Coordination Number Of Atoms Research Articles

In English

By the method of density functional theory with exchange-correlation functional B3LYP and basis set 3‑21G (d), the structural and energy characteristics have been considered of the molecular models of SnO2 nanoclusters of different size and composition with the number of Sn atoms from 1 to 10. Incompletely coordinated surface tin atoms were terminated by hydroxyl groups. It has been shown that the Sn–O bond length in nanoclusters does not depend on the cluster size and on the coordination number of Sn atoms, but is determined by the coordination type of neighboring oxygen atoms. Namely, the bond length Sn–O(3) (@ 2.10 Å) is greater than that of Sn–O (2) (@ 1.98 Å). The calculated values of Sn–O (3) bond lengths agree well with the experimental ones for crystalline SnO 2 samples (2.05 Å). The theoretically calculated width of the energy gap decreases naturally with increasing cluster size (from 6.14 to 3.46 eV) and approaches the experimental value of the band gap of the SnO 2 crystal (3.6 eV). The principle of additivity was used to analyze the energy characteristics of the considered models and to estimate the corresponding values for a cassiterite crystal. According to this principle, a molecular model can be represented as a set of atoms or atomic groups of several types that differ in the coordination environment and, therefore, make different contributions to the total energy of the system. The calculated value of the atomization energy for SnO2 is 1661 kJ/mol and corresponds satisfactorily to the experimentally measured specific atomization energy of crystalline SnO2 (1381 kJ/mol). It has been shown that a satisfactory reproduction of the experimental characteristics of crystalline tin dioxide is possible when using clusters containing at least 10 state atoms, for example, (SnO2)10×14H2O.

Si⋯O proximity in imidosilanes – absence of orbital interactions

Abstract New N-silylated phthalimides, succinimides, and 1,8-napthalimides were synthesised by reactions of the alkali imides with chlorosilanes in THF. Six different mono-, di-, tri- and tetra-iminosilanes of the type (CH3)4–n Si(imide) n were obtained and the products analysed with 1H, 13C, 29Si NMR, and Raman spectroscopy. The molecular structures of four imidosilanes have been determined by single-crystal X-ray diffraction. A characteristic structural feature of the compounds is the fact that all intramolecular Si⋯O distances are significantly below the sum of the van-der-Waals radii of silicon and oxygen of 3.62 Å. Experimentally found values for Si⋯O distances range from 2.813 to 3.030 Å. However, there are no significant orbital interactions between silicon and oxygen atoms, as shown by quantum chemical analysis with AIM and NBO methods. The short Si⋯O distances in these molecules are caused by the geometry of the rigid imide group bound to the silicon atom, and there is no evidence for an increase of the coordination number of the Si atoms.

Locating Single-Atom Optical Picocavities Using Wavelength-Multiplexed Raman Scattering.

Transient atomic protrusions in plasmonic nanocavities confine optical fields to sub-1-nm3 picocavities, allowing the optical interrogation of single molecules at room temperature. While picocavity formation is linked to both the local chemical environment and optical irradiation, the role of light in localizing the picocavity formation is unclear. Here, we combine information from thousands of picocavity events and simultaneously compare the transient Raman scattering arising from two incident pump wavelengths. Full analysis of the data set suggests that light suppresses the local effective barrier height for adatom formation and that the initial barrier height is decreased by reduced atomic coordination numbers near facet edges. Modeling the system also resolves the frequency-dependent picocavity field enhancements supported by these atomic scale features.

High valence states of Pd supported on ferroelectric BaTiO3 driven by electric polarization

At a metal–ferroelectric junction, it has been considered that the electric polarization of the ferroelectric material can affect the electronic structure of the neighboring metal. Here, we demonstrate that the valence state of Pd can be shifted to the unstable high value of 4+ by the electric field of electric polarization in ferroelectric BaTiO3. Study of the absorption fine structure of both hard and soft x rays revealed the existence of Pd4+ states on the surface of Pd oxide nanoparticles. The positions of Pd and oxygen atoms are shifted in opposite directions by the electric field due to the electric polarization of ferroelectric BaTiO3. The atomic displacement of Pd and O forms a zigzag structure, in which the coordination number of Pd atoms is changed from four to six, producing a quadrivalent state. This report presents experimental evidence that ferroelectric polarization can control the electronic states of neighboring metal atoms, and we suggest that using the ferroelectric support effect may produce a new type of catalyst.

