First-principles Molecular Dynamics Study Research Articles

Analysis of ion conduction behavior of Nb- and Zr-doped Li3InCl6-based materials via material simulation

The Li-ion conductivities of Li3InCl6 (LIC), which is a promising chloride solid electrolyte, and its compositional derivatives, Nb5+- and Zr4+-doped LIC, i.e., Li3−2xIn1−xNbxCl6 and Li3−yIn1−yZryCl6, respectively, were experimentally and computationally investigated. An increase in the ionic conductivity caused by Nb5+ or Zr4+ doping, which was due to the increase in Li vacancies, was observed in both the experimental and computational results. Nb5+ doping yielded a larger increase in conductivity at 60 °C. First-principles molecular dynamics studies indicated two factors affecting the Li-ion conductivity under doping with higher-valent ions: (1) the vacancy trapping effect and (2) the reduction in the phase-transition temperature from a Li/vacancy ordered structure to a disordered structure. In particular, in factor (2), the effect of Nb5+ doping is larger than that of Zr4+ doping, which supports the improvement in ionic conductivity at 333 K in the experiment.

Thermodynamics, Kinetics, and Optical Properties of Rotaxane: A First-Principles Molecular Dynamics Study.

Through molecular mechanics using the force field along with the quantum dynamical aspect of mechanically interlocked compounds, rotaxanes (defined as macromolecular rings that are threaded on a dumbbell-shaped axle molecule) are investigated with advanced quantum mechanical methods, including the atom-centered density matrix propagation simulation technique, at different temperatures like 300, 500, 700, 900, 2000, and 2500 K for 1.2 ps. Ab initio molecular dynamics simulation is carried out. In addition to, we investigate the noncovalent interaction present in the rotaxane compound 2R-D-2PF6 with the help of reduced density gradient, average reduced density gradient, density overlap region indicator, and interaction region indicator as well as Hirshfeld surface analyses. Furthermore, the stability of 2R-D-2PF6 at room temperature and higher temperatures is elucidated by analyzing the thermal fluctuation index through a dynamic process. In order to check the optical behavior of our selected rotaxane compound, an evaluation of the electronic dipole moment, static and frequency-dependent average polarizability, and first- and second-order hyperpolarizability is carried out. The rotaxane compound shows very promising linear and nonlinear optical responses, which indicates its utility as a very good optical material. The calculation of the time-dependent density-functional theory highlights the broad absorption band of rotaxane spanning the UV-visible domain. Therefore, we also unravel that this can tap into solar radiation or harnessing of solar energy.

Oxidation kinetic mechanism of n-decane under high temperature and pressure: a first-principles molecular dynamics study.

The n-decane/air (C10H22/air) combustion reaction kinetics has attracted much research attention because of its potential application in the aerospace field. In this work, C10H22 oxidation in O2 under high temperature and pressure is simulated based on the first-principles molecular dynamics method for the first time. Our results show that C-C bond breaking and H-abstraction are the two main initial reactions in the oxidation process of C10H22. However, there exists an obvious difference under high and atmospheric pressures. Under high pressure, C-C bond dissociation reactions of hydrocarbon molecules are the main reaction types, while H-abstraction reactions are the main reaction types under atmospheric pressure. The radicals (HO2, OH, O, etc.) play key roles in promoting the oxidation of hydrocarbon molecules. A detailed chemical kinetic model (76 species and 435 elementary reactions), the FP-C10H22 model, of C10H22/air mixture combustion is constructed and verified. The predicted values of FP-C10H22 model on the ignition delay time, laminar flame speed and species concentration of jet stirred reactor (JSR) species concentration are in good agreement with the experimental data.

Partition model for trace elements between liquid metal and silicate melts involving the interfacial transition structure: An exploratory two-phase first-principles molecular dynamics study

Separation of trace elements has become a major obstacle for preparation of high-purity metal materials. Effective design of the separation medium depends on accurate partitioning prediction of the trace elements in the two phases. However, development of a separation prediction model is difficult because of the limitations of recognition of the interfacial transition structures of the trace elements. Here, a partition model for trace elements between metal and silicate melts involving an interfacial transition structure was developed by exploratory two-phase first-principles molecular dynamics simulation. The results showed that the distribution strongly depends on the local coordination structure in the cluster (LCSC) of the impurity element, which is the key for investigating the interfacial transition structure. A computational strategy for the trace-element distribution ratio is proposed to quantify the contribution of the trace element at the interface. The LCSC partition model was demonstrated for trace element boron removal from silicon for molten silicon and silicate melts. The LCSC model gave a better predicted value of the experimental value than the traditional activity model. The new model assists in explaining the transformation mechanism of B atoms at the silicate–silicon interface at the atomic scale. The LCSC partition model allows prediction of the trace element partitioning behavior solely from first-principles calculations.

