D-electron Shell Research Articles

Influence of Ionization and Spin Transitions on Electron Delocalization in the Molecules of Transition Metal Sandwich Complexes

A quantum-chemical study of the structures of symmetric 3d metal sandwich complexes with benzene and cyclopentadienyl ligands has been carried out within the framework of the electron density of delocalized bonds (EDDB) model. Neutral and ionized molecules in various spin states were considered. It is shown that successive population of the d-electron shell by varying the metal atom in a series of similar complexes, as a rule, leads to a decrease in the degree of electron density delocalization. The detachment of an electron from neutral molecules also reduces the number of delocalized electrons in the sandwich system, butthe contribution of the metal atom to delocalization increases in most cases. Singlet-triplet transitions in metallocenes and bis-benzene complexes decrease the electron density of delocalized bonds, but to a lesser extent than in the free and C6H6 ligands.

In English

In this work, a nanoscale structure consisting of contacting layers of a metal of the iron subgroup and a rare earth metal oxide (REM) is considered. Such nanostructures have an interesting feature, which is that as a result of the contact of these layers, an increase in the galvanomagnetic, magneto-optical and kinetic properties of ferromagnetic metals are observed. Presumably, the enhancement is due to an increase in the magnetization of these metals, caused by the exchange f - d interaction between the unfilled f- and d-electron shells of the atoms that make up the contacting layers. The aim of this work is to find the possibility of such f - d exchange interaction by the EPR method. To compose the studied nanostructure, Fe used as it has the strongest magnetic properties in its subgroup. Gd2O3 was used as an REM oxide as one of the few oxides giving a significant signal in the EPR region. The Gd2O3/Fe nanostructure created by sequential electron-beam deposition of Gd2O3 and Fe layers on a sitall substrate. The thickness of the oxide and metal layers was 68 and 112 nm, respectively. EPR spectra were recorded at room temperature on a computerized spectrometer Radiopan 2547 SE / X at the frequency of 9.3 GHz. The set of the obtained spectra was processed using the OriginPro and MatLab programs, which confirmed their compliance with the Lorentz model. From the experimentally obtained EPR linewidth, the parameter of the exchange f - d interaction is determined under the condition of a number of assumptions. The value of the g-factor is also found. Comparison of the EPR parameters of the spectra of individual layers of Gd2O3 and Fe with the spectra of the Gd2O3/Fe nanostructure composed of them, including the value of the g factor and the exchange interaction parameter, suggests that the presence of an iron layer affects the EPR spectrum of the REM oxide layer Gd2O3. The exchange interaction parameter increases from 985 to 4685 (rel. units), the g-factor decreases from 3.5 to 2.4. The most probable reason for the change in the spectrum is the exchange f - d interaction between atoms with unfilled f- and d-electron shells that are parts of the contacting layers.

Segregation of alloying elements to stabilize θ′ phase interfaces in Al-Cu based alloys

Interactions of alloying elements (Si, Mg, Mn, Zr, Zn) and vacancies with coherent interfaces of θ′ phase in Al-based alloys have been systematically studied by means of ab initio supercell calculations. The interface structure with a half-filled interfacial Cu layer is calculated to be lower in energy (by 0.1 eV per structural vacancy) than the structure with a filled Cu layer; the degree of interface reconstruction depends on the availability of vacancies. The presence of vacancies in the interfacial Cu layer plays a crucial role in the interaction of solutes with coherent θ′ phase interfaces. The solute–interface interaction energies are calculated to be much weaker for elements having closed (Cu, Zn) or empty (Mg, Si) d-electron shells than for d-transition metals (Mn, Zr). To clarify the roles of alloying elements and interface structure in the stability of θ′ phase precipitates, we analyze the solute–interface interactions in terms of electronic-structure and atomic-size contributions to interatomic bonding.

Segregation of Alloying Elements to Stabilize Teta Prime Phase Interfaces in Al-Cu Based Alloys

Interactions of alloying elements (Si,Mg,Mn,Zr,Zn) and vacancies with coherent interfaces of θ′ phase in Al-based alloys have been systematically studied by means of ab initio calculations. The interface structure with a filled interfacial Cu layer is calculated to be lower in energy than the structure with a half-filled Cu layer (by 0.1 eV per structural vacancy), which implies that a temperature-induced reconstruction of the interface may take place. The presence of vacancies in the interfacial Cu layer structure plays a crucial role in the interaction of solutes with a coherent θ′ phase interface. The solute–interface interaction energies are calculated to be much weaker for elements having closed (Cu,Zn) or empty (Mg,Si) d-electron shells than for d-transition metals (Mn,Zr). To clarify the roles of alloying elements and interface structure in the stability of θ′ phase precipitates, we analyze the solute–interface interactions in terms of electronic-structure and atomic-size contributions to interatomic bonding.

