Atomic Sphere Research Articles

Infrared Polarizabilities of 3d-Transition and Rare-Earth Metals

A transition or rare-earth metal is modeled as the atom immersed in a jellium at intermediate electron gas densities specified by rs=4.0. The ground states of the spherical jellium atom are constructed based on the Hohenberg-Kohn-Sham density-functional formalism with the inclusion of electron-electron self-interaction corrections of Perdew and Zunger. Static and dynamic polarizabilities of the jellium atom are deduced using time-dependent linear response theory in a local density approximation as formulated by Stott and Zaremba. The calculation is extended to include the intervening elements In, Xe, Cs, and Ba. The calculation demonstrates how the Lindhard dielectric function can be modified to apply to non-simple metals treated in the jellium model.

АНАЛИЗ ТЕТРАГОНАЛЬНОСТИ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ МАРТЕНСИТА СПЛАВОВ Fe–C В МОДЕЛИ ЖЁСТКИХ АТОМОВ-ШАРОВ

The model of packaging solid atoms-spheres was used in the article to clarify a number of issues affecting martensitic transformation in carbon steels. The deformation that occurs when a spherical carbon atom with a radius of 0.77 Å is placed in the octahedral pore of the z-sublattice of iron incorporation is considered. It is shown that the displacements of iron atoms lying on the oz axis are so significant that they block the next two octahedral pores from possible filling. They will be released only when four carbon atoms occupy similar pores on the edges of the unit cell and again remove iron atoms from the blocked pores. The calculation was made of the number of carbon atoms in the unit cell, depending on the carbon content in the steel. It turned out that the number of carbon atoms in the cell equal to 0.5 is achieved at a carbon concentration of 5.13 wt. %. Consequently, almost all hardened structural and tool steels on average do not fill even one octahedral pore in the center of the face. Within the framework of this model, the lattice parameters a and c are calculated, which are compared with the experimental data obtained by academician G.V. Kurdyumov. Their very good agreement is shown for parameter c and less accurate for parameter a, which, from our point of view, is due to the lack of information about the Poisson ratio for processes on the atomic scale. A method has been developed for determining the lattice parameters of an ideal martensite, in which all z-pores are occupied by carbon atoms.

First-principles study of structural, electronic and magnetic properties of Co-based quaternary Heusler compounds: CoFeCrAl, CoFeTiAs, CoFeCrGa and CoMnVAs

We study the Structural, Electronic and Magnetic properties of Co-based LiMgPdSn-types of quaternary Heusler compounds (CoFeCrAl, CoFeTiAs, CoFeCrGa, and CoMnVAS) using Density Functional Theory (DFT) implemented on Tight Binding Linear Muffin-Tin Orbital within Atomic Sphere Approximation(TB-LMTO-ASA) Code. The optimized value of lattice parameter for CoFeCrAl, CoFeTiAs, CoFeCrGa and CoMnVAs are found to be 5.61A˚, 5.76 A˚, 5.61A˚ and 5.71A˚ respectively. From the calculation of electronic band structure and spin polarized total density of states (DOS), we found that CoFeCrAl and CoFeCrGa are spin-gapless semiconductor with half-metallic gap of 0.82eV and 0.25eV respectively. CoFeTiAs half-metals (Nearly spin-gapless semiconductor) with half-metallic gap 0.38 eV and CoMnVAs is found to be nearly gapless half-metal. Magnetic moment of these compounds almost obey the Slater-Pauling rules. All these compounds expected to have high curie temperature which makes them significant for spintroincs/magnetoelectroincs applications.BIBECHANA 15 (2018) 50-59

Exchange-correlation energies of atoms from efficient density functionals: influence of the electron density

The primary goal of Kohn–Sham density functional theory is to evaluate the exchange-correlation contribution to electronic properties. However, the accuracy of a density functional can be affected by the electron density. Here we apply the nonempirical Tao–Mo (TM) semilocal functional to study the influence of the electron density on the exchange and correlation energies of atoms and ions, and compare the results with the commonly used nonempirical semilocal functionals local spin-density approximation (LSDA), Perdew–Burke–Ernzerhof (PBE), Tao–Perdew–Staroverov–Scuseria (TPSS), and hybrid functional PBE0. We find that the spin-restricted Hartree–Fock density yields the exchange and correlation energies in good agreement with the Optimized Effective Potential method, particularly for spherical atoms and ions. However, the errors of these semilocal and hybrid functionals become larger for self-consistent densities. We further find that the quality of the electron density have greater effect on the exchange-correlation energies of kinetic energy density-dependent meta-GGA functionals TPSS and TM than on those of the LSDA and GGA, and therefore, should have greater influence on the performance of meta-GGA functionals. Finally, we show that the influence of the density quality on PBE0 is slightly reduced, compared to that of PBE, due to the exact mixing.

