction energies (Eint) and show that for most cases, the 1st-neighboring I­I interaction energies (E 1) are dominant. We found that fundamental features of phase diagrams of Fe-based binary alloys, such as segregation, solid solution and order, known experimentally, may be classified by use of the sign and magnitude of E 1. Second we discuss the calculated results for the 1st- and 2nd-neighboring interaction energies of 3d and 4sp impurities with perturbed-angular-correlation (PAC)-probe Sn in Fe. The comparison of the calculated results with available experimental results shows that the observed attraction for Sn­Co, Sn­Ni and Sn­Cu may be understood by the 1st-neighboring interaction energies of these impurity pairs, while the obsreved repulsion for Sn­Ga, and Sn­Ge by the 2nd-neighboring interaction energies of these impurity pairs. We also discuss the magnetism of single impurities X (= Sc­Cu) in Fe. The anti-parallel coupling to the bulk magnetization of the neighboring Fe atoms is stable for Sc­Mn, while the parallel coupling for Fe­Cu. [doi:10.2320/matertrans.M2013176]

ChemInform Abstract: Magnetic Effects Induced by Sp Impurities and Defects in Nonmagnetic Sp Materials

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Electronic structure and magnetism in BeO nanotubes induced by boron, carbon and nitrogen doping, and beryllium and oxygen vacancies inside tube walls

We have performed ab initio calculations to systematically investigate electronic properties and magnetism of insulating non-magnetic beryllium monoxide nanotubes (BeO NTs) induced by non-magnetic sp impurities: boron, carbon and nitrogen, as well as by nanotube wall defects: Be or O vacancies. We found that in the presence of these sp impurities, which replace oxygen atoms, the non-magnetic BeO NTs transform into magnetic semiconductors, which acquire magnetization caused by spin splitting of (B, C, N) 2p states located in the forbidden gap of a BeO tube. The magnetic moments of the impurities vary from 0.65 to 1.60 μ B. It was also found that a beryllium vacancy leads to vacancy-induced magnetic moments (at about 0.6 μ B) arising on the nearest oxygen atoms and beryllium-deficient BeO nanotubes adopt half-metallic-like properties. On the contrary, when (B, C, N) dopants substitute for Be atoms or in the presence of an oxygen vacancy, the non-magnetic state of the BeO tubes is retained. The results obtained have been compared with theoretically predicted magnetization effects for doped and non-stoichiometric crystalline BeO.

The full potential Korringa–Kohn–Rostoker method and its application in electric fieldgradient calculations

We have developed a full potential Korringa–Kohn–Rostoker (KKR) Green functionmethod. Three improvements which make the full potential treatment efficient andpractical are reported. One is a method for constructing the Green function which satisfiesthe Wronskian relation exactly. The second is including the contribution of thenon-spherical part of the potential in the wavefunctions correctly by use of a modifiedrecursive integral equation. Thirdly, we propose a method that completely eliminatesthe contribution of irregular solutions of the Schrödinger equation to charge/spindensities. In order to check the reliability of the method, we calculated the electricfield gradient (EFG), which is sensitive to how the potential is treated. We haveperformed EFG calculations for hcp metals and sp impurities in Zn and Cd with thepresent method. The results are in good agreement with experimental data andshow that the full potential KKR method is reliable enough for EFG calculations.

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.

Coordination dependence of hyperfine fields of 5sp impurities on Ni surfaces

We present first-principles calculations of the magnetic hyperfine fieldsHhf of 5sp impurities on the (001), (111), and (110) surfaces of Ni. We examine the dependence ofHhf on the coordination number by placing the impurity in the surfaces, on top of them at theadatom positions, and in the bulk. We find a strong coordination dependence ofHhf,different and characteristic for each impurity. The behaviour is explained in terms of theon-site s–p hybridization as the symmetry is reduced at the surface. Our results are inagreement with recent experimental findings.