Engineering the coordination environment enables molybdenum single-atom catalyst for efficient oxygen reduction reaction

With a half filled d-electron shell, molybdenum (Mo) plays an important role as catalysts in the petrochemical industry. However, Mo is generally regarded as not catalytically active for oxygen reduction reaction (ORR) compared with other transition metals such as Fe and Co. Inspired by molybdoenzymes, herein, we successfully endow Mo single-atom catalyst with highly ORR catalytic activity though engineering the coordination environment. This unique Mo single-atom catalyst consists of oxygen and nitrogen dual-component coordinated central Mo atom anchored on porous carbon (Mo-O/N-C), showing prominent ORR catalytic performance compared to the state-of-the-art Pt/C under alkaline condition. The extraordinary performance of Mo-O/N-C electrocatalyst is also demonstrated in Zn-air batteries as an air cathode. Density functional theory (DFT) calculations reveal the oxygen and nitrogen dual-component coordination could tailor the d-band center of Mo, subsequently optimizing its binding capability with reaction intermediates (O*, OH* and OOH*), hence accelerating overall ORR process. This work not only provides an efficient and commercially competitive ORR catalyst, but advancing further development of other electrocatalysts through engineering the coordination environment.

Effect of transition metal (M) and M–C slabs on equilibrium properties of Al-containing MAX carbides: An ab initio study

The effect of transition metal (M) and M–C slabs (n) on crystal structure and equilibrium properties of Al-containing MAX-phase carbides Mn+1AlCn (n=1–3) was investigated. There is a linear relation between lattice parameters and Goldschmid diameter of M atoms. For a given n, c/a ratios always fall into a narrow range. The theoretical density almost increases with increasing VEC (valence electron concentration), n and d-electron shell number. In general, the formation energy and cohesive energy increase with the increase of VEC. The bond length decreases with increasing VEC. The bond length of 3d compounds is much higher than those of 4d and 5d compounds. Among M–C bonds, the lowest bond length is present in the M–C bonds with the M atoms near to Al atoms. Overall, the M–Al bond angle decreases with increasing d-electron shell number. In addition, M–C bond angles increase with increasing the corresponding M–C bond length. Overall, the total density of states at Fermi energy increases with increasing VEC and n, but decreases from 3d, 4d to 5d for M3AlC2, and M4AlC3. In addition, a weak M1–M2 bond was found in M4AlC3 with VEC=5 and 6.

Active electronic states of silver catalysts for methanol selective oxidation

The results of investigations of the electronic and catalytic properties of silver catalysts by IR spectroscopy of adsorbed carbon monoxide, diffuse reflectance electron spectroscopy, electron microscopy and catalytic testing have been combined and examined. Of those metals of which the configuration of the valence electronic shell is d ns 1 (Cu, Ag, Au, Pt, Pd, Rh, Ru), silver has the highest effective charge as M + cation due to the most resistant d-electron shell. Supports and modifying additions of metal oxides have a pronounced effect on the electronic state and red-ox properties of silver as a result of donor-accepter interaction of the metal oxide. The factors that stabilize the ionic state of silver and increase its effective charge seem to improve the catalytic properties of silver in methanol oxidation. A hypothesis on the active electronic state of silver in this process — Ag + cations with maximum effective charge — is suggested based on comparison of spectroscopic and catalytic data.

Nonlinear refractive index of optical crystals.