Huygens–Fresnel picture for electron-molecule elastic scattering

The elastic scattering cross sections for a slow electron by C2 and H2 molecules have been calculated within the framework of the non-overlapping atomic potential model. For the amplitudes of the multiple electron scattering by a target the wave function of the molecular continuum is represented as a combination of a plane wave and two spherical waves generated by the centers of atomic spheres. This wave function obeys the Huygens–Fresnel principle according to which the electron wave scattering by a system of two centers is accompanied by generation of two spherical waves; their interaction creates a diffraction pattern far from the target. Each of the Huygens waves, in turn, is a superposition of the partial spherical waves with different orbital angular momenta l and their projections m. The amplitudes of these partial waves are defined by the corresponding phases of electron elastic scattering by an isolated atomic potential. In numerical calculations the s- and p-phase shifts are taken into account. So the number of interfering electron waves is equal to eight: two of which are the s-type waves and the remaining six waves are of the p-type with different m values. The calculation of the scattering amplitudes in closed form (rather than in the form of S-matrix expansion) is reduced to solving a system of eight inhomogeneous algebraic equations. The differential and total cross sections of electron scattering by fixed-in-space molecules and randomly oriented ones have been calculated as well. We conclude by discussing the special features of the S-matrix method for the case of arbitrary non-spherical potentials.

Electronic and Magnetic Properties of Half Metallic Heusler Alloy Co2mnsi: A First-Principles Study

Heusler alloys have been of great interest because of their application in the field of modern technological word. Electronic and magnetic properties of Co, Mn, Si and the Heusler alloy Co2MnSi have been studied using Density functional theory based Tight Binding Linear Muffin Tin Orbital with Atomic Sphere Approximation (TB-LMTO-ASA) approach. From the calculation lattice parameter of optimized structure of Co, Mn, Si and Co2MnSi are found to be 2.52A0 , 3.49A0 , 5.50A0 , 5.53A0 respectively. Band structure calculations show that Co and Mn are metallic, Si as semi-conducting while the Heusler alloy Co2MnSi as half-metallic in nature with band gap 0.29eV. The charge density plot indicates major bonds in Co2MnSi are ionic in nature. Magnetic property has been studied using the density of states (DOS), indicating that Co and Co2MnSi are magnetic with magnetic moment 2.85μB and 4.91μB respectively. The contribution of orbitals in band, DOS and magnetic moment are due to d-orbitals of Co and Mn and little from s and p-orbital of Si in Co2MnSi.The Himalayan Physics Vol. 6 & 7, April 2017 (31-36)

Isolated spherical impurity in noble gas crystals

To explain the experimentally observed structural anomalies in diluted solid solutions of argon in krypton, we calculated the positions of atoms in a nine-layer spherical cluster of krypton with an argon atom in the center. The interaction was approximated with the Lenard–Jones potential. The substitution volume ω of an argon atom in the krypton crystal was calculated. It was shown that compared to the generally accepted estimates obtained within the nearest-neighbor approximation, the value of ω is approximately twice as small. This effect is explained by an additional repulsion of the first coordination sphere of krypton atoms arising as the krypton atoms approach each other upon their inward motion.

