First-principles Molecular Dynamics Simulations Research Articles

5,6-Biheterocyclic pentazolate salts as promising energetic materials: a new design strategy

Pentazole ( cyclo -N 5 − ) energetic salts have always been of high research value in the energetic materials field. However, the low density of the available cyclo -N 5 − salt seriously affects its comprehensive performance. Therefore, it is of great significance to effectively design and screen out pentazole salts with excellent properties. In this work, a cation design strategy based on a 5,6-biheterocycle is proposed. Based on the predicted density, heat of formation and detonation properties, the nitrogen-rich bicyclic heterocycle with -N-O − bond and -NO 2 or -NHNO 2 substituent as a cation is an effective method to improve the comprehensive properties of cyclo -N 5 − salts, and four salts of F2 , G2 , H2 and H7 with superior comprehensive properties to RDX were selected. Monte Carlo method was used to predict the stable crystal structure of four salts. Atoms in molecules theory (AIM) was used to expound the contribution of hydrogen bond and π-π superposition to the stability of cyclo -N 5 − . The cohesive energy density and mechanical properties show that the thermal stability of H2 (with best detonation performance; D : 9.16 km s −1 , P : 38.52 GPa) is similar to 3,6,7-triamino-7 H -[1,2,4]triazolo[4,3-b][1,2,4]triazol-2-ium pentazolate (decomposition temperature: 394.1 K, impact sensitivity: greater than 40 J). First-principles molecular dynamics simulation for H2 thermal decomposition was performed to explore the underlying mechanism of its excellent detonation performance. This work is expected to provide inspiration for the design and synthesis of cyclo -N 5 − salts.

Dielectric Relaxation and Dielectric Switching Behaviors in (N,N-Diisopropylethylamine) Tetrachloroantimonate(III).

Dielectric switches have drawn renewed attention to the study of their many potential applications with the adjustable switch temperatures (Ts ). Herein, a novel antimony-based halide semiconductor, (N,N-diisopropylethylamine) tetrachloroantimonate ((DIPEA)SbCl4 , DIPEA+ =N,N-diisopropylethylamine), with dielectric relaxation behavior and dielectric switches has been successfully synthesized. This compound, consisting of coordinated anion chains and isolated DIPEA+ cations, undergoes an isostructural order-disorder phase transition and shows a step-like dielectric anomaly, which can function as a frequency-tuned dielectric switch with highly adjustable switch temperature (Ts ). Variable-temperature single-crystal structure analyses and first-principles molecular dynamics simulations give information about the general mechanisms of molecular dynamics. This work enriches the dielectric switch family and proves that hybrid metal halides are promising candidates for switchable physical or chemical properties.

Defect-free and crystallinity-preserving ductile deformation in semiconducting Ag2S

Typical ductile materials are metals, which deform by the motion of defects like dislocations in association with non-directional metallic bonds. Unfortunately, this textbook mechanism does not operate in most inorganic semiconductors at ambient temperature, thus severely limiting the development of much-needed flexible electronic devices. We found a shear-deformation mechanism in a recently discovered ductile semiconductor, monoclinic-silver sulfide (Ag2S), which is defect-free, omni-directional, and preserving perfect crystallinity. Our first-principles molecular dynamics simulations elucidate the ductile deformation mechanism in monoclinic-Ag2S under six types of shear systems. Planer mass movement of sulfur atoms plays an important role for the remarkable structural recovery of sulfur-sublattice. This in turn arises from a distinctively high symmetry of the anion-sublattice in Ag2S, which is not seen in other brittle silver chalcogenides. Such mechanistic and lattice-symmetric understanding provides a guideline for designing even higher-performance ductile inorganic semiconductors.

Remarkable enhancement in catechol sensing by the decoration of selective transition metals in biphenylene sheet: A systematic first-principles study