The nonlinear refractive indices (${n}_{2}$) of a large number of optical crystals have been measured at a wavelength near one micrometer with use of nearly degenerate three-wave mixing. The measurements are compared with the predictions of an empirical formula derived by Boling, Glass, and Owyoung. This formula, which relates ${n}_{2}$ to the linear refractive index and its dispersion, is shown to be accurate to within about 30% for materials with nonlinear indices ranging over 3 orders of magnitude. Measurements for a number of binary oxide and fluoride crystals have been analyzed under the assumption that the hyperpolarizability of the anion is much larger than that of the cation. It is found that the hyperpolarizability of oxygen varies by a factor of 10, and that of fluorine varies by a factor of 7, depending on the size of the coordinating cation. This behavior is similar to that of the linear polarizability, although the hyperpolarizability is much more sensitive than the linear polarizability to the identity of the cation. The measured halide ion hyperpolarizabilities for several alkali-halide crystals are in reasonable agreement with recent self-consistent calculations. A semiempirical model was proposed by Wilson and Curtis to account for the dependence of the linear anionic polarizability on the radius of the cation. This model also accounts quite well for the variation of the hyperpolarizability of both fluorine and oxygen, except for cation partners that have filled or unfilled d-electron shells. The nonlinear indices of a number of complex oxides (i.e., those with more than one cation) have been calculated from the partial hyperpolarizabilities deduced from the data for the binary oxides. The calculated and measured values of ${n}_{2}$ agree to within an average error of 13%.

Tarnish and corrosion behaviour of palladium-silver alloys

Binary and commercial palladium-silver dental alloys show a greatest resistance to both tarnish and corrosion at compositions of between 50 and 75% Ag, most probably due to saturation of the d-electron shell in this composition range. The commercial Pd-Ag alloys have more corrosion resistance, but a lower tarnish resistance than the binary Pd-Ag alloys, due in part to the minor alloying with indium or zinc. The addition of copper increases the segregtion of silver, which adversely affects both tarnish and corrosion resistance. A solutionization heat treatment degrades tarnish resistance, but does not affect corrosion resistance.

Investigations in the field of metallochemistry Communication 6. Suboxides of the transition metals

1. The solubility of oxygen in the transition metals depends upon the position of the element in the Periodic System; it is a maximum for metals with a great degree of incompletion of the d-electron shell and decreases as it is filled. 2. When a definite critical concentration of oxygen in the solid solutions formed has been reached, there is a very slow formation of suboxides with an ordered structure and an individual type of crystal lattice. The formation of suboxides is possible according to a peritectoid reaction. 3. The existence of two suboxide compounds of titanium has been detected (Ti6O and Ti3O); the existence of a series of suboxides in the transition metal-oxygen systems has been hypothesized.

Effect of nonmetallic atoms on the electric properties of high-melting compounds of transition metals

The authors generalize the data on the electrophysical properties of borides, carbides and nitrides of transient metals of groups IV–VI of the periodic table and point out the principal regularities of the changes in these properties with a change in the acceptor capacity of d-electron shells of the atoms of transient metals and the ionizing potentials of the nonmetals.

The sintering of nitrides of the transition metals

1. An investigation was made into the conditions of sintering of nitrides of the refractory transition metals by the methods of vacuum heating of precompacted blanks, hot pressing, and reactive sintering, consisting in the nitriding of precompacted blanks from metal powders. The optimum conditions have been established for the sintering of nitrides by all these methods, and the advantages of the reactive sintering method, which permits the preparation of impurity-free nitride parts, have been demonstrated. 2. It has been shown that the process of sintering nitrides of the transition metals is directly linked with the character of electron density distribution in the lattices of nitride phases; this, in turn, is governed by the degree of incompleteness of the d-electron shells of the corresponding transition metals and the nitrogen content of the nitride phases.

The boronization of refractory transition metals

1. An investigation was made of the diffusional saturation process of some d-transition metals with boron. The phase composition of the boride layers forming on the metals was determined, and their structure resulting from different boronizing conditions was studied. 2. It is shown that growth of the layers occurs as a result of one-sided diffusion of boron atoms from the boron-containing medium, via the phases being formed, into the metal. The kinetics of the process are described by a law which, is close to that of a quadratic parabola. 3. On the basis of a metallographic analysis, it was established that, with increasing thickness of the diffusion layers, the latter's adhesion to the base metal becomes weaker, their brittleness increases, and cracks in them grow in quantity and size. When selecting practical boronizing conditions, one should aim at the minimum temperature (1100–1200°C) and holding time which secure layers of the required thickness. 4. Determinations were made of the reaction diffusion constants and of the effect of temperature on the coefficients of diffusion of boron into titanium, zirconium, columbium, tantalum, molybdenum, and tungsten. The view expressed in [6], according to which the processes of diffusion of nonmetals into d-transition metals are governed by the acceptor capacity of the d-electron shells of the metallic atoms, was confirmed.

843. Stereochemistry of metals of the B sub-groups. Part I. Ions with filled d-electron shells

843. Stereochemistry of metals of the B sub-groups. Part I. Ions with filled d-electron shells