Dilithium and Magnesium Alkanediides and 1‐Oxaalkanediides

Dilithium and magnesium alkanediides of the types Li(CH2)nLi and [Mg(CH2)n]x represent substance classes with a long tradition, however, the interest in these compounds ceased in recent years. Numerous reasons account for the vanishing attention such as challenging synthetic procedures, poor to moderate yields, complex structures in solution and the solid state, and complex equilibria in solution. Degradation processes (ether degradation, β‐hydride elimination, indirect reduction reactions) increase the intricacy of these compounds because these degradation products take part in the solution equilibria. Thus, chloride and hydride ions are able to act as templates for lithium cages yielding e.g. [(Et2O)4Li]+ [Li12(C4H8)6(@‐X)]– (X = H, Cl) with an X‐centered Li12 icosahedron. Despite these challenges, the dilithium and magnesium alkanediides are a fascinating substance class with a rich chemistry by its own (such as Schlenk‐type equilibria) and as metalating and reducing reagents. The enormous sensitivity toward moisture (hydrolysis reactions) and air (oxidation processes) and reactivity toward ethereal media (ether degradation reactions) also interferes with the dianionic alkanes because alkoxide and oxide ions are strong Lewis bases and as such, are easily integrated in the structures as bridging anions or centers of metal cages. Thus, aerial oxidation leads to insertion of an oxygen atom into the metal–carbon bond yielding 1‐oxaalkanediides. Alternatively, magnesium 1‐oxaalkanediides can be prepared by a two‐step protocol from chloroalkanol, starting with deprotonation of the alcohol functionality with a Grignard reagent and a subsequent reduction of the C–Cl unit with magnesium turnings. This protocol leads to compounds of the type [{MgO(CH2)n}x(MgCl2)y] with additional ether bases saturating the coordination spheres of the magnesium atoms. All these reactions (increasing degradation processes and reactions with inadvertently introduced traces of oxygen and moisture) limit the durability of such solutions, subsumed as aging of these dilithium and magnesium dialkanediide stock solutions, which lead to a changing (decreasing) reactivity of such reagents.

Invariom-model refinement and Hirshfeld surface analysis of well-ordered solvent-free dibenzo-21-crown-7.

Crown ethers and their supramolecular derivatives are well-known chelators and scavengers for a variety of cations, most notably heavier alkali and alkaline-earth ions. Although they are widely used in synthetic chemistry, available crystal structures of uncoordinated and solvent-free crown ethers regularly suffer from disorder. In this study, we present the X-ray crystal structure analysis of well-ordered solvent-free crystals of dibenzo-21-crown-7 (systematic name: dibenzo[b,k]-1,4,7,10,13,16,19-heptaoxacycloheneicosa-2,11-diene, C22H28O7). Because of the quality of the crystal and diffraction data, we have chosen invarioms, in addition to standard independent spherical atoms, for modelling and briefly discuss the different refinement results. The electrostatic potential, which is directly deducible from the invariom model, and the Hirshfeld surface are analysed and complemented with interaction-energy computations to characterize intermolecular contacts. The boat-like molecules stack along the a axis and are arranged as dimers of chains, which assemble as rows to form a three-dimensional structure. Dispersive C-H...H-C and C-H...π interactions dominate, but nonclassical hydrogen bonds are present and reflect the overall rather weak electrostatic influence. A fingerprint plot of the Hirshfeld surface summarizes and visualizes the intermolecular interactions. The insight gained into the crystal structure of dibenzo-21-crown-7 not only demonstrates the power of invariom refinement, Hirshfeld surface analysis and interaction-energy computation, but also hints at favourable conditions for crystallizing solvent-free crown ethers.

A theoretical-electron-density databank using a model of real and virtual spherical atoms

A database describing the electron density of common chemical groups using combinations of real and virtual spherical atoms is proposed, as an alternative to the multipolar atom modelling of the molecular charge density. Theoretical structure factors were computed from periodic density functional theory calculations on 38 crystal structures of small molecules and the charge density was subsequently refined using a density model based on real spherical atoms and additional dummy charges on the covalent bonds and on electron lone-pair sites. The electron-density parameters of real and dummy atoms present in a similar chemical environment were averaged on all the molecules studied to build a database of transferable spherical atoms. Compared with the now-popular databases of transferable multipolar parameters, the spherical charge modelling needs fewer parameters to describe the molecular electron density and can be more easily incorporated in molecular modelling software for the computation of electrostatic properties. The construction method of the database is described. In order to analyse to what extent this modelling method can be used to derive meaningful molecular properties, it has been applied to the urea molecule and to biotin/streptavidin, a protein/ligand complex.

