Analysis of the valence electronic structures and calculation of the physical properties of Fe,Co, and Ni

The valence electronic structures of Fe, Co and Ni have been investigated with Empirical Electron Theory of Solids and Molecules. The magnetic moments, Curie temperature, cohesive energy and melting point have been calculated according to the valence electronic structure. These calculations fit the experimental data very well. Based on the calculations, the magnetic moments are proportional to the number of 3d magnetic electrons. Curie temperatures are related to the magnetic electrons and the bond lengths between magnetic atoms. Cohesive energies increase with the increase of the number of covalent electrons, and the decrease of the number of magnetic and dumb pair electrons. The melting point is mainly related to the number of covalent electron pairs distributed in the strongest bond. The contribution from the lattice electrons is very small, the dumb pair electrons weaken the melting point; however, the contribution to melting point of the magnetic electrons can be neglected. It reveals that the magnetic and thermal properties are closely related to the valence electronic structures, and the changes or transitions between the electrons obviously affect the physical properties. Supported by the National High Technology Development Program of China (Grant No. 2007AA03Z458)     

The contribution of 5d electrons to the saturation magnetic moments and magnetic hyperfine fields in the heavy rare earth metals

The main results of a model for 5d electrons in the heavy rare earth metals are presented. The model involves the use of wave functions based on published analyses for 4fⁿ5d6s² atomic configurations, and the spreading of each of these energy levels uniformly over a band of width W in the metals. Excess saturation magnetic moments above those of the tripositive ions can be explained by the model with W in the range 0.84±0.16eV in the five metals Gd, Tb, Dy, Er and Tm. The magnetic hyperfine fields in the metals include negative contributions from the 5d electrons which have been shown to amount to about ?250koe in Gd, Er and Tm.     

Sputtering of metals at ion-electron irradiation

It has been found that, in contrast to the commonly accepted opinion, simultaneous irradiation by 15-keV Ar⁺ ions and 2.5-keV electrons at temperatures above 0.5T _m (T _m is the melting temperature) induces much larger sputtering of metallic copper, nickel, and steel than irradiation only by Ar⁺ ions. The effect increases with the temperature. At T = 0.7T _m, the sputtering coefficients in the case of ion-electron irradiation are more than twice as large as the sputtering coefficients in the case of irradiation by Ar⁺ ions. The experiments on the sublimation of copper show that the sublimation rate in the case of the heating of a sample by an electron beam is higher than that in the case of heating in an electric vacuum oven. The revealed effects are explained by the electron-induced excitation of adatoms (atoms stuck over the surface, which appear owing to ion bombardment). Excited adatoms have a smaller binding energy with the surface and are sputtered more easily.     

Theoretical study of the adsorption of 3d- and 4d-metals on a WC(0001) surface

The interaction of 3d- and 4d-metals with a WC(0001) surface has been studied theoretically by density-functional theory methods depending on surface termination and adsorbate position. The most stable sites of metal adsorption on the surface have been determined. The binding energy of d-metals with the surface is shown to be higher in the case of carbon terminated surface. This is explained by the predominant ionic-covalent contribution to the chemical bond at the interface, with the bond ionicity being determined by charge transfer from the metals to the electronegative carbon. Analysis of the electronic and structural characteristics has revealed the factors affecting the bonding energetics at the metal-carbide interface depending on the metal d-shell filling with electrons.     

Theory for the 2p-shell sxaps for the 3d-transition metals

The 2p-shell Soft X-Ray Appearance Potential Spectra (SXAPS) of the 3d-transition metals have been described as a scattering process whereby both the impinging and the 2p-core electrons go into the empty conduction band states above the Fermi level (E_F) of the metals. It is shown that the resonant contribution to the ordinary bremsstrahlung emission process can be of comparable importance to the usual characteristic one.     

