Hydrogen Diffusion in Aluminum Melts: An ab initio Molecular Dynamics Study

The diffusion process of hydrogen in aluminum melts was investigated by molecular dynamics simulation. The pair correlation function, first peak position, and coordination number was calculated and differences in the structural properties among Al-H, Cl-H, and Al-Cl pair were examined. The mechanism of chlorine on improving hydrogen diffusion was discussed. From an ab initio molecular dynamics calculations, the diffusivity of hydrogen in liquid aluminum as D（T）=（0.118×10-4 m2/s）exp（-0.316 eV/kT） is obtained, which is in good agreement with the experimental data. Correspondingly the diffusivity with presence of chlorine is promoted as D（T）=（0.09×10-4 m2/s）exp（-0.251 eV/kT）. It can be concluded that the diffusion of hydrogen in aluminum melts can be enhanced in the presence of chlorine.

Ab initio molecular dynamics simulations of nano-crystallization of Fe-based amorphous alloys with early transition metals

The addition of early transition metals（ETMs） into Fe-based amorphous alloys is practically found to be effective in reducing the α-Fe grain size in crystallization process. In this paper, by using ab initio molecular dynamics simulations, the mechanism of the effect of two typical ETMs（Nb and W） on nano-crystallization is studied. It is found that the diffusion ability in amorphous alloy is mainly determined by the bonding energy of the atom rather than the size or weight of the atom. The alloying of B dramatically reduces the diffusion ability of the ETM atoms, which prevents the supply of Fe near the grain surface and consequently suppresses the growth of α-Fe grains. Moreover, the difference in grain refining effectiveness between Nb and W could be attributed to the larger bonding energy between Nb and B than that between W and B.

Zero-point fluctuation of hydrogen bond in water dimer from ab initio molecular dynamics

Dynamic nature of hydrogen bond (H-bond) is central in molecular science of substance transportation, energy transfer, and phase transition in H-bonding networks diversely expressed as solution, crystal, and interfacial systems, thus attracting the state-of-the-art revealing of its phenomenological edges and sophisticated causes. However, the current understanding of the ground-state fluctuation from zero-point vibration (ZPV) lacks a firm quasi-classical base, concerning three basic dimensions as geometry, electronic structure, and interaction energy. Here, based on the ab initio molecular dynamics simulation of a ground-state water dimer, temporally separated fluctuation features in the elementary H-bond as the long-time weakening and the minor short-time strengthening are respectively assigned to two low-frequency intermolecular ZPV modes and two O–H stretching ones. Geometrically, the former modes instantaneously lengthen H-bond up to 0.2 Å whose time-averaged effect coverages to about 0.03 Å over 1-picosecond. Electronic-structure fluctuation crosses criteria' borders, dividing into partially covalent and noncovalent H-bonding established for equilibrium models, with a 370% amplitude and the district trend in interaction energy fluctuation compared with conventional dragging models using frozen monomers. Extended physical picture within the normal-mode disclosure further approaches to the dynamic nature of H-bond and better supports the upper-building explorations towards ultrafast and mode-specific manipulation.

Dissociation of liquid water on defective rutile TiO2 （110） surfaces using ab initio molecular dynamics simulations

In order to obtain a comprehensive understanding of both thermodynamics and kinetics of water dissociation on TiO2, the reactions between liquid water and perfect and defective rutile TiO2 （110） surfaces were investigated using ab initio molecular dynamics simulations. The results showed that the free-energy barrier （-4.4 kcal/mol） is too high for a spontaneous dissociation of water on the perfect rutile （110） surface at a low temperature. The most stable oxygen vacancy （VOl） on the rutile （110） surface cannot promote the dissociation of water, while other unstable oxygen vacancies can significantly enhance the water dissociation rate. This is opposite to the general understanding that Vol defects are active sites for water dissociation. Furthermore, we reveal that water dissociation is an exothermic reaction, which demonstrates that the dissociated state of the adsorbed water is thermodynamically favorable for both perfect and defective futile （110） surfaces. The dissociation adsorption of water can also increase the hydrophilicity of TiO2.

Study of structural and magnetic properties of Fe(80)P-9B(11) amorphous alloy by ab initio molecular dynamic simulation

The structural and magnetic properties of Fe80P9B11 amorphous alloy are investigated through ab initio molecular dynamic simulation. The structure evolution of Fe（80）P9B（11） amorphous alloy can be described in the framework of topological fluctuation theory, and the fluctuation of atomic hydrostatic stress gradually decreases upon cooling. The left sub peak of the second peak of Fe–B partial pair distribution functions（PDFs） becomes pronounced below the glass transition temperature, which may be the major reason why B promotes the glass formation ability significantly. The magnetization mainly originates from Fe 3d states, while small contribution results from metalloid elements P and B. This work may be helpful for developing Fe-based metallic glasses with both high saturation flux density and glass formation ability.