Initial hydration process of calcium silicates in Portland cement: A comprehensive comparison from molecular dynamics simulations

As the main components of Portland cement, calcium silicates show substantial differences in their hydration reactivity which have not been fully explained. A comprehensive comparison of the initial hydration process of calcium silicates, namely dicalcium silicate (C2S) and tricalcium silicate (C3S), was conducted using molecular dynamics simulations. The initial hydration process was divided into three stages using cut-off times of 0.001 ns and 3 ns. The hydration of M3-C3S (010) was more evident than that of β-C2S (100), supported by the hydroxylation degree, radial distribution function, atomic density profile, etc. The coordination number of the surface Ca atoms might be the underlying reason for such a difference. Interactions between cement surfaces and water molecules were mainly characterised by solid OH bonding and Ca-water O bonding. Dissolution of Ca atoms was observed, although quite scarce, while no dissolution of Si atoms was observed.

Electronic and Magnetic Properties of Building Blocks of Mn and Fe Atomic Chains on Nb(110).

We present results for the electronic and magnetic structure of Mn and Fe clusters on Nb(110) surface, focusing on building blocks of atomic chains as possible realizations of topological superconductivity. The magnetic ground states of the atomic dimers and most of the monatomic chains are determined by the nearest-neighbor isotropic interaction. To gain physical insight, the dependence on the crystallographic direction as well as on the atomic coordination number is analyzed via an orbital decomposition of this isotropic interaction based on the spin-cluster expansion and the difference in the local density of states between ferromagnetic and antiferromagnetic configurations. A spin-spiral ground state is obtained for Fe chains along the [] direction as a consequence of the frustration of the isotropic interactions. Here, a flat spin-spiral dispersion relation is identified, which can stabilize spin spirals with various wave vectors together with the magnetic anisotropy. This may lead to the observation of spin spirals of different wave vectors and chiralities in longer chains instead of a unique ground state.

Effect of irradiation on the atomic structure of borosilicate glasses

AbstractBorosilicate glasses incorporating high‐level nuclear waste are exposed to high‐energy radiations during their storage in the deep geological repositories. However, the effect of radiation on the atomic structure of borosilicate glasses remains poorly understood. Herein, using molecular dynamics simulations, we study the irradiation‐induced structural changes of a series of calcium‐sodium borosilicate glasses with varying Si/B molar ratios—ranging from pure silicate to pure borate glasses. We observe that irradiation leads to an increase in disorder, both in the short‐ and medium‐range, as evidenced by the enthalpy, coordination number, and ring distribution. In particular, the impact of the change in the atomic structure (due to radiation) on the glass volume is investigated. Interestingly, we observe a composition‐dependent transition in the volumetric response of borosilicate glasses under irradiation—wherein borate‐rich compositions tend to swell, whereas silica‐rich glasses tend to densify. Through a detailed analysis of the structure, we demonstrate two competing mechanisms contributing to the volume change, i.e., a decrease in the coordination number of boron atoms and a reduction in the average silicon inter‐polytope angle. We also show that the increase in the disorder in the medium‐range order may play a major role in governing the volumetric changes in the irradiated structure in a non‐trivial fashion. Altogether, the present study highlights that irradiation has a non‐trivial effect on borosilicate glasses, which, in turn, could impact their corrosion kinetics.

Atomic Regulation of PGM Electrocatalysts for the Oxygen Reduction Reaction.