On the importance of local orbitals using second energy derivatives for [formula omitted] and [formula omitted] electrons

The all-electron linearized augmented plane wave (LAPW) methods are among the most accurate to solve the Kohn–Sham equations of density functional theory for periodic solids. In the LAPW methods, the unit cell is partitioned into spheres surrounding the atoms, inside which the wave functions are expanded into spherical harmonics, and the interstitial region, where the wave functions are expanded in Fourier series. Recently, Michalicek et al. (2013) reported an analysis of the so-called linearization error, which is inherent to the basis functions inside the spheres, and advocated the use of local orbital basis functions involving the second energy derivative of the radial part (HDLO). In the present work, we report the implementation of such basis functions into the WIEN2k code, and discuss in detail the improvement in terms of accuracy. From our tests, which involve atoms from the whole periodic table, it is concluded that for ground-state properties (e.g., equilibrium volume) the use of HDLO is necessary only for atoms with d or f electrons in the valence and large atomic spheres. For unoccupied states which are not too high above the Fermi energy, HDLO systematically improve the band structure, which may be of importance for the calculation of optical properties.

The first copper(I)-olefin complexes bearing a 1,3,4-oxadiazole core: Alternating-current electrochemical crystallization, X-ray experiment and DFT study

By means of alternating-current electrochemical technique, four new π-complexes, namely [Cu2(C11H10N2OS)2Br1.91Cl0.09] (1), [Cu(C11H10N2OS)NO3] (2), [Cu2(C11H10N2OS)2(H2O)2](BF4)2 (3) and [Cu2(C11H10N2OS)2(H2O)2](ClO4)2 (4), were obtained using copper(II) salts and the 2-(allylthio)-5-phenyl-1,3,4-oxadiazole (C11H10N2OS) ligand. The metal and halogen centers in 1 form Cu2X2 dimers; the N-atom from the oxadiazole ring and the CC bond of the allyl group from the same ligand complete the copper coordination environment, giving [Cu(C11H10N2OS)X]2 isolated fragments. The ligand plays the same chelating role in 2, whereas the O (NO3) atom occupies the third position in the copper atom’s equatorial plane. Two more elongated Cu–O(NO3) contacts associate the Cu(C11H10N2OS)NO3 fragments into 1D chains. The geometries of the [Cu(C11H10N2OS)]22+ cationic units in 3 and 4 are affected by the position of two water molecules in the coordination spheres of the copper atoms with respect to the plane containing the oxadiazole rings and the copper atoms. The molecular structures and Raman spectra of the compounds were computed using the DFT/B3LYP/cc-pVDZ level of theory. The results are compared with the experimental data obtained and used for vibrational band assignment.

Electronic and Magnetic Properties of Half Metallic Heusler Alloy Co2MnSi: A First-Principles Study

<p class="Default">Heusler alloys have been of great interest because of their application in the field of modern technological applications. Electronic and magnetic properties of Co, Mn, Si and the Heusler alloy Co<sub>2</sub>MnSi have been studied using Density functional theory based Tight Binding Linear Muffin Tin Orbital with Atomic Sphere Approximation (TB-LMTO-ASA) approach. From the calculation lattice parameter of optimized structure of Co, Mn, Si and Co<sub>2</sub>MnSi are found to be 2.52Å, 3.49Å, 5.50Å, 5.53Å respectively. Band structure calculations show that Co and Mn are metallic, Si as semi-conducting while the Heusler alloy Co<sub>2</sub>MnSi as half-metallic in nature with band gap 0.29eV. The charge density plot indicates major bonds in Co<sub>2</sub>MnSi are ionic in nature. Magnetic property has been studied using the density of states (DOS), indicating that Co and Co2MnSi are magnetic with magnetic moments 2.85μ<sub>B</sub> and 4.91μ<sub>B</sub> respectively. The contribution of orbital in band structure, DOS and magnetic moments are due to d-orbital of Co and Mn and little from s and p-orbital of Si in Co<sub>2</sub>MnSi alloy.</p><p><strong>Journal of Nepal Physical Society</strong><em><br /></em>Volume 4, Issue 1, February 2017, Page: 60-66</p>

