Valence Electron Density Research Articles

Investigation into the strength of β-Sn/α-CoSn3 and Co/α-CoSn3 interfaces with Ni-doped α-CoSn3 using first-principles calculations

The strength of Pb-free solder/Co-Ni alloy joints was investigated using first-principles calculations. The adhesion work was computed, revealing that the inclusion of Ni in the α-CoSn3 phase reduced the adhesion work for both β-Sn/α-CoSn3 and Co/α-CoSn3 systems. Tensile and shear simulations further demonstrated distinct differences in the mechanical properties between β-Sn/α-(Co,Ni)Sn3 and Co/α-(Co,Ni)Sn3 systems. The results demonstrate that Co/α-(Co,Ni)Sn3 exhibited superior tensile and shear strength. The analysis suggests that failure in β-Sn/α-(Co,Ni)Sn3 structures was more likely to originate either at the interface or near the β-Sn region, as opposed to within the α-(Co,Ni)Sn3 phase for the Co/α-(Co,Ni)Sn3 structures. This observation is consistent with predictions made by Griffith’s fracture theory. Additionally, the valence electron density map illustrates the electron transfer under varying tensile strains.

The dominant roles of “electron-density-mechanism” and “chemical-bonding-mechanism” for hydrogen in molybdenum and lithium

Using first-principles calculations, we have systematically studied structures and thermodynamic stability of interstitial H as well as the H-vacancy interaction in molybdenum (Mo) and lithium (Li). Single H atom prefers to occupy tetrahedral interstitial position (TIP) and octahedral interstitial position (OIP) in Mo and Li, respectively, and the solution energies are 0.87 eV and −0.66 eV, respectively. In Mo, mono-vacancy can capture as many as seven H atoms and each H atom prefers to bind onto an isosurface of valence electron density. However, H atoms detach from vacancy to occupy the OIPs outside vacancy in Li. Based on these results, we reveal that the electron-density-mechanism (EDM) and chemical-bonding-mechanism (CBM) cause different properties of H in Mo and Li, respectively. In Mo, since the valence electron density everywhere in interstitial lattice is much high, H atom has to search a place where the valence electron density must be suitable. Accordingly, vacancy can provide an optimal valence electron density region for H dissolution, and the optimal valence electron density is 0.10 electron/Å3 at vacancy. In Li, H atom exhibits the negative solution energy in the interstitial lattice, which promotes H atom to form ionic bond with neighboring Li atom. H atoms do not combine inside vacancy but stay at the OIPs outside vacancy to form ionic bonds with neighboring Li atoms. We believe that the EDM and CBM can be generalized to other transition metals and other alkali metals, respectively.

The piezoelectric activity in the HfO2 nanoclusters

In this paper the piezoelectric effects in infinite free-standing HfO2 film and HfO2 cluster under mechanical action are studied by methods of the density functional theory and first-principles pseudopotential based on own program code. The spatial distribution of valence electron density, the forces that act on an individual atom from the electronic and ionic subsystems, and Coulomb potential along transverse direction are calculated. It was found that a polarization charge distribution of valence electrons, which leads to piezoelectric effects, was observed in HfO2 nanofilms, while in HfO2 nanoclusters the charge distribution remained uniform both in the uncompressed and in the compressed state.

MB38(M=Be and Zn): A Quasi-Planar Structure Rather Than a Core-Shell Octahedral Borospherene Structure.

In a recent paper (ChemPhysChem, 2023, 24, e202200947), based on the results computed using DFT method, the perfect core-shell octahedral configuration Be@B38 and Zn@B38 was reported to be the global minima of the MB38(M=Be and Zn) clusters. However, this paper presents the lower energy structures of MB38(M=Be and Zn) clusters as a quasi-planar configuration, the Be atom is found to reside on the convex surface of the quasi-planar B38 isomer, while the Zn atom tends to be attached to the top three B atoms of the quasi-planar B38 isomer. Our results show that quasi-planar MB38(M=Be and Zn) at DFT method have lower energy than core-shell octahedral configuration M@B38(M=Be and Zn). Natural atomic charges, valence electron density, electron localization function (ELF) analyses identify the MB38(M=Be and Zn) to be charge transfer complexes (Be2+B38 2-and Zn1+B38 1-) and suggest primarily the electrostatic interactions between doped atom and B38 fragment. The photoelectron spectra of the corresponding anionic structures were simulated, providing theoretical basis for future structural identification.