With the increasing enthusiasm for the hydrogen economy and zero-emission fuel cell technologies, intensive efforts have been dedicated to the development of high-performance electrocatalytic materials for the cathodic oxygen reduction reaction (ORR). Some major fundamental breakthroughs have been made in the past few years. Therefore, reviewing the most recent development of platinum-group-metal (PGM) ORR electrocatalysts is of great significance to pushing it forward. It is known that the ORR on the fuel cell electrode is a heterogeneous reaction occurring at the solid/liquid interface, wherein the electron reduces the oxygen along with species in the electrolyte. Therefore, the ORR kinetic is in close correlation with the electronic density of states and wave function, which are dominated by the localized atomic structure including the atomic distance and coordination number (CN). In this review, the recent development in the regulation over the localized state on the catalyst surface is narrowed down to the following structural factors whereby the corresponding strategies include: the crystallographic facet engineering, phase engineering, strain engineering, and defect engineering. Although these strategies show distinctive features, they are not entirely independent, because they all correlate with the atomic local structure. This review will be mainly divided into four parts with critical analyses and comparisons of breakthroughs. Meanwhile, each part is described with some more specific techniques as a methodological guideline. It is hoped that the review will enhance an insightful understanding on PGM catalysts of ORR with a visionary outlook.

Local Structure of Li+ in Superconcentrated Aqueous LiTFSA Solutions.

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li+, which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li+, the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of 6Li/7Li and H/D. The Li+ local structure changes with the increase of the LiTFSA concentration; the oligomer ([Lip(TFSA)q](p-q)+ (q > 2) forms at the molar fraction of LiTFSA (xLiTFSA) > 0.25. The average structure can be determined in which two water molecules and two oxygen atoms of TFSA anion(s) coordinate to the Li+ in the superconcentrated LiTFSA aqueous solution (LiTFSA)0.25(H2O)0.75. In addition, the intermolecular interaction between the neighboring water molecules was not found, and the hydrogen-bonded interaction in the solution should be significantly weak. According to the coordination number of the oxygen atom (TFSA or H2O), a variety of TFSA- and H2O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li+.

SANS, RAMAN and SEM studies of lead-germanate glasses doped with the manganese oxide

The purpose of this work was to justify the induced structural modifications of the increase of MnO content in lead-germanate glasses and to explore the structural role of doping level. The local structure of the obtained glasses in the xMnO·(100-x)[7GeO2·3PbO2] composition where x = 10–50 mol% MnO was characterized by the analysis of X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) micrographs, Raman spectra and Small Angle Neutron Scattering (SANS) data. The Raman analysis shows that manganese-lead-germanate glasses are composed of [GeO4], [GeO6], [PbOn] and [MnOn] structural units and the number of [GeO6] octahedral units attains maximum values for the smaller MnO contents up to 20 mol%. Further higher MnO concentration in the host matrix implies the change in the coordination number of the germanium atoms from 6-fold to 4-fold and the breaking of short GeOGe chains from germanate network. The predicted coordination number of germanium atoms in the Raman spectra was also compared with SANS data which show strong evidence in germanate anomaly. The combining the results provided from Raman, SEM and SANS data we can conclude the maximum of tetrahedral units into octahedral germanate units occurs at compositions up to 20 mol% MnO. A further increase of doping level in host matrix up to 30 mol% MnO results in a decrease in the coordination number of germanium atoms with a contraction of skeleton of GeO2 network, an increase of non-bridging oxygen concentration and a maximum value of refractive index. For higher MnO content a depolimerisation of the disordered network lead due to the breaking of GeOGe bridges and the formation of [GeOn] structural units with 4 ≥ n < 6.

PC-silicene: A two-dimensional silicene allotrope with strong anisotropic conductance

Among the reported two-dimensional silicon materials, only hexagonal and kagome-like silicene have been reported experimentally. Thus, designing such materials with another strategy is desirable. Based on the experimentally synthesized pentagonal silicon cluster recently and via a swarm structure search method, herein we report a two-dimensional silicene allotrope by combining pentagonal silicon clusters. Further calculations show that this silicene allotrope exhibits great anisotropic conductance, it behaves as a metal with linear energy dispersions along the b -direction and Dirac point located at about 0.10 eV below the Fermi energy level, while it exhibits semiconducting state along the a -direction. This strong anisotropic conductance is driven by the different coordination numbers of silicon atoms. Through hydrogenating the three-fold coordinated silicon atoms, the silicene allotrope would turn into a semiconductor with a direct band gap of 1.60 eV (2.05 eV under HSE level). Particularly, the hydrogenated model possesses high carrier mobility of and at room temperature for electrons and holes, respectively. This finding promotes potential applications of silicon nanomaterials in in-plane anisotropic nanoelectronic, high-speed electronic, and photovoltaic devices.