FPMD studies on the microstructures and transport properties of molten MgCl2-NaCl-KCl with addition of active metals

A purification protocol of molten MgCl2-NaCl-KCl (MNK) by adding active metals has been proposed to reduce the corrosion of alloy materials in the application of generation III concentrating solar power plants. In this work, the ionic transport properties and micro structural characteristics of metals (Mg, Na, and K) and molten MNK mixtures (metal-MNK) are investigated at 723 K by first principles molecular dynamics (FPMD) simulations. According to the evolution of total energy, the Mg-MNK is the most stable mixture, while the Na-MNK mixture has larger energy fluctuations resulting in a higher specific heat capacity. Studies have shown that the small amount of metal existing in the molten MNK has almost no effect on the ionic diffusion order, but total diffusion coefficients and thermal conductivities of metal-MNK mixtures are improved. Meanwhile, the intensity of diffracted light of molten MNK is weakened by metals, and the microstructures of molten MNK added with alkali metals are more susceptible than that with metallic Mg by analyzing the total and partial static structure factors. Furthermore, the intermediate range orders of metal-MNK are demonstrated, especially for Mg-Mg pairs hinting the formation of the Mg2+-void-Mg2+ structure or Mg2+-centered clusters. Overall, the FPMD results deeply elaborate the dynamic interactions of molten MNK and metallic particles, which give insights for the development of salt purification and corrosion inhibition technology.

Dissociation of the Watson-Crick base pairs in vacuum and in aqueous solution: a first-principles molecular dynamics study

The damage of the DNA structure can affect the correct functioning of the cellular processes. This work investigates the required forces to dissociate the Watson-Crick (WC) base pairs AT into A and T, and GC into G and C. The WC base pairs are immersed in water under realistic conditions of temperature, volume, and density that reproduce the main characteristics of a biological system. The simulations are based on first-principles molecular dynamics combined with steering atomic forces. In addition to the force intensities, the charge transfers between the nucleic acid bases, energy variations, and temperature fluctuations in the cleavage moments are reported. With the purpose of evaluating the effects of the aqueous medium, simulations of the WC base pairs in vacuum are included. The results considering the solvated medium are consistent with the experimental measurements, and show the importance of the aqueous solution to regulate the structural modifications of the nucleic acid bases. The investigation contributes with a novel molecular model in molecular simulations, and to better understand the biological processes where the DNA compounds play an active role in life forms. Communicated by Ramaswamy H. Sarma

Microstructural and diffusive properties of Cr solute in MgCl2–NaCl–KCl eutectic: A First-Principles molecular dynamics study

The hot-corrosion-related mutual effect between the MgCl2–NaCl–KCl eutectic (MNK) and structural alloy materials is important for next-generation concentrated solar power (CSP) plants. However, little attention has been given to the feedback effect of in-situ corrosion impurities on eutectics, in which the Cr solute is among the most common corrosion products. In this work, first-principles molecular dynamics (FPMD) were introduced to investigate the interaction between the Cr solute and the MNK eutectic. The results showed inhibition of the native covalent Mg2+ bonding priority in the MNK eutectic after the introduction of the Cr solute. An oxidation-state-induced microstructural evolution from a metastable Cr cluster to a stable Cr coordination complex was revealed. The overall native ion diffusion was suppressed by Cr impurities, and the diffusion priority of specific ions was also affected. Furthermore, the valence-dependent intrinsic diffusion of Cr ions in the MNK showed a trade-off relationship with Cr concentration. This work elucidates the effect of the Cr solute on the microstructural and diffusive properties of the MNK eutectic and provides further guidance on experimental and simulation studies concerning Cr-related corrosion in MNK eutectics.