Motivated by the recent successful synthesis of biphenylene structure (Fan et al 2021 Science 372 852), we have explored the sensing properties of this material towards the catechol biomolecule by performing the first-principles density functional theory and molecular dynamics simulations. Pristine biphenylene sheet adsorbs catechol molecule with a binding energy of −0.35 eV, which can be systematically improved by decorating the transition metals (Ag, Au, Pd, and Ti) at various possible sites of biphenylene. It is observed that the catechol molecule is adsorbed on Pd and Ti-decorated biphenylene sheets with strong adsorption energies of −1.00 eV and −2.54 eV, respectively. The interaction of the catechol molecule with biphenylene and metal-decorated biphenylene is due to the charge transfer from the O-2p orbitals of the catechol molecule to the C-2p orbitals of biphenylene and d-orbitals of metals in metal-decorated biphenylene, respectively. From the Bader charge calculation, we found that 0.05e amount of charge is transferred from the catechol molecule to pristine biphenylene, which gets almost double (∼0.1e) for the Ti-decorated biphenylene sheet. The diffusion energy barrier for the clustering of the Pd and Ti atoms comes out to be 2.39 eV and 4.29 eV, computed by performing the climbing-image nudged elastic band calculations. We found that the catechol molecule gets desorbed from the pristine biphenylene sheet at 100 K but remains attached to metal (Pd, Ti) decorated biphenylene sheets at room temperature by performing the ab-initio molecular dynamics simulations. The Ti-decorated biphenylene sheet has more sensitivity toward catechol adsorption while the Pd-decorated biphenylene sheet has a suitable recovery time at 500 K. The results suggest that the Pd and Ti-decorated biphenylene sheets are promising materials for catechol detection.

The impact of conformational sampling on first-principles calculations of vicinal COCH J-couplings in carbohydrates.

Dihedral angles in organic molecules and biomolecules are vital structural parameters that can be indirectly probed by nuclear magnetic resonance (NMR) measurements of vicinal J-couplings. The empirical relations that map the measured couplings to dihedral angles are typically determined by fitting using static structural models, but this neglects the effects of thermal fluctuations at the finite temperature conditions under which NMR measurements are often taken. In this study, we calculate ensemble-averaged J-couplings for several structurally rigid carbohydrate derivatives using first-principles molecular dynamics simulations to sample the thermally accessible conformations around the minimum energy structure. Our results show that including thermal fluctuation effects significantly shifts the predicted couplings relative to single-point calculations at the energy minima, leading to improved agreement with experiments. This provides evidence that accounting for conformational sampling in first-principles calculations can improve the accuracy of NMR-based structure determination for structurally complex carbohydrates.

Deposition mechanism of molecular S8 on the dolomite surface

Sulfur deposition threatens the production efficiency and mining safety in the development of high-sulfur natural gas reservoirs, but its occurrence in the pore throats remains unclear. The adsorption and migration mechanism of S8 molecules on the dolomite surface is studied by means of first-principles calculations and molecular dynamics simulations. The S8 molecule prefers to lie horizontally on the dolomite (104) surface, apt to slide, but hard to be exfoliated from the surface. Our calculations address the strong S8–surface interaction, which is however ignored in the in-use sulfur deposition models.

Accelerating equilibration in first-principles molecular dynamics with orbital-free density functional theory

We introduce a practical hybrid approach that combines orbital-free density functional theory (DFT) with Kohn-Sham DFT for speeding up first-principles molecular dynamics simulations. Equilibrated ionic configurations are generated using orbital-free DFT for subsequent Kohn-Sham DFT molecular dynamics. This leads to a massive reduction of the simulation time without any sacrifice in accuracy. We assess this finding across systems of different sizes and temperature, up to the warm dense matter regime. To that end, we use the cosine distance between the time series of radial distribution functions representing the ionic configurations. Likewise, we show that the equilibrated ionic configurations from this hybrid approach significantly enhance the accuracy of machine-learning models that replace Kohn-Sham DFT. Our hybrid scheme enables systematic first-principles simulations of warm dense matter that are otherwise hampered by the large numbers of atoms and the prevalent high temperatures. Moreover, our finding provides an additional motivation for developing kinetic and noninteracting free-energy functionals for orbital-free DFT.

Theoretical investigation of CO oxidation by single cobalt atom within two-dimensional porphyrin sheet

In this paper, the oxidation mechanism of carbon monoxide (CO) on two-dimensional porphyrin sheet within a single cobalt atom (Co-TDPs) was studied by density functional theory with dispersion (DFT-D). The stability of Co-TDPs at different temperatures was verified by first-principle molecular dynamics simulations. Absorption energies of reactant and product to anchor to the Co–N4 site showed CO and O2 adsorption to be stronger than the CO2 adsorption. In addition, the Langmuir–Hinshelwood, Eley–Rideal (ER), and ter-molecular Eley–Rideal (TER) mechanisms were used to investigate the reaction mechanisms of CO oxidation on Co-TDPs. The Langmuir–Hinshelwood (LH) and ER mechanisms were feasible reaction profiles of CO oxidation because of their smaller energy barrier. The results suggested that the Co-TDPS was acting as a catalyst for CO oxidation in the mild condition.