The structure of lithium silicate melts revealed by high-temperature Raman spectroscopy

ABSTRACTThe structure of lithium silicate glasses and melts was studied by in situ Raman spectroscopy at temperatures from 293 to 1423 K. The composition range of the studied melts varied from 33 to 60 mol. % of lithium oxide. A curve-fitting procedure was applied for data analysis taking into account the second coordination sphere of silica atoms. To obtain a correct interpretation of all bands in the Raman spectra, additional types of structural unit, carrying the information about the neighbors, were introduced into a description of glasses and melts structure. It was shown that structural units possessing two non-bridging oxygen atoms are typical of almost all the melts of the lithium silicate system, with their content in the system being higher than that in the melts of the sodium silicate system of the same composition. Such an unusual behavior of these melts under increased temperature as well as the elevated concentration of species with two non-bridging oxygen atoms may significantly affect the viscosity value of lithium silicate melts.

Modeling the p–v–T behavior of furfural compounds using perturbed linear Yukawa isotherm regularity

The p–v–T behavior of furfural compounds (FCs) such as 5–methylfurfural (5–MF) and tetrahydrofurfuryl alcohol (THF–A) by using the perturbation-based linear Yukawa isotherm regularity (LYIR) were modeled. The LYIR is a perturbation–based equation of state (EoS) which was recently derived by making use of the attraction part of hard–core Yukawa (HCY) potential as perturbation average effective pair potential (AEPP) in the framework of liquid's thermodynamic perturbation theory. To use the LYIR EoS for modeling FCs, each molecule of the FC was considered as an one–center spherical united atom with temperature–dependent effective diameter. The results showed that the LYIR model can predict the p–v–T behavior of the selected FCs very well and the reproduced densities obtained by the LYIR EoS are in very good agreements with the available experimental densities. The range of the relative deviations percentage between the experimental and calculated density are from −0.006% to 0.02% for 5–MF and 0.009% to −0.01% for THF–A. In addition, the parameters of the HCY potential (σeff, λeff), as AEPP, were obtained for the selected FCs using the LYIR model and provided valuable information on the structure, intermolecular interactions, and the role of substitution groups on the p–v–T behavior of this class of green fluids. Moreover, the results of the study mirror the effect of the hydrogenation and the dehydrogenation processes on the value of σeff. Additionally, the contribution of internal pressure in compression factor and also the effect of both pressure and temperature on the internal compression factor were investigated.

Transferable Aspherical Atom Modeling of Electron Density in Highly Symmetric Crystals: A Case Study of Alkali-Metal Nitrates.

A comparative electron density study (from X-ray diffraction and periodic quantum chemistry) of sodium and potassium nitrates is performed to test the performance of a transferrable aspherical atom model, which is based on the invarioms, to describe chemical bonding features of ions occurring in sites of different symmetry typical of inorganic salts and in different crystal environments. Relying on tabulated entries for the isolated ions (although tailor-made to account for different site symmetries), it takes the same time to employ as the spherical atom model routinely used in X-ray diffraction studies but provides an electron density distribution that faithfully reveals all the interionic interactions-even the weakest ones (such as between the nitrate anions or a K···N interaction found in the metastable form of KNO3) yet important for properties of inorganic materials-as if obtained from high-resolution X-ray diffraction data.

Synthesis, structure and catalytic olefin epoxidation activity of a dinuclear oxo-bridged oxodiperoxomolybdenum(VI) complex containing coordinated 4,4′-bipyridinium

