Coordination Number Of Atoms Research Articles

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.

THE PHASE EQUILIBRIA OF THE PBS – PR2S3 – ER2S3 SYSTEM

In this work the theoretical analysis of structural features of initial phases of quasi-ternary system PbS – Pr2S3 – Er2S3 is carried out. It is important to note that they are characterized by a congruent type of melting and can use as components of the above pointed quasi-binary system. The crystalline structure of PbS and Er2S3 is described by octahedral polyhedra of cations, and in the structure of Pr2S3, due to the increase in the radius of the Pr atom compared to the radius of the Er atom, the coordination number increases to 8. Thus, a prism with two additional atoms is obtained. During the transition to ternary phases, we observe some changes in the coordination numbers of Pb and Pr atoms. In the structure of Er2PbS4, the coordination number of Er atoms remains octahedral, but in Pb atoms it becomes prismatic with one additional atom. In the structure of Pr2PbS4 a dense packing of atoms is formed and as a result a mixture of atoms {0.667 La + 0.333 Pb} is formed, which occupies the site 12a. This motif of coordination transitions ends with the formation of only ternary phases in the quasi-ternary system PbS - Pr2S3 - Er2S3. In addition, more than 40 samples were synthesized and their X-ray phase analysis was performed. According to its results, the existence of ternary phases Er2PbS4 and Pr2PbS4 was confirmed. The maximum synthesis temperature was 1323 K. The synthesis of the initial samples of the system was carried out using solid-phase reactions in vacuum quartz ampoules at a residual pressure of 10-2. The presence of new ternary phases has not been established. Experimental studies point to the existence of quasi-binary equilibria Er2PbS4 - Pr2PbS4 and a sufficiently bulky two-phase region Er2S3 + Pr2+2/3xPb1-xS4 (x = 0 ÷ 0.54). According to the results of the study, an isothermal section of the quasi-ternary system PbS - Pr2S3 - Er2S3 at a temperature of 770 K was constructed.

Синтез и строение бис(2,5-дифторбензоата) трис(2-метоксифенил)сурьмы

Tris(2-methoxyphenyl)antimony bis(2,5-difluorobenzoate) (1) has been obtained by the oxidative addition reaction between tris(2-methoxyphenyl)antimony, 2,5-difluorocarboxylic acid and tertiary butyl hydroperoxide in diethyl ether with the 72% yield. The compound has been identified by IR spectroscopy and X-ray diffraction analysis. According to the X-ray diffraction data, the crystal of compound 1 contains two types of crystallographically independent molecules a and b, the geometric parameters of which are slightly different. In molecules a and b the antimony atoms have a distorted trigonal-bipyramidal coordination with the oxygen atoms of the carboxylate ligands in the axial positions. The crystal also contains a solvate molecule of diethyl ether, disordered in two positions. The updated ratios of position contributions are 0.50/0.50. The sums of the CSbC angles in the equatorial plane of molecules 1a and 1b are 359.9(3)° and 359.8(3)°, respectively. The OSbO axial angles are 174.60(16)° (1a) and 175.51(17)° (1b). The antimony atom departs from the equatorial plane [C3] by 0.040 Å for both molecules. The conformation of the aryl ligands with respect to the equatorial plane [C3] is propeller-like. The SbC bond lengths have close values: 2.097(7)2.127(6) Å in 1a, 2.107(7)2.115(6) Å in 1b. The SbO distances (2.112(5), 2.131(5) Å in 1a, 2.107(5), 2.128(5) Å in 1b) are commensurate with the covalent SbO bond lengths. Bidentate carboxylate ligands are coordinated to the metal atom less symmetrically in molecule 1а, while the SbO(=С) intramolecular distances are 3.116(7), 3.063(7) Å in 1a, and 3.120(6) Å, 3.126(7) Å in 1b, which is less than the sum of the van der Waals radii of the Sb and O atoms. In molecules 1a and 1b short distances are observed between the oxygen atoms of the methoxy groups and the antimony atoms (3.080(6), 3.138(7), 3.164(4) Å for 1a, 3.023(6), 3.085(5), 3.194(7) Å for 1b), which increases the coordination number of the antimony atom. The structure of the crystal is presented in such a way that the solvate molecules of diethyl ether, arranged in one chain of the form (∙∙∙СH3CH2OCH2CH3∙∙∙F∙∙∙)n, are surrounded by molecules of tris(2-methoxyphenyl)antimony bis(2,5-difluorobenzoate), forming a shell of sorts for a chain of ether molecules. This interaction is due to hydrogen bonds: –ОСH(СH3)–H∙∙∙F, Et2O∙∙∙HMeO–, –ОСH2CH2–H∙∙∙F. Molecules of the a and b types contact each other through hydrogen bonds С=О∙∙∙HMeO–, F∙∙∙HMeO–, F∙∙∙HAr, and Sb–O∙∙∙HAr. Also, the СН∙∙∙π interactions and the stacking effect are observed in the packing of molecules. Complete tables of atom coordinates, bond lengths and valence angles are deposited at the Cambridge Crystallographic Data Center (No. 2077192 (1), deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/data_request/cif).