Supermolecule-mediated defect engineering of porous carbons for zinc-ion hybrid capacitors

Zinc ion hybrid capacitors hold great potential for future energy storage that requires both high energy density and high power capability. However, the charge storage mechanism of porous carbon cathode is ambiguous in Zn2+ ion-containing aqueous solutions, albeit porous carbon usually stores charge by electric double-layer capacitance. Herein, we developed a supermolecule-mediated direct pyrolysis carbonization strategy to convert sustainable sodium lignosulfonate resources into three-dimensional highly heteroatom-doped porous carbons with large mesopores. Through this strategy, we obtained lignin-derived porous carbons with high heteroatom dopings (14.9 at% nitrogen and 4.7 at% oxygen) and relatively high specific surface areas. Furthermore, the nitrogen doping configurations were mainly edge-nitrogen dopants even under high pyrolysis temperatures (> 900 °C). Lignin-derived nitrogen-doped porous carbon showed a high gravimetric specific capacitance of 266 F g−1 with high rate capability, which is endowed by the increased surface pseudocapacitance. First-principles calculations and molecular dynamics simulations indicate that the edge nitrogen and oxygen dopants contribute to the reversible adsorption/desorption of zinc ions and protons. Pores size less than 6.8 Å can cause a significant diffusion energy barrier for the hydrated zinc ions, thus degrading the capacitance and rate capability.

Diffusion behaviors of HF in molten LiF-BeF2 and LiF-NaF-KF eutectics studied by FPMD simulations and electrochemical techniques

The hydrofluorination reaction has proven to be a very effective approach to remove impurities in the preparation and usage of molten LiF-BeF 2 (Flibe) and LiF-NaF-KF (Flinak) eutectic salts, however, the diffusion behaviors of HF in two molten fluorides are incompletely understood. Here we design two systems, HF+Flibe and HF+Flinak, to reveal the diffusion properties and local structures as well as their correlations by first principles molecular dynamics (FPMD) simulations and electrochemical methods . It is shown that the strong interaction between Be 2+ and F – in molten Flibe reduces their diffusivities while providing more space for Li + migration. The diffusion coefficient of H + in molten Flinak is higher than that in molten Flibe determined by the fast exchange of H + between two F anions, and the larger diffusivity of F – in molten Flinak is related to the irregular movement of free F anions. Besides, the quasi-gaseous transport behavior of HF in two molten fluorides is proposed in this study according to the stable H-F bonds. Moreover, the experimental diffusion coefficient of HF in molten Flinak estimated by the method of convolution linear sweep voltammetry is consistent with our simulation value, and the larger diffusivity is well explained by the higher stretching vibration frequency between H + and F – in molten Flinak. Finally, the ionic diffusion coefficients are descripted by the first sharp diffraction peaks of structure factors, the nearest neighbor peaks of radial distribution functions , and the major bond angels of angular distribution functions, and it is speculated from these FPMD simulations that the peak position has a higher weight on diffusion coefficient. Overall, in-depth understandings of the structure-property relationships of HF+Flibe and HF+Flinak systems provide insights into the molten salt purification technology and the daily operation of molten salt reactors .

Implementing an “Impracticable” Copolymerization to Fabricate a Desired Polymer Precursor for N-doped Porous Carbons

It is common that a proof-of-concept of a desired reaction, which might generate materials with new functions or application potential, is eventually proved impracticable or commercially unfeasible. Considerable efforts have been made but wasted in searching for unknown reaction conditions in solvent environments because it was believed that the activity of reactants can be enhanced to facilitate reactions by dissolving them in solvents. However, an abnormal case was discovered in this study. A desired copolymerization reaction between 1,3,5-tris(chloromethyl)-2,4,6-trimethylbenzene and melamine was confirmed to be impracticable under various solvent conditions; however, it was successfully implemented using a solvent-free method. Using first-principle calculations and molecular dynamics simulations, two decisive factors that the reaction in solvents cannot possess, namely the reaction equilibrium being pushed by the timely release of by-products and the confined thermal motions of the activated monomer molecules in the solid phase, were demonstrated to make the copolymerization successful in the solvent-free method. Owing to the high aromaticity and azacyclo-content, the as-synthetic copolymer exhibited good application potential as a precursor to fabricate N-doped porous carbons with satisfactory carbon yields, ideal N contents, desired textural properties, and competitive CO2 capture abilities compared to other representative counterparts reported recently.