Electronic structure and magnetic susceptibility of nearly magnetic metals (palladium and platinum)

A self-consistent approach to calculations of the electronic structure and the magnetic susceptibility of nearly magnetic metals, such as palladium and platinum, has been developed in terms of the generalized s(p)d Hubbard model. The energy band structure has been calculated using the ab initio LDA + U + SO method with the additional inclusion of the interstitial s(p)d exchange interaction and spin-fluctuation renormalizations of the electronic spectra, which appear at finite temperatures. The developed approach makes it possible to quantitatively describe the density of states and unusual temperature dependences of the magnetic susceptibility of the nearly magnetic metals under consideration and to evaluate the basic parameters of the electron-electron interactions. The role of the spin-orbit interaction in the formation of the electronic and magnetic properties is enhanced when going from palladium (4d period) to platinum (5d period). The effects of the temperature redistribution of electrons between the s(p) and d states have been revealed.     

Pseudopotential in metals and alloys

In this communication, the pseudopotential investigation of, the various properties of non-transition metals and alloys, is discussed. Various one parametric model pseudopotentials, derived from well known spherical functions s_l(x), are employed in the calculations. Many recurrence relations of the s_l(x) function have been described. The effects of exchange and correlation on conduction electrons are also considered separately by using different dielectric screenings in various properties. The ion-ion interaction, force constants, phonon spectrum, temperature coefficient of Knight shift and electronic transport coefficients of certain metals and alloys are evaluated. The results are compared with available experimental values. Generally good agreement is achieved. The screening charge density of certain metals in low and high density region are also determined.     

Predominance of d electrons in the conduction bands of rare earths and intermetallic compounds

With a view to study the nature of the conduction electrons in rare earth metals and intermetallic compounds self consistent augmented-plane-wave calculations have been performed for DyZn. The results indicate that 72 per cent of conduction electrons inside the APW sphere of Dy have d character. The dependence of the nature of the conduction electrons on the exchange potential has also been studied.     

Melting of lead and zinc to 60 kbar

The melting curves for lead and zinc were determined to 60 kbar. Our melting data for lead is in good agreement with that of Millet up to 22 kbar, beyond which Millet's values are significantly higher than ours. The melting curve for zinc is almost linear to 60 kbar and our values are lower than the values reported by other workers. The melting relationship proposed by Kennedy as well as the Lindemann law have been examined in the light of the new melting data for these two metals. A straight line can be fitted through the T_m vs ΔV/V₀ plots for zinc within the limits of experimental precision, but the data for lead shows a departure from the straight line fit. The lead melting curve is concave toward the T_m axis, as predicted by the Lindermann law and, in this respect, resembles previously studied Van der Waals solids.     

Structural and electronic relationships between the lanthanide and actinide elements

The similarity and difference between the solid state properties of the 4f and 5f transition metals are pointed out. The heavier 5f elements show properties which have direct correspondence to the early 4f transition metals, suggesting a localized behaviour of the 5f electrons for those metals. On the other hand, the fact that Pu metal has a 30% lower volume than its neighbour heavier element, Am, suggests a tremendous difference in the properties of the 5f electrons for this element relative to the heavier actinides. This change in behaviour between Pu and Am can be viewed as a Mott transition within the 5f shell as a function of the atomic number Z. On the metallic 5f side of the Mott transition (i.e., early actinides), the elements show most unusual crystal structures, the common feature being their low symmetry. An analogous behaviour for the lanthanides is found in cerium metal under compression, where structures typical for the light actinides have been observed experimentally. A generalized phase diagram for the actinides is shown to contain features comparable to the individual phase diagram of Ce metal. The crystal structure behaviour of the lanthanides and heavier actinides is determined by the number of 5d (or 6d) electrons in the metallic state, since for these elements the f electrons are localized and nonbonding. For the earlier actinide metals electronic structure calculations - where the 5f orbitals are treated as part of the valence bands - account very well for the observed ground state crystal structures. The distorted structures can be understood as Peierls distortions away from the symmetric bcc structure and originate from strongly bonding 5f electrons occupying relatively narrow 5f states. High pressure is an extremely useful experimental tool to demonstrate the interrelationship between the lanthanides and the actinides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Untersuchung der Zustandsdichte und der charakteristischen Energieverluste von Ir,Rh, Pt und Pd mit Isochromatenmessungen

X-ray isochromats of the above metals measured with high resolution are presented. Assuming a rigid band model for the four metals they give — with some usual approximations — the qualitative form of the density of states. It turns out that the densities of statesZ(ε) at the Fermi limits decrease in the succession Ir, Rh, Pt, Pd. This is in contradiction to low temperature measurements of the electronic specific heatc _e which for the same succession give an appreciable raising inZ(ε). It is assumed that this discrepancy is caused by a special coupling of the electrons to the transverse phonons. In consequence of this,Z(ε) of the renormalized electron system measured byc _e can be very different from that of the bare electrons measured by the isochromats. Assuming that the transition temperature of elemental superconductors is related according to BCS-theory to the latter density of states the absence of superconductivity of Rh, Pt and Pd can be understood. In addition the isochromats give values for the characteristic energy losses of the four metals.     