Unveiling the Nature of Chemical Bonds in Real Space.

Recent advent of diverse chemical entities necessitates a re-evaluation of chemical bond concepts, underscoring the importance of experimental evidence. Our prior study introduced a general methodology, termed Core Differential Fourier Synthesis (CDFS), for mapping the distribution of valence electron density (VED) in crystalline substances within real space. In this study, we directly compare the VED distributions obtained through CDFS with those derived from high-accuracy theoretical calculation using long-range corrected density functional theory, which quantitatively reproduces accurate orbital energies. This comparison serves to demonstrate the precision of the CDFS in replicating complex details. The VED patterns observed experimentally exhibited detailed structures and phases of wave functions indicative of sp3 hybrid orbitals, closely aligning with theoretical predictions. This alignment underscores the utility of our approach in gathering quantum chemical data experimentally, a crucial step for discussing the chemical properties, such as reaction mechanisms.

Exploring the Bonding Nature of Iron(IV)-Oxo Species through Valence Bond Calculations and Electron Density Analysis.

Compound I (Cpd I) plays a pivotal role in substrate transformations within the catalytic cycle of cytochrome P450 enzymes (P450s). A key constituent of Cpd I is the iron(IV)-oxo unit, a structural motif also found in other heme enzymes and nonheme enzymes. In this study, we performed ab initio valence bond (VB) calculations, employing the valence bond self-consistent field (VBSCF) and breathing orbital valence bond (BOVB) methods, to unveil the bonding nature of this vital "Fe(IV)═O″ unit in bioinorganic chemistry. Comparisons were drawn with the triplet O2 molecule, which shares some electronic characteristics with iron(IV)-oxo. Additionally, Cpd I models of horseradish peroxidase (HRP) and catalase (CAT) were analyzed to assess the proximal ligand effect on the electronic structure of iron(IV)-oxo. Our VB analysis underscores the significant role of noncovalent resonance effects in shaping the iron(IV)-oxo bonding. The resonance stabilization within the π and σ frameworks occurs to comparable degrees, with additional stabilization resulting from resonance between VB structures from these frameworks. Furthermore, we elucidated the substantial influence of proximal and equatorial ligands in modulating the relative significance of different VB structures. Notably, in the presence of these ligands, iron(IV)-oxo is better described as iron(III)-oxyl or iron(II)-oxygen, displaying weak covalent character but enhanced by resonance effects. Although both species exhibit diradicaloid characters, resonance stabilization in iron(IV)-oxo is weaker than in O2. Further exploration using the Laplacian of electron density shows that, unlike O2, which exhibits a charge concentration region between its two oxygen atoms, iron(IV)-oxo species display a charge depletion region.

Structure and chemical bonding in high-pressure potassium silver alloys

The high-pressure structures of K-Ag alloys were examples of pressure-induced electron transfer from the electropositive potassium to the electronegative silver. We re-examined the crystal and electronic structures of KAg2, K2Ag, and K3Ag using powder X-ray diffraction and theoretical calculations. Our findings establish a connection between the morphologies of these three phases and the precursor face-centered cubic Ag. For K2Ag, we discovered a disordered structure that better matches the X-ray pattern. Valence electron density distributions obtained from the maximum entropy method, along with charge density calculations, provide a comprehensive understanding of the evolution of chemical bonding in these systems. It was found that K atoms share their valence electrons during alloy formation, contributing to K-Ag and Ag-Ag bonds in K2Ag and KAg2, while no Ag-Ag bonds are present in K3Ag. These results indicate the Zintl-Klemm model may be too simplistic to describe the structure and bonding in high-pressure binary intermetallic compounds.

A polarizable valence electron density based force field for high-energy interactions between atoms and molecules.