Ab Initio Two-Phase Molecular Dynamics on the Melting Curve of SiO_2

Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20-160 GPa and temperatures of 2 500-6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature （Tin） of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary （CMB） of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD （molecular dynamics） simulations. For liquid silica, the O-O coordination number is found to be 12 along the Tm and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10^-9-3.3×10^-9 m2/s, and the viscosity is 0.02-0.03 Pa＇s along the Tin. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.

Excited electron and spin dynamics in topological insulator: A perspective from ab initio non-adiabatic molecular dynamics

We perform an ab initio non-adiabatic molecular dynamics simulation to investigate the non-equilibrium spin and electron dynamics in a prototypical topological insulator(TI)Bi,Ses.Different from the ground state,we reveal that backscattering can happen in an oscillating manner between time-reversal pair topological surface states(TSSs)in the non-equilibrium dynamics.Analysis shows the phonon excitation induces orbital composition change by electron-phonon interaction,which further stimulates spin canting through spin-orbit coupling.The spin canting of time-reversal pair TSSs leads to the non-zero non-adiabatic coupling between them and then issues in backscattering.Both the spin canting and backscattering result in ultrafast spin relaxation with a timescale around 10o fs.This study provides critical insights into the non-equilibrium electron and spin dynamics in TI at the ab initio level and paves a way for the design of ultrafast spintronic materials.

Identifying the crystal structure of T 1 precipitates in Al-Li-Cu alloys by ab initio calculations and HAADF-STEM imaging

Controversial experimental reports on the crystal structure of T 1 precipitates in Al-Li-Cu alloys have ex-isted for a long time,and all of them can be classified into five models.To clarify its ground-state atomic structure,herein,we have combined high-throughput first-principles calculations and CALPHAD,as well as aberration-corrected HAADF-STEM experiments.Employing the special quasi-random structure(SQS)and supercell approximation(SPA)methods to simulate the local disorder on Al-Cu sub-lattices,we find that none of the present models can satisfy the phase stability in Al-Li-Cu ternary system based on temperature-dependent convex hull analysis.Using the cluster expansion(CE)formulas,structural predic-tions derived from the five-frame models were performed.Subsequently,by introducing the vibrational contribution to the free energy at aging temperatures,we proposed a novel ground-state T 1 structure that maintains a coherent relationship with Al-matrix at the<112>Al orientation.The underlying phase transition between the variants of T 1 precipitates was further discussed.By means of ab initio molecular dynamics(AIMD)simulations,we resolved the controversy regarding the number of atomic layers con-stituting the T 1 phase and acknowledged the existence of Al-Li corrugated layers.The root cause of this structural distortion is triggered by atomic forces and bondings.Our work can have an positive impact on the novel fourth generation of Al-Cu-Li alloy designs by engineering the T 1 strengthening phase.

Static and dynamic evolution of CO adsorption onγ-U(100)surface with different levels of Mo doping using DFT and AIMD calculations

Alloys of uranium and molybdenum are considered as the future of nuclear fuel and defense materials.However,surface corrosion is a fundamental problem in practical applications and storage.In this study,the static and dynamic evolution of carbon monoxide(CO)adsorption and dissociation onγ-U(100)surface with different Mo doping levels was investigated based on density functional theory and ab initio molecular dynamics.During the static calculation phase,parameters,such as adsorption energy,configuration,and Bader charge,were evaluated at all adsorption sites.Furthermore,the time-dependent behavior of CO molecule adsorption were investigated at the most favorable sites.The minimum energy paths for CO molecu-lar dissociation and atom migration were investigated using the transition state search method.The results demonstrated that the CO on the uranium surface mainly manifests as chemical adsorption before dissociation of the CO molecule.The CO molecule exhibited a tendency to rotate and tilt upright adsorption.However,it is difficult for CO adsorption on the surface in one of the configurations with CO molecule in vertical direction but oxygen(O)is closer to the surface.Bader charge illustrates that the charge transfers from slab atoms to the 2π*antibonding orbital of CO molecule and particularly occurs in carbon(C)atoms.The time is less than 100 fs for the adsorptions that forms embryos with tilt upright in dynamics evolution.The density of states elucidates that the overlapping hybridization of C and O 2p orbitals is mainly formed via the d orbitals of uranium and molybdenum(Mo)atoms in the dissociation and re-adsorption of CO molecule.In conclusion,Mo doping of the surface can decelerate the adsorption and dissociation of CO molecules.A Mo-doped surface,created through ion injection,enhanced the resistance to uranium-induced surface corrosion.