On the relationship between the ionization potential and the work function: Metals

A relationship is analyzed between the ionization energy of atoms I and the work function φ of a metal composed of these atoms. The transition energy ∑=I?φ is assumed to be the sum of kinetic K and coulombic C contributions. Contribution K is calculated in the framework of a model of uniform gas of quasi-free electrons, and C is determined from experimental values of Σ. Calculations performed for a wide range of metals have shown that the dimensionless coefficients determining the Coulombic contribution C differ insignificantly between various groups of metals. The relationships obtained have been used for determining the work function of binary alloys.     

Thermische Ausdehnung von Metallen bei tiefen Temperaturen

The thermal expansion coefficients of the following metals in the temperature region between 1,5° and 12°K have been measured: Al, Pb, Pt, Mo, Ta, W, Mg, Cd, Re, Ti, La, Ce, Nd, Gd, Yb. Except for tantalum all the specimens were polycrystalline. Is is found in accordance with theoretical prediction that the coefficient of thermal expansionβ=1/V (?V/?T) [whereV = volume,T = temperature] at sufficiently low temperatures is composed of an electronic component varying linearly with temperature, and a lattice component varying as the cube of the temperature. The electronic component is strongly modified in the superconducting state (Pb, Ta, La). The rare earth metals Gd, Ce and Nd have negative anomalies in their expansion. These are connected with the ferro- and antiferromagnetism of these substances. The results are discussed on the basis of lattice dynamics and the theory of electrons in metals.     

Low-dimensional electronic states at silicon surfaces

Self-assembled atomic chains can be triggered at stepped Si(111) surfaces by adding sub-monolayer amounts of metals, such as gold, silver, platinum, alkali metals, alkaline earths, and rare earths. A common feature of all these structures is the honeycomb chain, a graphitic strip of Si atoms at the step edge that is lattice matched in the direction parallel to the edge but completely mismatched perpendicular to it. This honeycomb chain drives one-dimensional surface reconstructions even on the flat Si(111) surface, breaking its three-fold symmetry. Particularly interesting are metallic chain structures, such as those induced by gold. The Au atoms are locked rigidly to the Si substrate but the electrons near the Fermi level completely decouple from the substrate because they lie in the band gap of silicon. The electronic structure of one-dimensional electrons is predicted to be qualitatively different from that of higher dimensions, since electrons cannot avoid each other when moving along the same line. The single-electron picture has to be abandoned, making way for collective excitations, such as spinons and holons, where the spins and charges of electrons become separated. Although such excitations have yet to be confirmed definitively, the band structure seen in angle-resoled photoemission exhibits a variety of unusual features, such as a fractional electron count and a doublet of nearly half-filled bands. Chains of tunable spins can be created with rare earths. The dimensionality can be controlled by adjusting the step spacing with intra- and inter-chain coupling ratios from 10:1 to >70:1. Thus, metal-induced chain structures on stepped silicon provide a versatile class of low-dimensional materials for approaching the one-dimensional limit and exploring the exotic electronic states predicted for one dimension. PACS 73.20.At; 71.10.Pm; 79.60.Jv; 81.07.Vb; 73.21.Hb     

Crystal structure prediction of uranium hydrides at high pressure: A new hydrogen-rich phase

Uranium hydride is a novel hydrogen-rich system which contains 5f electrons. Uranium hydride can not only be used in the nuclear fuel industry, but also be a candidate of high superconducting-temperature materials. In this paper, we have searched the stable uranium hydride structures by using particle swarm optimization method and first-principles calculations. Besides UH₈ and UH₉, we find that UH₁₇, which contains larger hydrogen content than most hydride materials reported before, is also stable at high pressure. The atomic structures, electronic structures and phase diagram of uranium hydrides are provided, and we find that all of the discovered uranium hydrides are metals with negligible magnetic moments.     