Stability and structure of platinum sulfide complexes in hydrothermal fluids

Knowledge of the chemical speciation of platinum and the solubility of Pt-bearing minerals in hydrothermal fluids is required to assess Pt transport, remobilization and concentration in the Earth’s crust. In this study, we combined PtS(s) solubility measurements in a hydrothermal reactor allowing fluid sampling, in situ X-ray absorption spectroscopy, and first-principles molecular dynamics simulations to systematically investigate the structure, composition and stability of Pt sulfide complexes in model aqueous H2S-bearing solutions up to 300 °C and 600 bar. The results demonstrate that tetrahydrosulfide, PtII(HS)42–, is the major Pt-bearing complex in aqueous solutions saturated with PtS(s) over a wide range of dissolved hydrogen sulfide concentrations, from < 0.2 to ∼ 2 molal. The equilibrium constants of the dissolution reaction of PtS(s) generated in this study, PtS(s) + 3 H2S(aq) = Pt(HS)42– + 2 H+ (β4), are described by the equation log10β4 = 0.9 × 1000/T(K) – 19.7 (± 0.5) over the temperature range 25–300 °C and pressure range Psat–600 bar. Furthermore, the stepwise formation constants of four PtII-HS complexes, Pt(HS)+, Pt(HS)20, Pt(HS)3–, and Pt(HS)42–, were estimated, for the first time, from molecular dynamics simulations. The generated constants indicate that the maximum solubility of platinum in the form of Pt(HS)42– in reduced H2S-dominated hydrothermal fluids at moderate temperatures (≤ 350 °C) is close to 1 ppb Pt at near-neutral pH of 6–8 and hydrogen sulfide concentrations of 0.1 molal. Although this solubility is much greater than that of Pt-Cl, Pt-OH and Pt-SO4 complexes at such conditions, it is yet too low to account for significant Pt transport in most shallow-crust hydrothermal settings, characterized by the presence of both sulfide and sulfate. Complexes with S-bearing ligands, very likely other than H2S/HS–, such as S3–, would be required to account for Pt hydrothermal mobility. Our results provide a basis for more systematic future studies, using combined approaches, of the role of hydrothermal fluids in the behavior of platinum group elements in nature.

Equilibrium boron isotope fractionation during serpentinization and applications in understanding subduction zone processes

Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (δ11B) of serpentinite is used in studying water-mediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The 11B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (β-factor) of aqueous B(OH)3 and B(OH)4− are 1000lnβB(OH)3 = −20.1 + 37.07(1000/T) + 8.2(1000/T)2 and 1000lnβB(OH)4- = −13.94 + 24.43(1000/T) + 9.12(1000/T)2 at the P-T range of 0–500 MPa and 298–1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Δ11Blizardite-fluid = 15.89–22.88(1000/T) - 0.19(1000/T)2 between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the δ11B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts.

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.

Melting curve and transport properties of ammonia ice up to the deep mantle conditions of Uranus and Neptune

Motivated by the poor knowledge of the implication of planetary ice on the internal convection and anomalous magnetic field of ice giants, we perform extensive first-principles molecular dynamics simulation on ammonia ice to investigate its thermophysical behaviors at high temperature ($T$) and pressure ($P$) conditions relevant to the deep mantle of Neptune and Uranus. The melting curve, transport properties, and sound velocity of ammonia up to 350 GPa and 5500 K, spanning from fluid mixtures to a highly compressed superionic regime, are determined. A first-order phase transition of ammonia from the plastic state to the superionic state is observed explicitly, which is associated with the reduction of entropy. In contrast to previous predictions, the melting curve elucidates the existence of superionic ammonia in deep planetary interiors. Inspection of the reduction for transport properties and sound velocity along the isentrope of ice giants further evidences that superionic ammonia can sufficiently sustain the planetary dynamo mechanism and may contribute to the internal stratification which is responsible for the generation of the multipolar magnetic field. Finally, a comprehensive phase diagram of ammonia extended to a higher T-P regime is constructed, which provides a clear picture for studying the fundamental behavior and phase transition of ammonia in deep planets, and enhances our understanding of the interior structure and thermal convection of these planetary systems.