First-principles molecular dynamics study of the behavior of tritium in molten LiF-BeF2 eutectic

Molten LiF–BeF2 (FLiBe) eutectic (67 mol% LiF and 33 mol% BeF2) plays the fundamental role in the molten salt reactors (MSRs). However, the residual 6Li nuclides capture neutrons and generate the radioactive tritium nuclides, thus the tritium management is essential for the safe operation of MSRs. In this work, first-principles molecular dynamics (FPMD) simulations are applied to the investigation of the behavior of tritium in molten FLiBe. It is proved that the calculated temperature dependence of density and the coordination structure of molten FLiBe agree well with the available experimental information. Thus, it is confident that FPMD can be applied to the investigation of behavior of tritium in molten FLiBe. Finally, the coordination and electronic structures, as well as transport characteristics of various tritium moieties are studied for the understanding of the behavior of tritium in molten FLiBe.

Surface Chirality Influences Molecular Rotation upon Desorption.

Chiral surfaces offer great potential as a medium for enantioselective synthesis or separation, yet their dynamic enantiospecific interactions with adsorbates are not well understood. Here, the influence of chiral surfaces on the molecular rotations of desorbing molecules is investigated. Formic acid desorption from Cu{531} and Cu{110} serve as model systems for desorption processes of an achiral adsorbate from a chiral and an achiral surface. Our first-principles molecular dynamics study reveals a much larger and more directed angular momentum for molecules desorbing from the chiral surface and a clear preference for one sense of rotation. This result provides new insight into desorption and adsorption processes and propensities on chiral surfaces.

Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5 X Argyrodites.

The rational development of fast-ion-conducting solid electrolytes for all-solid-state lithium-ion batteries requires understanding the key structural and chemical principles that give some materials their exceptional ionic conductivities. For the lithium argyrodites Li6PS5X (X = Cl, Br, or I), the choice of the halide, X, strongly affects the ionic conductivity, giving room-temperature ionic conductivities for X = {Cl,Br} that are ×103 higher than for X = I. This variation has been attributed to differing degrees of S/X anion disorder. For X = {Cl,Br}, the S/X anions are substitutionally disordered, while for X = I, the anion substructure is fully ordered. To better understand the role of substitutional anion disorder in enabling fast lithium-ion transport, we have performed a first-principles molecular dynamics study of Li6PS5I and Li6PS5Cl with varying amounts of S/X anion-site disorder. By considering the S/X anions as a tetrahedrally close-packed substructure, we identify three partially occupied lithium sites that define a contiguous three-dimensional network of face-sharing tetrahedra. The active lithium-ion diffusion pathways within this network are found to depend on the S/X anion configuration. For anion-disordered systems, the active site–site pathways give a percolating three-dimensional diffusion network; whereas for anion-ordered systems, critical site–site pathways are inactive, giving a disconnected diffusion network with lithium motion restricted to local orbits around S positions. Analysis of the lithium substructure and dynamics in terms of the lithium coordination around each sulfur site highlights a mechanistic link between substitutional anion disorder and lithium disorder. In anion-ordered systems, the lithium ions are pseudo-ordered, with preferential 6-fold coordination of sulfur sites. Long-ranged lithium diffusion would disrupt this SLi6 pseudo-ordering, and is, therefore, disfavored. In anion-disordered systems, the pseudo-ordered 6-fold S–Li coordination is frustrated because of Li–Li Coulombic repulsion. Lithium positions become disordered, giving a range of S–Li coordination environments. Long-ranged lithium diffusion is now possible with no net change in S–Li coordination numbers. This gives rise to superionic lithium transport in the anion-disordered systems, effected by a concerted string-like diffusion mechanism.

First-principles molecular dynamics study on the surface chemistry and nanotribological properties of MgAl layered double hydroxides.

Layered double hydroxides (LDHs) are promising materials for lubrication. However, the underlying mechanism that leads to the low friction of the material is not well-understood. In this study, density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations have been used to study the reduced friction mechanism of MgAl-LDH. Our results indicate that the introduction of trivalent cations has a significant impact on the friction reduction of the LDH. Besides, the lateral force shows a strong correlation with the coverage of the hydroxyl group on the surface. By using AIMD simulation, we show that the water/hydroxide molecules interact with the surface through strong hydrogen bonds that confine the movement and the orientation of the intercalated molecules on the surface. Furthermore, the friction is reduced when the water thickness is increased. The reaction pathways of water with the LDH surface has been investigated using well-tempered metadynamics simulation. We found that the LDH can promote proton transfer, leading to the formation of hydroxide intermediates (OH), which then chemically adsorb on the surface. The chemical adsorption of the hydroxide intermediates can cleave the O-H bonds on the LDH surface.