Magnetic Hyperfine Fields at Sp-Impurities in Cobalt and Nickel

The understanding of magnetic hyperfine fields (B hf) at impurity nuclei in magnetic materials has been a central problem in nuclear solid state physics for the last decades. Recent progress in density functional calculations has enabled a virtually quantitative description ofB hf at substitutional impurities in iron with the supercell approach. The present calculations were performed in an equivalent manner for the sp elements Fe-Kr, Ru-Xe, and Os-Rn in the other two simple ferromagnetic matrices, Co and Ni. The used M15X cell in optimized geometry allows for volume expansion and nearest neighbor relaxation. For Co the experimentalB hf data are quite well reproduced, though there are some remaining discrepancies at the end of the series. For Ni the qualitative trends are also accounted for, but the quantitative agreement is not satisfactory. Increasing the supercell size to Ni127X for the 5sp impurities does not lead to much better results.

Hyperfine Fields and Local Structure at 4d and 5sp Impurities in bcc Iron

The magnetic hyperfine fields Bhf for the 4d and 5sp elements Rb through Xe at the substitutional site in ferromagnetic iron were derived from scalar-relativistic ab-initio FLAPW calculations for Fe15X supercells with a gradient-corrected density functional. Lattice relaxation and volume expansion were calculated self-consistently. The Bhf values for all cases except Rb and Sr are in excellent agreement with the experimental results.

Mössbauer spectroscopy at ISOLDE

Applications of radioactive ion beams produced at the ISOLDE facility for Mossbauer studies of probe atoms in solids are presented. Examples are given for a site-selective incorporation on different substitutional sites in compound semiconductors by ion implantation and thermal annealing of the radiation damage resulting from the implantation. The interactions of the probe atoms with lattice defects created in the implantation process have been studied to elucidate likely causes for the site-selective implantation mechanism. The technique has enabled to determine the electronic densities at electrically active substitutional probe atoms, having shallow donor or acceptor states as well as states deeper in the band gap. The results are in good agreement with theoretical results from local density calculations. Methodological aspects of the Mossbauer emission techniques employed at ISOLDE are compared to alternative accelerator based techniques and the consequences of the application of different precursor isotopes to the 57Fe Mossbauer isotope are treated in detail for 57Fe in silicon. Finally, results obtained for the magnetic hyperfine interactions of 5 sp impurities associated with vacancies in ferromagnetic metals are discussed.

Hyperfine fields of probe atoms on the (001) surface of Ni

Abstract We present first-principles calculations of the electronic structure and hyperfine fields of 3d and 4sp impurities on the (001) surface of Ni. The calculations are based on the local-spin-density-functional theory and employ a Korringa-Kohn-Rostoker Green's function method for impurities at surfwes. The systematic behaviour obtained for the hyperfine fields of the 4sp adatoms or impurities in the first surface layer is completely different from that found in the bulk. Instead of a single maximum with a very large hyperfine-field value at about the end of an sp series, the adatoms exhibit two maxima with a pronounced minimum in between. This behaviour can be traced back to the reduced coordination number of the adatoms which leads to a much smaller relative splitting of the bonding and antibonding peaks, and to the lower symmetry at the surface which results in an on-site s-p: hybridization. The hyperfine fields found for the 3d impurities at the surface are determined basically by the ferromagnet...

Low-field galvanomagnetic properties of aluminium-based dilute alloys

We report a systematic study of low-field galvanomagnetic properties of aluminium-based dilute alloys with 3d and 4sp impurities. The low-field magnetoresistivity tensor is determined by exactly solving the linearized Boltzmann equation for the anisotropic vector mean free path, without using any adjustable parameter. Our method of calculation is based on the on-Fermi-sphere approximation which allows us to combine the full anisotropy of the host Fermi surface, obtained by the four-orthogonal-plane-wave method, with the impurity scattering phase shifts, evaluated by self-consistent local-density-functional impurity-in-jellium calculations. Our results for the Hall coefficient and the magnetoresistance are in good agreement with the experimental data.

Interaction energies of point defects in metals: Non-local effect beyond the LSDA

We examine the non-local effect (NLE) beyond the LSDA for defect interactions, such as I-I (I  3d and 4sp impurities) and V-I (V = vacancy) interactions, in metals (Al, Ni, Cu). The calculations are based on the generalized-gradient approximation proposed by Perdew and Wang. We find that the NLE is very small for I-I interactions, while it becomes large for V-I interactions and depends on the host; the NLE is attractive for V-I interactions in Al, almost zero for V-I in Ni, and repulsive for V-I in Cu. The micromechanism of this host dependence is discussed.