The incompressibility of atoms at high pressures

Abstract The structures of the silica polymorphs α-quartz and stishovite have been geometry optimized at highly simulated isotropic pressure within the framework of Density Functional Theory. The atoms of the high-pressure polymorph stishovite are virtually incompressible with the bonded radii for Si and O atoms decreasing by only 0.04 and 0.08 Å, respectively, at 100 GPa. In compensating for the increase in the effective interatomic potential associated with the compression of the Si-O bonded interactions, the electron density at the bond critical point between the bonded pair increases from 0.69 to 0.89 e/Å 3. The bonded radii of the Si and O atoms for α-quartz decrease by 0.006 and 0.008 Å, respectively, between 1 bar and 26.4 GPa. The impact of simulated, isotropic pressure on the bonded radii of the atoms for three perovskites YAlO3, LaAlO3, and CaSnO3 was also examined at high pressure. For the YAlO3 perovskite, the bonded radii for Y and Al decrease by 0.06 and 0.05 Å, respectively, at 80 GPa, while the electron density between the bonded atoms increases by 0.12 and 0.15 e/Å3, on average. The calculations also show that the coordination number of the Y atom increases from 9 to 10 while the coordination number of the O1 atom increases concomitantly in the structure from 5 to 6 at 20 GPa. Hence pressure not only promotes an increase in the coordination number of the metal atoms but also a necessary concomitant increase in the coordination number of the O atoms. The bonded radii, determined at a lower pressure between 0.0 and 15 GPa for LaAlO3 and CaSnO3, decrease a smaller amount with the radii for the La and Ca atoms decreasing by 0.03 and 0.04 Å, respectively, while the radii for the smaller Al and Sn atoms decrease by 0.01 and 0.02 Å, respectively. In general, O atoms are more compressible than the metal atoms, but overall the calculations demonstrate that the bonded radii for the atoms in crystals are virtually incompressible when subjected to high pressure. The reason that the bonded radii change little when subjected to high pressure is ascribed to the changes in the effective interatomic potentials that result in increased repulsion when the atoms are squeezed together.

P21/c Postorthopyroxene γ-LiScGe2O6, a New Dense High-Pressure Polymorph and Its Direct Transformation from the Pbca Structure.