Interfacial structures and acidity constants of goethite from first-principles Molecular Dynamics simulations

Abstract In this paper, we report a first-principles Molecular Dynamics (FPMD) study of interfacial structures and acidity constants of goethite. The pKa values of the groups on (010), (110), and (021) surfaces (space group Pbnm) are derived with the FPMD based vertical energy gap technique. The results indicate that major reactive groups include ≡Fe2OH2 and ≡FeOH2 on (010), ≡FeOH2, ≡Fe3OLH, and ≡Fe3OUH on (110), and ≡FeOhH2 and ≡Fe2OH on (021). The interfacial structures were characterized in detail with a focus on the hydrogen bonding environment. With the calculated pKa values, the point of zero charges (PZCs) of the three surfaces are derived and the overall PZC range of goethite is found to be consistent with the experiment. We further discuss the potential applications of these results in future studies toward understanding the environmental processes of goethite.

First-Principles Molecular Dynamics Study of Silicon-Based Ceramics: Different Tribochemical Reaction Mechanisms during the Running-in Period of Silicon Nitride and Silicon Carbide

Because silicon-based ceramics show superlow friction in aqueous environments, these materials have attracted much attention for the development of water lubrication systems. The superlow friction ...

The damage of the Watson–Crick base pairs by nickel nanoparticles: A first-principles molecular dynamics study

The nickel nanoparticles are harmful atmospheric pollutants, and the damage caused by them in humans has become a topic of great relevance. In this study we investigate the interaction of the Ni2 and Ni3 clusters with the AT and GC Watson–Crick base pairs in an aqueous medium. Molecular dynamics in combination with density functional theory are employed. A novel method is implemented to create realistic thermodynamic conditions (NVT) in the simulations. The energies, the charges of the interacting compounds, the temperature changes, and the geometric rearrangements are reported. The results show the formation of stable organometallic compounds of the nickel nanoparticles with the DNA nucleic acid bases. In this respect, the biological processes where the DNA is implicated may be altered by the formation of such super-structures.

Structure and dynamical properties of liquid Au under pressure

The structures and properties of disordered systems are very challenging in material science. In the present first-principles molecular dynamics study, the results show that the liquid gold at 2000 K undergoes a continuous liquid–liquid transition under pressure. The icosahedral short range order was found to be initially enhanced by compression and then started to decay and became less dominant. At 18 GPa, it was solidified and became a crystal. The present theoretical results provide a new perspective to understand the evolution of the atomic packing in liquid metals under compression and may be generalized for other disordered systems.

Molecular polarizabilities as fingerprints of perturbations to water by ions and confinement.

Perturbations to water, both by ions and confining media, have been the focus of numerous experimental and theoretical studies. Yet, several open questions remain, including the extent to which such perturbations modify the structural and dielectric properties of the liquid. Here, we present a first-principles molecular dynamics study of alkali cations in water (Li+, Na+, and K+) as well as of water and LiCl and KCl solutions under confinement within carbon nanotubes (CNTs) of small diameter (1.1-1.5 nm). Our simulations support the view that the water structure is only modified locally in the presence of cations. We found that molecular polarizabilities are fingerprints of hydrogen bonding modifications, which occur at most up to the second solvation shell for all cations in bulk water. Under confinement, we found that the overall value of the molecular polarizability of water molecules near the surface is determined by the balance of two effects, which are quantitatively different in CNTs of different radii: the presence of broken hydrogen bonds at the surface leads to a decrease in the polarizabilities of water molecules, while the interaction with the CNT enhances polarizabilities. Interestingly, the reduction in dipole moments of interfacial water molecules under confinement is instead driven only by changes in the water structure and not by interfacial interactions. As expected, confinement effects on water molecular polarizabilities and dipole moments are more pronounced in the case of the 1.1 nm CNT.

Structural and Thermophysical Anomalies of Liquid Water: A Tale of Molecules in the Instantaneous Low- and High-Density Regions.