High-accuracy molecular force field models suited for hot gases and plasmas are not as abundant as those geared toward ambient pressure and temperature conditions. Here, we present an improved version of our previous electron-density based force field model that can now account for polarization effects by adjusting the atomic valence electron contributions to match abinitio calculated Mulliken partial charges. Using a slightly modified version of the Hohenberg-Kohn theorem, we also include an improved theoretical formulation of our model when applied to systems with degenerate ground states. We present two variants of our polarizable model, fitted from abinitio reference data calculated at CCSD(T)/cc-pVTZ and CCSD(T)/CEP-31G levels of theory, that both accurately model water dimer interaction energies. Further improvements include the additional interaction components with fictitious non-spherically symmetric, yet atom-centered, electron densities and fitting the exchange and correlation coefficients against analytical expressions. The latter removes all unphysical oscillations that are observed in the previous non-polarizable variant of our force field.

Important role of carbon doping in photocatalytic peroxymonosulfate activation of carbon nitride for efficient 4-Chlorophenol degradation

In order to improve the photocatalytic activity of pristine carbon nitride while maintaining its metal-free property to achieve the activation of peroxymonosulfate (PMS), a series of carbon-doped g-C3N4 (BCN) with different carbon contents were synthesized in this paper by adding benzotriazole to urea. The catalyst obtained by this approach significantly promoted PMS activation and enhanced the degradation efficiency of 4-chlorophenol (4-CP). Experiments and theoretical calculations showed that the introduction of carbon not only narrowed the band gap, but also accelerated the separation of photoinduced electron-hole pairs, which greatly improved the photocatalytic activity of g-C3N4. In the degradation of pollutants, free radicals and non-free radicals acted simultaneously, with non-free radicals playing a dominant role. This study explained the origin of 1O2 by Density functional theory (DFT) calculations, visible light excited photocatalysts to form holes, while carbon introduction decreased the valence-electron density around the area, inducing electron transfer from PMS toward g-C3N4 directed to generate 1O2. The 1O2-dominated degradation process improved the application of the catalysts in complex aqueous matrices. In addition, we proposed 4-CP degradation pathways, and the toxicity of intermediates derived from 4-CP degradation process reduced.

Spectroscopic screening and performance parameters of hybrid perovskite (CH3CH2PH3PbI3) using WIEN2k and SCAPS-1d

Herein, we propose that the commonly used methylammonium cation (CH3NH3+) in CH3NH3PbI3 (MAPbI3) be replaced with ethyl-phosphonium cation CH3CH2PH3+ (EP+), allowing stronger electronic coupling between the PbI6 octahedra and the organic cation and thereby increasing its stability. This paper will examine EP + based hybrid perovskite (CH3CH2PH3PbI3 or EPPbI3) as an alternative absorber material for photovoltaic cells of high efficiency and that too at affordable processing costs. By using FP-LAPW + lo methodology as used in density functional theory (DFT), we have examined physical features such as the bandgap energy, distribution of valence electron density, DOS, and optical and thermodynamical coefficients. We also simulated the performance of solar cells by employing EPPbI3 as PVA material using SCAPS-1D. The findings of the current study viz. an absorption coefficient surpassing 104 cm−1, a direct forbidden energy gap of 1.388 eV and simulated PCE of 31.8%, is enough to support applicability EPPbI3 perovskite as a PVA material.

Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale.

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Temperature-dependent work function, thermionic emission and bulk modulus of TiC: a study on the identification of free valence electrons and localized valence electrons and their roles played in carbides.

By analyzing the projected states of valence electrons (fatband structures), the localized valence electrons and the free valence electrons of TiC were identified, respectively. After defining the volumes and the magnitudes of localized valence electrons and free valence electrons, the influences of the temperature, including the thermal expansion and the atomic thermal vibration, on the localized valence electron density and the free valence electron density were investigated, respectively. Based on the metallic plasma model (MPM), the temperature-dependent work functions and the thermionic emission current densities of TiC were calculated in terms of temperature-dependent free valence electron densities. The results were in good agreement with experimental results. Furthermore, as it was observed, the linear dependence of the bulk modulus on the localized valence electron density demonstrated that the bulk modulus of TiC was determined by the localized valence electron density. The different roles played by the free valence electrons and the localized valence electrons in the work function and the bulk modulus of TiC could be attributed to their different contributions to the kinetic energy density of valence electrons. The influences of the temperature on the work function, thermionic emission and bulk modulus of TiC indicated that the transition metal carbides with lower free valence electron density, higher localized valence electron density and heavier atomic mass were desired to achieve lower work function, higher current density and higher stability.