Orthorhombic β-LiScGe2O6 single crystals were compressed hydrostatically up to 10.35 GPa using a diamond anvil cell and investigated in situ by means of X-ray diffraction and Raman spectroscopy. Crystal-structure investigations at ambient conditions and at high pressure show a structural transition from an orthopyroxene-type Pbca structure (a ≈ 18.43 Å, b ≈ 8.85 Å, and c ≈ 5.34 Å at 8.6 ± 0.1 GPa) to a postorthopyroxene type P21/c structure of the new dense γ-LiScGe2O6 (a ≈ 18.62 Å, b ≈ 8.85 Å, c ≈ 5.20 Å, and β ≈ 93.1° at 9.5 ± 0.1 GPa). The structure refinements reveal displacive shifts of O atoms associated with a rotation of every other tetrahedral-chain unit from the O- to S-type position similar to the postorthopyroxene-type MgSiO3. As a consequence of the oxygen displacement, the coordination number of Li atoms is changing from [5 + 1] to a proper 6-fold coordination. The transition around Pc = 9.0 ± 0.1 GPa is associated with a volume discontinuity of ΔV = −1.6%. This orthopyroxene (OEn-Pbca) to postorthopyroxene (pOEn-P21/c) transition is the second example of this type of transformation. Precise lattice parameters have been determined during isothermal compression. The fit of the unit-cell volumes of β-LiScGe2O6, using a third-order Birch–Murnaghan equation of state, yields V0 = 943.63 ± 0.11 Å3, K0 = 89.8 ± 0.6 GPa, and dK/dP = 4.75 ± 0.18 as parameters. Evaluation of the data points beyond the critical transition pressure using a second-order Birch–Murnaghan equation suggests V0 = 940.6 ± 4.4 Å3 and K0 = 82.4 ± 4.8 GPa. A series of high-pressure Raman spectra confirm the symmetry-related structural transition, with band positions shifting in a noncontinuous manner, thus confirming the proposed first-order transition.

Product-Specific Active Site Motifs of Cu for Electrochemical CO2 Reduction

<h2>Summary</h2> Aqueous electrochemical CO₂ reduction (CO₂R) on Cu can generate a variety of valuable fuels, yet challenges remain in the improvement of electrosynthesis pathways for highly selective fuel production. Mechanistically, understanding CO₂R on Cu, particularly identifying the product-specific active sites, is crucial. Herein, we rationally designed and fabricated nine large-area single-crystal Cu foils with various surface orientations as electrocatalysts and identified the voltage- and facet-dependent CO₂R selectivities. <i>Operando</i> grazing incidence X-ray diffraction (GIXRD) and electron back-scattered diffraction (EBSD) were applied to track the top-surface reconstructions of Cu, and we correlate the structural evolution with the change of product selectivities. We extracted three distinct structural descriptors, including crystal facet, atomic coordination number, and step-terrace angle, to reveal the intrinsic structure-function relationships and uniquely identify the specific product-producing sites for CO₂R. Our work guides the rational design of Cu-based CO₂R electrocatalysts and, more importantly, establishes a paradigm to understand the structure-function correlation in catalysis.

Conductive all-carbon nanotube layers: Results on attractive physicochemical, anti-bacterial, anticancer and biocompatibility properties

Physicochemical, electrochemical and biological performance of 4 types of all-carbon nanotube layers was studied. Higher oxidation state of carbon was responsible for micro-scaled uniformity of the layers and excellent electrical conductivity, while nitrogen containing functional groups yielded materials with anisotropy similar to natural tissues and reduced work function. All materials were cytocompatible with mammalian fibroblasts (viability >80%, cytotoxicity <3% at day 7) and human dermal fibroblast (viability of cells >70% at day 1), while reducing bacterial and cancer cells proliferation without adding any drug. After 8 h culture, a ~50% depletion in the number of Gram-positive bacteria was observed on materials with lower work function, while Gram-negative bacteria were more sensitive towards carbon coordination number and presence of nitrogen atoms (cell depletion of up to 48% on amidized carbon nanotubes). After 1-day culture, >80% reduction in the melanoma cells number, connected with enhanced production of reactive oxygen species (ROS) was observed. All-carbon nanotube layers decreased bacteria and cancer cell functions without negatively influencing mammalian cells nor using drugs and we believe that this can be explained by various sensitivity of the tested cells towards exogenous ROS overproduction. As the concerns over implant-related infections as well as rates of antibiotic-resistant bacteria and chemotherapeutic-resistant cancer cells are growing, such materials should pave the way for a wide range of biomedical applications.

Predicting Cytotoxicity of Metal Oxide Nanoparticles using Isalos Analytics Platform.