Water is believed to be a heterogeneous liquid comprising multiple density regions that arise because of the presence of interstitial molecules and can be differentiated by their structure as well as the existence of hydrogen-bonded pairs with varying strengths. First-principles molecular dynamics studies were performed at six different temperatures to investigate the effect of temperature on the thermophysical, structure, dynamics, and vibrational spectral properties of the water molecules using dispersion-corrected density functional theory. The variation of properties like density, cohesive energy, and compressibility with a change in temperature produces a trend that matches with the experiments and resembles the experimentally observed anomalous behavior. We explore the possibility of explaining the trends in calculated properties by analyzing the structure and dynamics of the water molecules in terms of instantaneous low- and instantaneous high-density regions that are found during the simulation time. The dynamics of these two types of water molecules were studied by calculating the lifetime from the proposed autocorrelation functions. The lifetime of formation of instantaneous low-density water is found to decrease with an increase in temperature, whereas the lifetime of instantaneous high-density water is found to be maximum at 298 K among all the considered temperatures. The presence of more interstitial water molecules is observed at this temperature. The signature of these water molecules is found in the radial distribution function, spatial distribution function, void distribution, configurational space, orientational dynamics, and spectral diffusion calculations. It is also found that around 298 K, these water molecules are present distinctively that mix up with the first and second solvation shells with the rise of the temperature. The outlook of the reported results can be extended to other thermodynamic conditions to explain some of the anomalous properties, which can be related to the presence of the interstitial molecules in water.

Thermal stability of zirconia-doped ceria surfaces: A first-principles molecular dynamics study

Ceria, one of the most important functional rare-earth oxides, is widely used in many technically important industry fields. However, pure ceria is known to have poor thermal stability. Zirconia doping is usually employed to improve its thermal stability and achieve high OSC (oxygen storage capacity) performances under severe conditions. We have performed first-principles molecular dynamics simulations at a series of temperatures and proposed two parameters, i.e. the thermal displacement rate and the phase segregation degree, to describe the structural change of the ceria-zirconia solid solution surfaces. Based on our molecular dynamics simulations the temperature predicted at which the specific surface area dramatically decreases is in good agreement with previous experimental observations.

Deuterium addition to liquid Li–Sn alloys: implications for plasma-facing applications

Liquid metals are being explored actively as candidates for plasma-facing components (PFCs) in fusion reactors. Recently, Li–Sn alloys have appeared as promising alternatives that could overcome some of the challenges faced by the well-studied liquid Li system, namely, a vapor pressure that limits the operating temperature and a high hydrogen isotope retention. However, only scarce data (experimental or theoretical) are available concerning the performance of Li–Sn alloys, specifically only for the compositions of Li30Sn70 and Li20Sn80, related to their bonding and retention of deuterium (D). Here, we present a comprehensive, first-principles molecular-dynamics study of static and dynamic properties of liquid Li30Sn70 at various D concentrations. We observe the formation of D2 gas bubbles for β in Li30Sn70Dβ greater than 22.5 along with Li segregation towards D2 bubbles. To understand the effect of Sn addition on D retention in Li–Sn alloys, we perform a thermodynamic evaluation of maximum D retention in Li-rich Li–Sn alloys. Overall, this work will provide useful data and guidance in the development of Li–Sn PFCs in fusion reactors.

Theoretical prediction of stable cluster-assembled CdSe bilayer and its functionalization with Co and Cr adatoms

In this article, we present our results on bilayers assembled upon strategic placement of Cd6Se6 clusters. These bilayers are studied for their stability and electronic structure with the help of density functional theory and are further analyzed using Bardeen, Tersoff and Hamann formalism for their tunneling properties. Our calculations show that the hexagonal arrangement of these clusters prevails as the most stable geometry showing all real phonon modes. First-principles molecular dynamics studies on this 2D structure show that it remains intact even at room temperature. This bilayer shows an indirect semiconducting band gap of 1.28 eV with the current–voltage (I–V) characteristics similar to a tunnel diode. Furthermore, we functionalized this bilayer using transition metal atoms, Co and Cr. The aim was to see whether the bilayer sustains magnetism and how the concentration affects its electronic and magnetic properties. Co functionalization brings ferromagnetic ordering in the bilayer which turns near half-metallic upon increasing the concentration. On the other hand, Cr functionalization shows a transition from antiferro- to ferromagnetic ordering upon increasing the concentration. The I–V characteristics of all these functionalized bilayers show negative differential conductance similar to a tunnel diode.