On silicon nanobubbles in space for scattering and interception of solar radiation to ease high-temperature induced climate change

A thin film of silicon-based nanobubbles was recently suggested that could block a fraction of the sun’s radiation to alleviate the present climate crisis. But detailed information is limited to the composition, architecture, fabrication, and optical properties of the film. We examine here the optical response of Si nanobubbles in the range of 300–1000 nm to evaluate the feasibility using semi numerical solution of Maxwell’s equations, following the Mie and finite-difference time-domain procedures. We analyzed a variety of bubble sizes, thicknesses, and configurations. The calculations yield resonance scattering spectra, intensities, and field distributions. We also analyzed some many-body effects using doublets of bubbles. We show, due to high valence electron density, silicon exhibits strong polarization/plasmonic resonance scattering and absorption enhancements over the geometrical factor, which afford lighter but more efficient interception with a wide band neutral density filtering across the relevant solar light spectrum. We show that it is sufficient to use a sub monolayer raft with ∼0.75% coverage, consisting of thin (∼15 nm) but large silicon nanobubbles (∼550 nm diameter), to achieve 1.8% blockage of solar light with neutral density filtering, and ∼0.78 mg/m2 silicon, much less than the mass effective limit set earlier at 1.5 g/m2. We evaluated solid counterpart nanoparticles, which may be produced in blowing/inflation procedures of molten silicon, as well as aging by including silicon oxide capping. The studies confirm the feasibility of a space bubble filtering raft, with insignificant imbalance of the correlated color temperature (CCT) and color rendering index characteristics of sunlight.

Electron screening in the 2H(19F,p)20F reaction

The dependence of electron screening potential on the position of the target nucleus in host-material lattice was investigated by measuring the rate of the 2H(19F,p)20F reaction in zirconium, titanium and palladium targets containing deuterium. Very different values of the screening potential were measured, thus showing the link with the valence electron densities around deuterium nuclei.

Evolution of valence electron density of compressed Na–Au alloys

Evolution of valence electron density of compressed Na–Au alloys

Interaction of Si Atom with the (001) Surface of TiN, AlN and TaN Compounds

Nowadays, the application of multicomponent coatings with multiphase nanocrystalline structure is the most promising direction in the search for wear-resistant protective coatings with a full set of necessary operational properties. Nanocrystalline multicomponent coatings based on the Ti-Al-Ta-Si-N system have a high hardness combined with thermal stability and oxidation resistance. Silicon atoms are weakly soluble in the TiN, Ti1−xAlxN, and TaN crystalline phases of the Ti-Al-Ta-Si-N system and interact preferentially with N atoms, forming the amorphous Si3N4 phase. In this context, it is important to first study the peculiarities of the interaction of Si atoms with the simplest structural units of the Ti-Al-Ta-Si-N system, such as TiN, AlN, and TaN compounds with the NaCl structure. This work is devoted to the study of the interaction of a Si atom with the (001) surface of AlN, TiN, and TaN compounds with the NaCl structure using ab initio calculations. This provides information for a deep understanding of the initial stages of the formation of different crystallites of the considered composite. It was established that the adsorption of silicon on the (001) surface of AlN, TiN, and TaN significantly increases the relaxation of the surface layers and leads to an increase in the corrugation observed on the clean surfaces. The largest corrugation is observed on the surface of the TaN compound. The most energetically favorable adsorption positions of Si atoms were found to be the position of Si above the N atom on the TiN and TaN surfaces and the quadruple coordinated position on the AlN surface. The valence electron density distribution and the crystal orbital Hamiltonian population were studied to identify the type of Si atom bonding with the (001) surface of AlN, TiN, and TaN compounds. It was found that silicon forms predominantly covalent bonds with the nearest metal and nitrogen atoms, except for the quadruple coordinated position on the surface of TiN and TaN, where there is a high degree of ionic bonding of silicon with surface atoms.