A literature curated dataset containing 24 distinct metal oxide (MexOy) nanoparticles (NPs), including 15 physicochemical, structural and assay-related descriptors, was enriched with 62 atomistic computational descriptors and exploited to produce a robust and validated in silico model for prediction of NP cytotoxicity. The model can be used to predict the cytotoxicity (cell viability) of MexOy NPs based on the colorimetric lactate dehydrogenase (LDH) assay and the luminometric adenosine triphosphate (ATP) assay, both of which quantify irreversible cell membrane damage. Out of the 77 total descriptors used, 7 were identified as being significant for induction of cytotoxicity by MexOy NPs. These were NP core size, hydrodynamic size, assay type, exposure dose, the energy of the MexOy conduction band (EC), the coordination number of the metal atoms on the NP surface (Avg. C.N. Me atoms surface) and the average force vector surface normal component of all metal atoms (v⊥ Me atoms surface). The significance and effect of these descriptors is discussed to demonstrate their direct correlation with cytotoxicity. The produced model has been made publicly available by the Horizon 2020 (H2020) NanoSolveIT project and will be added to the project’s Integrated Approach to Testing and Assessment (IATA).

Segregation of P and S Impurities to A Σ9 Grain Boundary in Cu

The segregation of P and S to grain boundaries (GBs) in fcc Cu has implications in diverse physical-chemical properties of the material and this can be of particular high relevance when the material is employed in high performance applications. Here, we studied the segregation of P and S to the symmetric tilt Σ9 (22¯1¯) [110], 38.9° GB of fcc Cu. This GB is characterized by a variety of segregation sites within and near the GB plane, with considerable differences in both atomic site volume and coordination number and geometry. We found that the segregation energies of P and S vary considerably both with distance from the GB plane and sites within the GB plane. The segregation energy is significantly large at the GB plane but drops to almost zero at a distance of only ≈3.5 Å from this. Additionally, for each impurity there are considerable variations in energy (up to 0.6 eV) between segregation sites in the GB plane. These variations have origins both in differences in coordination number and atomic site volume with the effect of coordination number dominating. For sites with the same coordination number, up to a certain atomic site volume, a larger atomic site volume leads to a stronger segregation. After that limit in volume has been reached, a larger volume leads to weaker segregation. The fact that the segregation energy varies with such magnitude within the Σ9 GB plane may have implications in the accumulation of these impurities at these GBs in the material. Because of this, atomic-scale variations of concentration of P and S are expected to occur at the Σ9 GB center and in other GBs with similar features.

Deformation mechanisms of Si-doped diamond-like carbon films under uniaxial tension conditions

Element-doped diamond-like carbon (DLC) films have been fabricated to achieve excellent mechanical properties but their elastic/plastic deformation mechanisms are unclear due to experimental limitations in characterizing the deformation process. This study investigates the deformation mechanisms of Si-doped DLC films by simulating their uniaxial tensile tests. It is found that these films show many strain-localized regions under tensile deformations. The coordination number (CN) of Si atoms in such regions decreases even with a small tensile strain. The sp3-sp2 bonding transition of C atoms also happen in these regions with a large tensile strain. As a result, these regions show highly-degraded structures with weak mechanical strength and can be regarded as the defects in the Si-DLC films. In this case, the tensile forces are mainly undertaken by the networks formed of both sp3C atoms and Si atoms with large CN, and hence such networks can be regarded as backbones of Si-DLC films. Their fracture finally initiates in the strain-localized regions. Furthermore, Si-DLC films with an increased Si content show large fraction of sp3C atoms but still exhibit a decreased strength due to the rapid presence of strain-localized regions.

Explainable machine learning for materials discovery: predicting the potentially formable Nd-Fe-B crystal structures and extracting the structure-stability relationship.

New Nd-Fe-B crystal structures can be formed via the elemental substitution of LA-T-X host structures, including lanthanides (LA), transition metals (T) and light elements, X = B, C, N and O. The 5967 samples of ternary LA-T-X materials that are collected are then used as the host structures. For each host crystal structure, a substituted crystal structure is created by substituting all lanthanide sites with Nd, all transition metal sites with Fe and all light-element sites with B. High-throughput first-principles calculations are applied to evaluate the phase stability of the newly created crystal structures, and 20 of them are found to be potentially formable. A data-driven approach based on supervised and unsupervised learning techniques is applied to estimate the stability and analyze the structure-stability relationship of the newly created Nd-Fe-B crystal structures. For predicting the stability for the newly created Nd-Fe-B structures, three supervised learning models: kernel ridge regression, logistic classification and decision tree model, are learned from the LA-T-X host crystal structures; the models achieved maximum accuracy and recall scores of 70.4 and 68.7%, respectively. On the other hand, our proposed unsupervised learning model based on the integration of descriptor-relevance analysis and a Gaussian mixture model achieved an accuracy and recall score of 72.9 and 82.1%, respectively, which are significantly better than those of the supervised models. While capturing and interpreting the structure-stability relationship of the Nd-Fe-B crystal structures, the unsupervised learning model indicates that the average atomic coordination number and coordination number of the Fe sites are the most important factors in determining the phase stability of the new substituted Nd-Fe-B crystal structures.