Ab initio study of structural distortion and its influence on the magnetic properties of metallic dilute alloys

We report a systematic study of lattice relaxation effects around 3d and 4sp impurities in aluminum, using the full-potential Korringa-Kohn-Rostoker Green function method. Our results for the magnetic properties of the impurities seem to resolve the discrepancy between experiment and previous calculations. In addition, the calculated atomic displacements and total volume changes are in good agreement with the corresponding experimental data.

The local magnetic properties of sp impurities in alkali metal clusters

The local magnetic properties of sp impurities (B, C, N, O) in alkali metal clusters (Li 12, Na 12, K 12, Rb 12) with I h and O h symmetries have been systematically studied within the density functional formalism, and the Kohn-Sham equation is solved self-consistently using the discrete variational method (DVM). It is found that all the impurities in clusters are magnetic, especially the local moment of impurity O in bulk is small, but a large moment appears in clusters. Of all the clusters, impurity N in Rb 12 has the largest moment of 2.50 μ B. For impurity C in K 12 and Rb 12, the moments are close to the values of interstitial impurity in bulk, for impurity N in Na 12, K 12 and Rb 12, the moments are close to the values of substitutional impurity in bulk. The stability of clusters is also studied: the I h structure is more stable than O h, Li 12O, Na 12O, K 12O and Rb 12O have closed shells, the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicates that the greater the gap, the greater the binding energy.

Interaction energies of 111In perturbed-angular-correlation probes with 3d and 4sp impurities in Ag, Pd, and Rh.

We present systematic ab initio calculations for the nearest-neighbor interaction energies of $^{111}\mathrm{In}$ perturbed-angular-correlation probe atoms with 3d and 4sp impurity atoms (Sc-As, with Z=21\char21{}33) in Ag, Pd, and Rh. The calculations are based on local-spin-density theory and apply the Korringa-Kohn-Rostoker Green's-function method for spherical potentials. The full nonspherical charge density is evaluated to calculate the double-counting contributions to the total energy. The present calculations reproduce very well the chemical trend of the available experimental interaction energies; attractive interaction of In with 3d impurities in Ag and repulsive interaction with 4sp impurities in Ag. For Mn-In pairs in Ag, the detailed comparison between the calculated results with and without spin-polarization energy shows the importance of magnetism for the 3d impurity-probe interaction enegies and is also useful to elucidate the physical mechanism for the magnetic energy anomalies in cohesive, surface, and solution energies of 3d systems. \textcopyright{} 1996 The American Physical Society.

Low-temperature thermopower of Al-based dilute alloys

We report systematic calculations of the low-temperature diffusion thermopower of Al-based dilute alloys with 3d and 4sp impurities, by solving self-consistently the linearized Boltzmann equation. The impurity scattering is described by the phase shifts obtained from self-consistent local-density-functional impurity-in-jellium calculations. Moreover, the influence of the full anisotropy of the Al Fermi surface on the scattering process is taken into account within the on-Fermi-sphere approximation. Our results explain successfully the experimentally measured variations in the thermoelectric power, except for Mn impurities. In this case, the presence of a narrow many-body resonance at the Fermi level in the localized spin-fluctuations regime seems to be responsible for the large negative value of thermopower observed.

Magnetic impurities in simple metals

The local magnetic behaviour of impurities in simple monovalent metal hosts is studied systematically by means of ab-initio, local-spin-density-functional electronic structure calculations. Our results predict that besides the 3d and 4d impurities also the 5d and some sp impurities are magnetic in the late alkali metals and that local magnetism can exist for impurities not only on substitutional but also on interstitial sites.

Calculation of the residual resistivity and the low-field Hall coefficient of 3d and 4sp impurities in aluminum.

We report systematic calculations of the residual resistivity and the low-field Hall coefficient of Al-based dilute alloys with 3d and 4sp impurities, by self-consistently solving the linearized Boltzmann equation. We employ the on-Fermi-sphere approximation, which allows us to combine the full anisotropy of the aluminum Fermi surface, obtained by the four-orthogonal-plane-wave method, with the phase shifts associated with isotropic impurity scattering, evaluated by self-consistent local-density-functional impurity-in-jellium calculations. Our results show that the anisotropic scattering increases the residual resistivity, thus obtaining better agreement with the experiment. Moreover, a consistent interpretation of the observed trends of the low-field Hall coefficient is presented.