A solution obtained by dissolving MoO3 in aqueous hydrogen peroxide reacts with 4,4′-bipyridine (4,4′-bipy) to give the dinuclear oxodiperoxo complex [{MoO(O2)2(4,4′-Hbipy)}2(μ-O)] (1). The unique structural feature of 1 with respect to other related complexes is the completion of the coordination sphere of the Mo atoms by a monoprotonated [4,4′-Hbipy]+ molecule, which is bound to the metal center through the unprotonated pyridine ring. The catalytic performance of 1 has been investigated for the epoxidation of the benchmark substrate cis-cyclooctene and three representative bio-olefins, namely limonene, methyl oleate and methyl linoleate, using either aq. H2O2 or tert-butylhydroperoxide (TBHP) oxidants. For the epoxidation of cis-cyclooctene, catalytic activity at 55°C with TBHP was comparable with that at 70°C with H2O2 (90–95% conversion at 24h; turnover number in the range 90–95molCymolMo−1; 100% selectivity towards cyclooctene epoxide). The catalytic reactions are homogeneously catalysed and compounds were isolated from the reaction mixtures in order to ascertain the nature of the active species present in solution. With H2O2 as oxidant, the liquid-phase conversion of 1 to β-octamolybdate anions, β-[Mo8O26]4−, takes place during the 24h catalytic batch run. The same transformation occurs when the molybdenum oxide-bipyridine hybrid compound [MoO3(4,4′-bipy)0.5] (2) is used as the (pre)catalyst instead of 1. With TBHP as oxidant, no species other than 1 were isolated from the recovered reaction solution. For the reactions with the bio-olefins (1, TBHP, 70°C, 24h), fairly high conversions of 68–92% were reached, with very high selectivity towards epoxide products (90–100%). Monoepoxides were the major products formed from limonene and methyl linoleate.

Li9Al4Sn5as a new ordered superstructure of the Li13Sn5type

Alloys from the ternary Li-Al-Sn system have been investigated with respect to possible applications as negative electrode materials in Li-ion batteries. This led to the discovery of a new ternary compound, a superstructure of the Li13Sn5 binary compound. The ternary stannide, Li9Al4Sn5 (nonalithium tetraaluminium pentastannide; trigonal, P-3m1, hP18), crystallizes as a new structure type, which is an ordered variant of the binary Li13Sn5 structure type. One Li and one Sn site have -3m. symmetry, and all other atoms occupy sites of 3m. symmetry. The polyhedra around all types of atoms are rhombic dodecahedra. The electronic structure was calculated by the tight-binding linear muffin-tin orbital atomic spheres approximation method. The electron concentration is higher around the Sn and Al atoms, which form an [Al4Sn5]m- polyanion.

Synthesis and structure of heptaaqua{nitrilotris(methylenephosphonato)}(dibarium)sodium monohydrate [Na(H2O)3{μ6-NH(CH2PO3)3}(μ-H2O)3Ba2(H2O)] · H2O

Crystals of the monohydrate form of heptaaqua{nitrilotris(methylenephosphonato)}(dibarium) sodium [Na(H2O)3{µ6-NH(CH2PO3)3}(µ-H2O)3Ba2(H2O)] · H2O are obtained; space group P21/c, Z = 4; a = 13.9117(10) A, b = 11.54030(10) A, and c = 24.1784(17) A, s = 148.785(18)°. The Na atom is coordinated octahedrally by one oxygen atom of a phosphonate group and five water molecules, including two bridging molecules. Ba atoms occupy two inequivalent crystallographic positions with coordination number eight and nine. The coordination spheres of both Ba atoms include two water molecules. Each ligand is bound to one Na atom and five Ba atoms forming three Ba–O–P–O and five Ba–O–P–C–N–C–P–O chelate cycles. In addition to the coordination bonds, molecules, including the solvate water molecule, are involved in hydrogen bonds in the crystal packing.

127I NMR calculations in binary metal iodides by PBE-GGA, YS-PBE0 and mBJ exchange correlation potentials

We have calculated Nuclear Magnetic Resonance (NMR) spectroscopy for 127I (quadrupolar nuclei I=5/2) in binary metal iodides XI (X=Li, Na, K, Rb and Cs) by using PBE- GGA, YS- PBE0 and mBJ exchange correlation potentials. The results show that the nature of bonds between Iodine and metal atoms are ionic. The main contribution in NMR spectroscopy is related to the induced current inside the atomic sphere and the remainder of the unit cell volume contributes only a few ppm. Obtained NMR shifts are compared with the NMR shielding data and the NMR shielding for metal-p band varies across the series about 221ppm. Density of states results indicate that the largest contribution in the shielding comes from the I-core electrons (1s and 4d). The NMR shielding graphs show that there are negative linear correlation with slope −1.18, −1.16 and −1.01 by PBE- GGA, YS- PBE0 and mBJ, respectively. The computed results by mBJ are in good agreement with the experimental values.