Mechanism of crack evolution in nano-indentation of single crystal silicon by atomistic simulations and theoretical analysis

The molecular dynamics (MD) model of nano-indentation process was established to study the crack evolution in single crystal during nano-indentation. Two workpieces with different cracks and one workpiece with no crack were selected for indentation simulation in this study. The parameters of atom displacement, coordination number (CN), temperature, potential energy and loading force in the indentation process are analyzed in detail. Cracks were found to close during nano-indentation. Two modes of crack closure are observed: cooperative displacement and indentation failure. The existence of cracks will affect the size of transformation zone and the coordination number of atoms after indentation. Besides, the existence of cracks will reduce the increase of temperature and potential energy, and the closing mode of cracks is found to affect the value of indentation load. In addition, the change of stress with indentation depth at crack tip is calculated by theoretical model. The calculated stress curves reveal the evolution trend of cracks during indentation. These results provide guidance for the production of silicon wafer with higher surface quality.

Available Active Sites on ε‐Fe3N Nanoparticles Synthesized by a Facile Route for Hydrogen Evolution Reaction

AbstractExploring efficient noble‐metal‐free water‐splitting electrocatalysts from earth‐abundant elements is of great importance to realize wide applications in the generation of hydrogen fuel for clean energy. Here, a facile route is reported to synthesize ε‐Fe3N single‐phase nanoparticles by thermal ammonolysis of Fe precursors. The roles of nitrogen atoms in tailoring the hydrogen evolution reaction (HER) activities of ε‐Fe3N have been systematically investigated. HER activity is enhanced by reducing the effective coordination number of nitrogen atoms from 2.61 to 1.67, where the standard coordination number in ε‐Fe3N is 2. Density functional theory calculations reveal that the reduction of nitrogen content lowers the energy of Tafel process on the (100)‐FeN‐exposed and (110) N‐exposed surfaces. Both surfaces are thermodynamically favored for the HER. Furthermore, the active sites of Tafel process change from the kinetically less favored hollow sites of Fe atoms to the kinetically more favored top site of N atoms and the bridge site of Fe atoms on both (100)‐FeN and (110) N‐exposed surfaces. The findings propose a novel strategy to enhance HER activity by using nitrogen deficiency, which is of great importance for the development of highly active transition metal based electrocatalysts.

Homogeneous-like Alkyne Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

Conflicting Roles of Coordination Number on Catalytic Performance of Single-Atom Pt Catalysts

Tailoring the coordination number (CN) of metal atoms has been increasingly recognized as one of the strategies to enhance the catalytic performance of single-atom catalysts (SACs). We here present...

First-principles study of the liquid and amorphous phases of Sb2Te phase change memory material

We have investigated the local structure of liquid and amorphous phases of Sb2Te phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid Sb2Te is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb–Sb bonds (∼53%) and less Sb–Te bonds (∼42%) with the presence of Te–Te bonds (∼5%) in liquid Sb2Te. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous Sb2Te, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb–Sb, Te–Te, and Sb–Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous Sb2Te increases the local structural similarity, facilitating the rapid low-energy crystallization.

Theoretical investigation of the reactivity of flat Ni (111) and stepped Ni (211) surfaces for acetic acid hydrogenation to ethanol