Boron cage effects on Nd–Fe–B crystal structure’s stability

In this study, we investigate the structure-stability relationship of hypothetical Nd-Fe-B crystal structures using descriptor-relevance analysis and the t-SNE dimensionality reduction method. 149 hypothetical Nd-Fe-B crystal structures are generated from 5967 LA-T-X host structures in the Open Quantum Materials Database by using the elemental substitution method, with LA denoting lanthanides, T denoting transition metals, and X denoting light elements such as B, C, N, and O. By borrowing the skeletal structure of each of the host materials, a hypothetical crystal structure is created by substituting all lanthanide sites with Nd, all transition metal sites with Fe, and all light element sites with B. High-throughput first-principle calculations are applied to evaluate the phase stability of these structures. Twenty of them are found to be potentially formable. As the first investigative result, the descriptor-relevance analysis on the orbital field matrix (OFM) materials' descriptor reveals the average atomic coordination number as the essential factor in determining the structure stability of these substituted Nd-Fe-B crystal structures. 19 among 20 hypothetical structures that are found potentially formable have an average coordination number larger than 6.5. By applying the t-SNE dimensionality reduction method, all the local structures represented by the OFM descriptors are integrated into a visible space to study the detailed correlation between their characteristics and the stability of the crystal structure to which they belong. We discover that unstable substituted structures frequently carry Nd and Fe local structures with two prominent points: low average coordination numbers and fully occupied B neighboring atoms. Moreover, there are only three popular forms of B local structures appearing on all potentially formable substituted structures: cage networks, planar networks, and interstitial sites. The discovered relationships are promising to speed up the screening process for the new formable crystal structures.

Evolution of nano-cracks in single-crystal silicon during ultraprecision mechanical polishing

Abstract The evolution of nano-cracks in mono-crystalline silicon during three-body polishing was investigated by three-dimension molecular dynamics (MD) simulations and theoretical calculations. Atomic displacement, coordination numbers (CN), temperature and stress during the polishing process are studied in detail. The results show that cracks will close rather than expand in ultraprecision mechanical polishing process. The simulation results show two different ways for crack closure due to different crack inclinations: the coordinated displacement of atoms and the phase transformation filling of atoms. Crack closure will result in significant changes in CN. Furthermore, the crack closure effect is the best at 600 K. The calculation shows an obvious uneven stress distribution along the crack line in the polishing process. In addition, the distribution of the normal stress and shear stress at the lower tip of cracks are related to the mode of closure, indicating that simulation is related to the calculated stresses in theoretical model.

The Crystallization Process of Vaterite Microdisc Mesocrystals via Proto-Vaterite Amorphous Calcium Carbonate Characterized by Cryo-X-ray Absorption Spectroscopy

Investigation on the formation mechanism of crystals via amorphous precursors has attracted a lot of interests in the last years. The formation mechanism of thermodynamically meta-stable vaterite in pure alcohols in the absence of any additive is less known. Herein, the crystallization process of vaterite microdisc mesocrystals via proto-vaterite amorphous calcium carbonate (ACC) in isopropanol was tracked by using Ca K-edge X-ray absorption spectroscopy (XAS) characterization under cryo-condition. Ca K-edge X-ray absorption near edge structure (XANES) spectra show that the absorption edges of the Ca ions of the vaterite samples with different crystallization times shift to lower photoelectron energy while increasing the crystallization times from 0.5 to 20 d, indicating the increase of crystallinity degree of calcium carbonate. Ca K-edge extended X-ray absorption fine structure (EXAFS) spectra exhibit that the coordination number of the nearest neighbor atom O around Ca increases slowly with the increase of crystallization time and tends to be stable as 4.3 (±1.4). Crystallization time dependent XANES and EXAFS analyses indicate that short-range ordered structure in proto-vaterite ACC gradually transform to long-range ordered structure in vaterite microdisc mesocrystals via a non-classical crystallization mechanism.