Polaronic excitations in the doped polyacene

The polyacene as two coupled chains of transpolyacetylene has been studied based on an extended SSHHubbard model. The dimerized displacement u ₀ is found to be similar to the case of trans-polyacetylene, and equals to 0.04 Å. The energy-band gap is 0.38 eV, in agreement with other authors. In particular, we have considered some cases where polyacene is doped with one and two donor electrons. In the case of doping with one electron, a polaron spreading over two chains has been found; in the case of doping with two electrons having different spins, a stable bipolaron has been obtained, which is different from the one of transpolyacetylene; in the case of doping with two electrons having the same spins, two stable polarons which spread over two chains have been found. The bound electronic states corresponding to these cases are obtained.     

First-principles calculation of the energy of compressed calcium

The energy of a calcium crystal with a simple cubic lattice as a function of the ratio (t/U) between two internal parameters of the Hubbard model has been calculated using the Hubbard model for the s bands, equations of motion, and direct algebraic method. The electronic spectra have been calculated for the 4s band of the crystal in two principal symmetry directions of the first Brillouin zone. The calculations have been performed at temperatures T ₁ = 0 K and T ₂ = 1000 K. All calculations have been carried out for different interaction energies U of s electrons, one angle, and their different concentrations n in the range 0 ≤ n ≤ 2. The calculations have demonstrated that the dependences of the energy and electronic spectra in this compressed state are very smooth. The occupation of the Ca 4s band is in good agreement with the results of the pioneering calculations of compressed Ca (and a number of other metals), which were carried out by Gandel’man and his colleagues in the Wigner-Seitz spherical cell approximation. It has been shown that the performed analysis accurately reproduces the data obtained on the superconductivity in terms of the Bardeen-Cooper-Schrieffer theory if the 4s band is half-occupied.     

Electron density and electron redistribution in alloys—II: Electron density in alloys and intermetallic phases

Electron density for alloys which have close-packed metallic structures is calculated by assigning valence electrons to octahedral and tetrahedral interstices, a method which has been previously used for elemental metals. Some localization of electron density is proposed for β -phases when there is considerable difference in ion core sizes. This method of characterizing electron density in alloys can be used to derive structures with the amount of electron transfer if an assumption is made for the volume fraction occupied by each component of the alloy. In general, the electronic structure of intermetallic phases appears to be dominated by the correspondence of a definite number of valence electrons with the number of interstices in the metallic structure (the Hume-Rothery $\text{e}\text{a}$ ratios). The model used can also accommodate electron distributions which include both ionic and covalent components of electron density. This is the case for Laves phases and the metallic A-15 compounds. There is a preponderance of intermetallic phases where one component is a d-shell metal. Evidence is presented that in several such alloys there is a change in d-shell configuration of the elemental metal which serves to minimize size differences of the ion cores of the alloy.     

Ab initio study on nonmetal and nonmagnetic metal atoms doped arsenene

The structural, electronic, and magnetic properties of arsenene doped with a series of nonmetals (B, C, F, N, and O) and nonmagnetic metals (Al, Ga, Li, Mg, and Na) are investigated using density functional theory. Magnetism is observed in the case of C. Among all the cases, the C-doped system is the most stable formed system. Hence, we study the ferromagnetic interaction in two-C-doped arsenene. Interestingly, both nonmagnetic (NM) and antiferromagnetic (AFM) states have been observed. As the increasing C?C distance, the magnetic coupling between the moments induced by two C is found to be AFM and the origin of the coupling can be attributed to the p?p hybridization interaction involving polarized electrons.     

Temperature dependence of the electrical resistivity in amorphous metals

In amorphous metals the electrical resistivity increases linearly in the temperature range from 2 to 40 K. This result differs fundamentally from the nonlinear behaviour known for crystalline metals and it suggests the conduction electrons not to be scattered by the vibrations of the amorphous point lattice. The temperature dependent part of the resistivity in amorphous metals is explained with scattering of conduction electrons by fluctuations ofp-electrons.