A Comparative XPS, UV PES, NEXAFS, and DFT Study of the Electronic Structure of the Salen Ligand in the H2(Salen) Molecule and the [Ni(Salen)] Complex.

A comparative study of the electronic structure of the salen ligand in the H2(Salen) molecule and the [Ni(Salen)] complex was performed using the experimental methods of XPS, UV PES, and NEXAFS spectroscopy along with DFT calculations. Significant chemical shifts of +1.0 eV (carbon), +1.9 eV (nitrogen), and -0.4 eV (oxygen) were observed in the 1s PE spectra of the salen ligand atoms when passing from a molecule to a complex, unambiguously indicating a substantial redistribution of the valence electron density between these atoms. It is proposed that the electron density transfer to the O atoms in [Ni(Salen)] occurred not only from the Ni atom, but also from the N and C atoms. This process seemed to be realized through the delocalized conjugated π-system of the phenol C 2p electronic states of the ligand molecule. The DFT calculations (total and partial DOS) for the valence band H2(Salen) and [Ni(Salen)] described well the spectral shape of the UV PE spectra of both compounds and confirmed their experimental identification. An analysis of the N and O 1s NEXAFS spectra clearly indicated that the atomic structure of the ethylenediamine and phenol fragments was retained upon passing from the free salen ligand to the nickel complex.

A simple, quantitative expression for understanding and evaluating the yield strength of amorphous alloys based on symbolic regression and dimensional calculation

High strength can be the most important and outstanding mechanical property of amorphous alloys as structural materials. To improve the limitations of current strength models, elucidate the key factors determining the yield strength (σy) of amorphous alloys and formulate an explicit, quantitative expression of σy, herein, we successfully developed an ML-based strength model by employing symbolic regression and dimensional calculation. 10 key descriptors were initially extracted from 22 candidate descriptors using random forest model and SHAP method, and dimensional calculation was simultaneously carried out with numerical calculation to ensure the dimensional consistency of final expression. The results shown that the SR-DC model was superior to SR-GP and SR-BGP models, with its satisfactory combination of accuracy (R2=0.93) and simplicity. Meanwhile, molar volume (Vm), melting point (Tm), Pauling electronegativity (χp), and valence electron density (VED) were identified as the key factors and were used to explicitly express the yield strength of amorphous alloys. The final expression was well verified by further theoretical and experimental verification. This work not only provided a general benchmark for the composition design and development of high-strength amorphous alloys but also demonstrated the great potential of symbolic regression combined with dimensional calculation to develop quantitative composition-structure-property relationship in future designing novel materials.

Ab initio study of the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeSe, GeS)

In this article, the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeS, GeSe) are studied by methods of the density functional theory and first-principles pseudopotential based on own program code. The spatial distribution of valence electron density, density of states, and Coulomb potential along transverse direction are calculated. It was found that a more significant rearrangement of the electron density was observed during compression of both GeS and GeSe bi-layer films than mono-layer films, but more significant in GeS films.

Development of Fe-containing BCC hydrogen storage alloys with high vanadium concentration

Vanadium-based alloys are promising hydrogen storage materials which reversibly absorb hydrogen > 2.0 wt% under ambient conditions. In present study, we systematically investigated the V-Ti-Cr-Fe quaternary alloys with V concentration of 65–80 at% and discussed the relationship between hydrogen sorption properties and lattice parameter (a) and valence electron density (e/a). When 3.023 ≤ a ≤ 3.031 Å and e/a ≤ 5.12, the alloys show the hydrogen desorption amount ≥ 2.30 wt% and desorption plateau pressure ≥ 0.1 MPa at 298 K. More interestingly, compared to the Fe-free V75-Ti-Cr (Ti/Cr = 0.9) alloy, addition of 1–3 at% Fe could improve the hydrogen desorption amount and plateau pressure. The optimized alloy, V75-Ti-Cr-Fe2 (Ti/Cr = 0.9), shows a reversible hydrogen capacity of 2.46 wt% at 298 K in the 1st cycle and the capacity retention rate of 96 % after 100 cycles.