Reactivity of two types of Ni surfaces-flat (111) and stepped (211) surfaces for acetic acid hydrogenation to ethanol was investigated using density functional theory method. The most stable configurations of the reactants, intermediates and products were obtained by investigating all the possible adsorption sites. Results showed that the adsorption of all the studied molecules on the Ni (211) surface are stronger than that on the Ni (111) surface, except for H atom (similar adsorption strength of H atom on the both surfaces was found). In addition, most of the molecules on the Ni (211) surface preferred to adsorb at the step edge, indicating that different coordination numbers of Ni atoms could result in different adsorption strength. Moreover, the elementary reactions with energy barriers related to ethanol and ethyl acetate formations were studied. The most favorable pathways for ethanol formation on the Ni (111) and (211) surfaces are CH3COOH → CH3CO → CH3CHO → CH3CHOH→ CH3CH2OH and CH3COOH → CH3CO → CH3COH → CH3CHOH → CH3CH2OH, respectively. The direct decomposition of acetic acid molecule to form acetyl species was the rate-determining step on the both surfaces. Slight difference for the rate-determining step barriers was observed (1.04 eV vs. 1.13 eV). However, the elementary step of ethyl acetate formation by CH3CO and CH3CH2O became much more difficult on the Ni (211) surface than that on the Ni (111) surface (1.06 eV vs. 0.67 eV). These results suggests that the Ni (211) surface is more likely to inhibit ethyl acetate formation compared with the Ni (111) surface. Meanwhile, the results of the rate constants and the effective barriers indicates that the Ni (211) surface presents a higher probability for higher ethanol selectivity.

Multi‐Center Hyperbonding in Phase‐Change Materials

A comprehensive understanding of chemical interactions underlying the network structure of chalcogenide materials is a crucial prerequisite for comprehending their microscopic structures, physicochemical properties, and capabilities for current or potential applications. However, for many chalcogenide materials, an inherent difficulty is often present in investigating their chemical properties, due to the involvement of delocalized bonding and non‐bonding (“lone‐pair”) electrons, which requires interaction mechanisms beyond that of conventional two‐center, two‐electron covalent bonding. Herein, some recent progress in the development of new interatomic interaction models for chalcogenides is reviewed, in particular focusing on the multi‐center hyperbonding model, proposed in an effort to resolve this issue. The capability of this model in elucidating a diversity of interesting material properties of phase‐change materials (PCMs) is highlighted, including Ge2Sb2Te5 (GST). These include the propensity of high coordination numbers of constituent atoms, linear triatomic bonding geometries with short and long bonds (often ascribed to the effect of a Peierls distortion), abnormally large Born effective charges of crystalline GST, large optical contrast between amorphous and crystalline GST, ultrafast crystallization speed of amorphous GST, and the chemical origin differentiating non‐PCM from PCM chalcogenide materials. Other bonding models for these materials are also briefly discussed.

Local atomic structures of Gd and Zn atoms in extruded Mg-Gd-Zn alloys

Minor addition of Gd and Zn can significantly improve the mechanical properties of extruded Mg-1 wt.%Gd-xZn (x = 0, 0.5, 1 wt.%) alloys. However, how they modify the structure of alloys remains elusive. By using x-ray absorption fine structure, we found that Zn atoms tend to bond with Gd and the 0.5 wt.% Zn addition greatly reduces the coordination number (CN) of Gd atoms, forming an enhanced double solid-solution strengthening. Whereas, 1 wt.% Zn not only increases the CNs of Zn and Gd but also promotes the formation of minor Mg3Zn3Gd2 phase. Compared with the grain refinement strengthening, it is found that the solid solution strengthening is not a dominant factor.

Synergizing Cu dimers and N atoms in graphene towards an active catalyst for hydrogen evolution reaction.

Moving forward from single atom catalysts, here we propose Cu mers coordinated with N atoms in graphene as a potential catalyst for hydrogen evolution reaction (HER) using first-principles calculations. Our study shows that Cu mers (monomer, dimer and trimer) with no N coordination adsorb H too strongly, whereas Cu mers with complete N coordination adsorb H too weakly, indicating that neither is catalytically active for HER. However, these results imply that Cu mers with partial N coordination may exhibit a better catalytic performance. Thus, we further explored all the Cu2Nx complexes with different atomic coordination numbers and spatial distributions and find that one of the Cu2N4 atomic configurations possesses a ΔGH* of −0.09 eV, exhibiting a superior catalytic performance for HER. The possible reason might be that this configuration tunes the p-band center to an optimum level. Our study here reveals a promising catalyst for HER and presents a practical route to design catalysts by introducing metal mers and tuning their coordination with high-valence non-metal elements.