Ordering in liquid and its heredity impact on phase transformation of Mg-Al-Ca alloys

It is a long-sought goal to achieve desired mechanical properties through tailoring phase formation in alloys,especially for complicated multi-phase alloys.In fact,unveiling nucleation of competitive crystalline phases during solidification hinges on the nature of liquid.Here we employ ab initio molecular dynamics simulations(AIMD)to reveal liquid configuration of the Mg-Al-Ca alloys and explore its effect on the transformation of Ca-containing Laves phase from Al2Ca to Mg_(2)Ca with increasing Ca/Al ratio(rCa/Al).There is structural similarity between liquid and crystalline phase in terms of the local arrangement environment,and the connection schemes of polyhedras.The forming signature of Mg_(2)Ca,as hinted by the topological and chemical short-range order originating from liquid,ascends monotonically with increasing rCa/Al.However,Al_(2)Ca crystal-like order increase at first and then decrease at the crossover of rCa/Al=0.74,corresponding to experimental composition of phase transition from Al_(2)Ca to Mg_(2)Ca.The origin of phase transformation across different compositions lies in the dense packing of atomic configurations and preferential bonding of chemical species in both liquid and solid.The present finding provides a feasible scenario for manipulating phase formation to achieve high performance alloys by tailoring the crystal-like order in liquid.

Li-ion transport in solid-state electrolyte of Li_(1-x)Al_(1-x)Si_(2+x)O_(6):an ab initio study

All-solid-state batteries are considered as nextgeneration technology for energy storage due to their high energy density and excellent s afety.However,only a few solid electrolytes exhibit ionic conductivities comparable to liquid electrolytes.Finding low-cost solid electrolytes with high Liion conductivity is in high demand.Based on the ab initio molecular dynamic simulations,the Li^(+)diffusion inβ-LiAISi_(2)O_(6),a type of cost-effective and naturally-available mineral,and its disordered systems Li_(1-x)Al_(1-x)Si_(2+x)O_(6)with-1.0≤x≤0.5 was studied.Our calculations show that the phases of Li_(1-x)Al_(1-x)Si_(2+x)O_(6)with nonzero x all possess much lower diffusion energy barriers than pristine LiAlSi_(2)O_(6).When x is positive,increased concentration of lithium vacancies accelerates the diffusion of Li-ions.When x is negative,additional Li-ions are inserted into structures and co-migration is stimulated among these Li-ions.In particular,the maximal ionic conductivity at 300 K(1.92×10^(-6)S·cm^(-1))is obtained in Li_(2)Al_(2)SiO_(6)(x=-1.0),which is five orders of magnitude larger than that of pristineβ-LiAlSi_(2)O_(6).In addition,the diffusion barrier can be further reduced to 0.38 eV by replacing Si with Ge,and the ionic conductivity for Li_(2)Al_(2)GeO_(6)can reach 3.08×10~(-5)S·cm^(-1)at 300 K.Our work facilitates the understanding of Li+conduction mechanisms in silicatebased electrolytes and the development of cost-effective and high-performance solid-s ate electrolytes.

Local atomic structure evolution of liquid gadolinium and yttrium during solidification:An ab initio study

Local atomic structure evolution of pure gadolinium(Gd)and yttrium(Y)during solidification was investigated by using ab initio molecular dynamics(AIMD)simulation.The calculated results indicate that the local short-range order(SRO)in liquid Gd and Y is similar to some transitional metals with an asymmetric shape of the second peak in static structure factors.Moreover,the formation of icosahedral local motifs as a function of temperature decreases the diffusivity,which explains the connection between structure evolution and dynamic properties.In examining the topological structures of both systems,we demonstrate that small atomic displacement leads to two different types of topological sixfold rings in liquid and solid states.All analyses yield a systematic study about rare earth metals Gd and Y at the atomic level.