Ab Initio Study of Hydrolysis Effects in Single and Ion-Exchanged Alkali Aluminosilicate Glasses

Hydrolysis in alkali-doped aluminosilicate glasses is one of the most complicated mechanisms in glass science. There remain many fundamental and unresolved issues with implications on their potential applications. Herein, we address this challenge by carrying out detailed calculations on the structure and properties of both anhydrate (dry) and hydrated alkali aluminosilicate glasses on carefully constructed models. Specifically, the Na-, (Na + K)- and K-doped aluminosilicate glasses with compositions (SiO2)0.6 (Al2O3)0.2 (Na2O)0.2-x (K2O)x (x = 0, 0.10 and 0.20) are simulated using ab initio molecular dynamics (AIMD). The local short- and intermediate-range order in these glasses are analyzed in terms of atomic pair distribution, coordination number, bond length and bond angle distributions to delineate the subtle variations due to different alkali size and hydrolysis. The electronic structure, interatomic bonding, mechanical and optical properties for these models are calculated and validated with available experimental data. We use the novel concept of total bond order density (TBOD), the quantum mechanically derived metric, to characterize the internal cohesion and strength in the simulated glasses. Detailed analysis of the hydrolysis mechanism enables us to provide information on the complex interplay of various participating elements and their interactions at the atomic level. Such detailed information provides new platform of knowledge which is crucial for understanding the issues related to glass corrosion and durability and ways and means for their special applications in commercial glass products. Both un-dissociated molecular water and dissociated water in the form of hydroxyl group exist in the hydrated models in the presence of alkali ions. For the first time, we observed the opposite mixed alkali effect in the Poisson's ratio for anhydrate and hydrated glasses.

Disorder-induced electron and hole trapping in amorphous TiO2

Thin films of amorphous (a)-TiO2 are ubiquitous as photocatalysts, protective coatings, photoanodes and in memory application, where they are exposed to excess electrons and holes. We investigate trapping of excess electrons and holes in the bulk of pure amorphous titanium dioxide, a-TiO2, using hybrid density functional theory (h-DFT) calculations. Fifty 270-atom a-TiO2 structures were produced using classical molecular dynamics and their geometries fully optimised using h-DFT simulations. They have the density, distribution of atomic coordination numbers and radial pair-distribution functions in agreement with experiment. The calculated average a-TiO2 band gap is 3.25 eV with no states splitting into the band gap. Trapping of excess electrons and holes in a-TiO2 is predicted at precursor sites, such as elongated Ti-O bonds. Single electron and hole polarons have average trapping energies (ET) of -0.4 eV and -0.8 eV, respectively. We also identify several types of electron and hole bipolaron states and discuss their stability. These results can be used for understanding the mechanisms of photo-catalysis and improving the performance of electronic devices employing a-TiO2 films.

Revealing the Correlation between Catalytic Selectivity and the Local Coordination Environment of Pt Single Atom

Single atom catalysts (SACs) have recently attracted great attention in heterogeneous catalysis and have been regarded as ideal models for investigating the strong interaction between metal and support. Despite the huge progress over the past decade, the deep understanding on the structure-performance correlation of SACs at a single atom level still remains to be a great challenge. In this study, we demonstrate that the variation in the coordination number of the Pt single atom can significantly promote the propylene selectivity during propyne semihydrogenation (PSH) for the first time. Specifically, the propylene selectivity greatly increases from 65.4% to 94.1% as the coordination number of Pt-O increases from ∼3.4 to ∼5, whereas the variation in the coordination number of Pt-O slightly influences the turnover frequency values of SACs. We anticipate that the present work may deepen the understanding on the structure-performance of SACs and also promote the fundamental research in single atom catalysis.