Multiscale simulations of surface adsorption characteristics of amino acids on zinc metal anode

Aqueous zinc-ion batteries(ZIBs) are considered promising power sources for grid storage,but they face several issues,including dendrite growth,corrosion,hydrogen evolution,etc.,which are related to the Zn metal/liquid electrolyte interface.To address these challenges,many researchers have focused on modifying the Zn anode with surface adsorption.However,the underlying mechanism between the Zn surface and adsorbed/protective molecules has not been thoroughly explored.In this study,we built a multiscale simulation platform that integrates state-of-art simulation methods to comprehensively investigate the adsorption process of amino acids on the Zn metal surface.Our major finding is that adsorption sites,adsorbate–surface angle,and average distance are critical parameters for the stability and strength of surface adsorption.Additionally,ab initio molecular dynamics reveal the kinetics of the surface adsorption and molecule reorientation processes.Specifically,it can be discovered that the amino acids prefer to align parallel to the Zn metal surface,leading to better surface protection against corrosion and preventing dendrite growth.These findings pave the way for an in-depth understanding of the surface adsorption process,as well as providing concrete design principles for stable Zn metal anodes.

Insights into the adsorption/desorption of CO_(2) and CO on single-atom Fe-nitrogen-graphene catalyst under electrochemical environment

Single-atom metal-nitrogen-graphene(M-N-Gra) catalysts are promising materials for electrocatalytic CO_(2) reduction reaction(CO_(2) RR). However, theoretical explorations on such systems were greatly hindered because of the complexity in modeling solid/liquid interface and electrochemical environment. In the current work, we investigated two crucial processes in CO_(2) RR, i.e. adsorption and desorption of CO_(2) and CO at Fe-N_(4) center, with an explicit aqueous model. We used the ab initio molecular dynamics simulations associated with free energy sampling methods and electrode potential analysis to estimate the energetics under electrochemical environment, and found significant difference in aqueous solution compared with the same process in vacuum. The effect of applied electrode potential on the adsorption structures,charge transfer and free energies of both CO_(2) and CO on Fe-N-Gra was thoroughly discussed. These findings bring insights in fundamental understandings of the CO_(2) RR process under realistic conditions, and facilitate future design of efficient M-N-Gra-based CO_2 RR catalysts.

Sulfurized-polyacrylonitrile in lithium-sulfur batteries:Interactions between undercoordinated carbons and polymer structure under low lithiation

Lithium-sulfur battery(LSB)represents an important candidate to be used in energy storage applications,due to its high specific capacities.Sulfurized-polyacrylonitrile(SPAN)is a candidate as a host material in LSB to replace graphite,due to its ability to chemisorb polysulfides(PSs).The sulfur chains attached to the polymer can reversibly form Li2S,and SPAN indicates to have a good cyclability and better performance than graphite,thus,SPAN acts partially as an active and also as a host material.In this study,we investigated the capacity of the solvent or the SPAN to lose a hydrogen atom from the backbone,to predict possible anodic reactions between solvent and host material.The simulation suggests that the photophilic salts may preferentially react with the solvent,and possibly building a cathode electrolyte interphase(CEI).We observed that an undercoordinated carbon(C_(uc))can be thermodynamically created,due to lithiation.The Cuccan react with the solvent on the polymer backbone through different mechanisms,however,the simulations indicated that the reaction should be affected by the interaction between the solvent and C_(uc),according to SPAN’s configuration.Moreover,C_(uc)reacts with long sulfur chains attached to SPAN,capturing sulfur and forming a C-S bond.A sulfur chain from one SPAN can connect to another polymer backbone,however,this process is affected by lithiation and vice-versa.Therefore,this work also investigates the formation of interconnected SPAN structures and the multiple C_(uc)effects.

Structural Origins for Enhanced Thermal Stability and Glass‑Forming Ability of Co–B Metallic Glasses with Y and Nb Addition

The effects of Y and Nb addition on thermal stability,glass-forming ability(GFA),and magnetic softness of Co75B25 metallic glass(MG)were comprehensively investigated.The experimental results indicated that the thermal stability,GFA,and magnetic softness of the studied MGs increase in the order Co_(75)B_(25)<Co_(73)Nb_(2)B_(25)<Co_(71.5)Y_(3.5)B_(25)<Co_(69.5)Y_(3.5)Nb_(2)B_(25).The structural origins of the improved properties were revealed by ab initio molecular dynamics(AIMD)simulations and density functional theory(DFT)calculations.Results showed that the B-centered prism units are the primary structure-forming units of the four MGs,connect through vertex-,edge-,and face-shared(VS,ES,and FS)atoms,and Co-centered units tend to connect with Co/B-centered units via the intercross-shared(IS)atoms.The addition of Y and Nb not only plays the role of connecting atoms but also enhances both bond strengths and the fractions of icosahedral-like units in increasing order Co_(75)B_(25)<Co_(73)Nb_(2)B_(25)<Co_(71.5)Y_(3.5)B_(25)<Co_(69.5)Y_(3.5)Nb_(2)B_(25),which is conducive to the enhancement of the structural stability,atomic packing density,and viscosity,thereby improving thermal stability and GFA.In addition,the improvement of structural stability and homogeneity leads to enhanced magnetic softness.

How water molecules occupying the active site of a single-atom catalyst affect the electrochemical reduction of carbon dioxide

In single-atom catalysts(SACs),the single atoms are often exposed as protrusions above the substrate.The solvent molecules in the electrocatalytic environment can interact or even bind to these coordination-unsaturated single atoms and thus influence the reaction process,but this has not been studied in depth.In this work,we systematically investigate the thermodynamics of CO_(2)reduction reaction(CO_(2)RR)to CO over MoS_(2)-supported single metal atom catalysts(TM@MoS_(2),TM=transition metal)under vacuum and explicit solvent environments using density functional theory.In addition,the ab initio molecular dynamics results show that explicit H_(2)O molecules can coordinate to the TM site and undergo competitive adsorption with the CO_(2)RR intermediates,which significantly affects the energy and conformation of the CO_(2)RR pathway.Electronic structure analysis reveals that the occupying H_(2)O molecules change the electronic state of single atom and further influence the adsorption strength of different CO_(2)RR intermediates.Our work shows that water molecules can not only act as ligands to influence the electronic state of TM,but also affect the energy and conformation of CO_(2)RR intermediates,which highlights the important role of occupying H_(2)O molecules at the single-atom sites in CO_(2)RR and provides useful insights for the design of SACs for efficient CO_(2)RR.

Repelling effects of Mg on diffusion of He atoms towards surface in SiC: Irradiation and annealing experiments combined with first-principles calculations

In this study,the effects of Mg on the formation of He bubbles and diffusion behavior of He atoms in cubic silicon carbide(3C–SiC)were investigated by irradiation and annealing experiments as well as first-principles calculations.TEM results indicated that two damage bands were formed in He&Mg irradiated SiC.During annealing,Mg could prevent He atoms from diffusing to the surface,resulting in the formation of the He bubbles in the deeper areas far from Mg-implanted regions,which is helpful in avoiding surface blisters.First-principles calculations were then performed to explore the effects of Mg on the He behavior in SiC.The solution energy,binding energy charge density,bond length,and crystal orbital Hamiltonian population of these elements were calculated to identify their states.The results suggested that the binding capacity between He and Mg was weak,and Mg could increase the diffusion energy barrier of He.Ab initio molecular dynamics(AIMD)simulation showed that Mg could make He in a high-energy unstable state,and force He atom to move toward the vacancy away from Mg,which explains the experimental results.

Signature of the hydrogen-bonded environment of liquid water in X-ray emission spectra from first-principles calculations

Based on ab initio molecular dynamics simulations and density functional theory, we performed a systematic theoretical study to elucidate the correlation between the H-bonded environment and X- ray emission spectra of liquid water. The spectra generated from excited water molecules embedded in an intact H-bonded environment yield broader spectral peaks and a larger spectral range than the spectra generated from water molecules in a broken H-bonded environment. Such differences are caused by the local electronic structures on the excited water molecules within the core-hole lifetime that evolve differently through the rearrangement of neighboring water molecules in different H-bonded environments.

Single Iron-dimer Catalysts on MoS_(2) Nanosheet for Potential Nitrogen Activation

Electrocatalytic nitrogen reduction reaction(NRR)is a promising way to produce ammonia(NH_(3))at ambient temperature and pressure.Herein,we have constructed single Fe dimer catalysts on a molybdenum disulfide monolayer for potential nitrogen activation.By employing ab initio molecular dynamics simulations,it is suggested that a dual iron-single atom site can be dynamically formed,which exhibits the similar Fe-S-Fe structure as the nitrogenase.We further identify an iron dimer with a sulfur vacancy as the active center for realistic nitrogen activation by the free energy calculations since the bridged sulfur is easy to be released in the form of H_(2)S during the reduction process.It is shown that N_(2)mainly adsorbs on the Fe_(2)dimer at the sulfur vacancies in the pattern of side-on configuration,and the nitrogen reduction reaction is proceeded by an enzymatic mechanism.Charge analyses further show that the Fe_(2)dimer mainly works as an electron reservoir while MoS_(2)substrate with one sulfur vacancy acts as an inert carrier to stabilize the Fe_(2)dimer.Overall,our work provides important insights into how N_(2)molecules were adsorbed and activated on Fe_(2)-doped MoS_(2),and provides new ideas for the transformation of actual reaction sites during electrochemical reactions.