Evaluation of the local structure and electrochemical behavior in the LiCl-KCl-SmCl3 melt

This present work evaluates the local structure and electrochemical behavior of the LiCl-KCl-SmCl3 melt to facilitate the efficient reprocessing of spent nuclear fuel (SNF). At 723 K, the radial distribution function, coordination number, and structure factors were adopted to understand the local structure of LiCl-KCl and LiCl-KCl-SmCl3 based on first-principles molecular dynamics (FPMD) simulation. We found the SmCl3 slight effect on LiCl-KCl melts in short-range order and its influence on the distribution of Li-Li intermediate-range order. Moreover, the diffusion coefficient of Sm(III) 2.012 × 10-5 cm2/s in LiCl-KCl melts was determined by analyzing the mean square displacement. The electrochemical behavior of Sm(III) was conducted in the temperature range from 673 K to 823 K by a series of electrochemical methods. According to the theory of Matsuda and Ayabe, the Sm(III) + e- = Sm(II) is a quasi-reversible process. The diffusion coefficient of Sm(III) was measured by the chronoamperometry method, which verified the credibility of the FPMD simulation. Furthermore, this work explored the exchange current density of Sm(III)/Sm(II), whose value is between 0.826 × 10-3 and 2.383 × 10-3 A·cm−2, by the linear polarization.

Method to determine the electron–ion temperature relaxation rate from test particle distributions

A method to calculate the electron–ion energy exchange rate from the forces on and velocities of an ensemble of test particles is described. The essence of the method is that the energy exchange rate is related to the covariance between the distribution of velocities of test particles and the forces acting on them. The energy exchange rate is related to the electron–ion temperature relaxation rate in the limit of a low test particle speed. A proof of concept is conducted with first-principles molecular dynamics simulations.

Passivating Lead Halide Perovskites Using Pyridinium Salts with Superhalogen Atoms

Passivating lead halide perovskites using pyridinium salts has attracted enormous attention, but the excellent surface passivation of the halide perovskites has not been achieved by using only a pyridinium salt until now. Herein, passivating the (001) planes of the cubic CsPbI3, CH3NH3PbI3, and NH2CHNH2PbI3 perovskites using the pyridinium salts of C5NH6X (X = Cl, Br, I, PF6, ClO4, or BF4) is systematically studied by high-throughput first-principle calculations and ab initio molecular dynamics simulations. The results show that the excellent surface passivation of the three perovskites is achieved by the pyridinium salt of C5NH6BF4 (i.e., shallow level, negative formation energy, unchanged band-edge construction, and stable dynamics property are obtained for the three passivated perovskites), which strongly imply that their devices can show excellent performances, such as long-term stability, low ion migration, and high efficiency. However, the C5NH6ClO4 and C5NH6PF6 pyridinium salts are only profitable for passivating the (001) PbI2 plane of the three perovskites, and other C5NH6X pyridinium salts have adverse effects.

Finite Temperature Ultraviolet-Visible Dielectric Functions of Tantalum Pentoxide: A Combined Spectroscopic Ellipsometry and First-Principles Study

Tantalum pentoxide (Ta2O5) has demonstrated promising applications in gate dielectrics and microwave communication devices with its intrinsically high dielectric constant and low dielectric loss. Although there are numerous studies on the dielectric properties of Ta2O5, few studies have focused on the influence of external environmental changes (i.e., temperature and pressure) on the dielectric properties and the underlying physics is not fully understood. Herein, we synthesize Ta2O5 thin films using the magnetron sputtering method, measure the ultraviolet-visible dielectric function at temperatures varying from 300 to 873 K by spectroscopic ellipsometry (SE), and investigate the temperature influence on the dielectric function from first principles. SE experiments observe that temperature has a nontrivial influence on the ultraviolet-visible dielectric function, accompanying the consistently decreased amplitude and increased broadening width for the dominant absorption peak. First-principles calculations confirm that the dominant absorption peak originates from the aggregated energy states near the valence band maximum (VBM) and conduction band minimum (CBM), and the theoretically predicted dielectric functions demonstrate good agreement with the SE experiments. Moreover, by performing first-principles molecular dynamics simulations, the finite-temperature dielectric function is predicted and its change trend with increasing temperature agrees overall with the SE measurements. This work explores the physical origins of temperature influence on the ultraviolet-visible dielectric function of Ta2O5, aimed at promoting its applications in the field of micro-/nanoelectronics.

An effect of nitrogen incorporation on the structure and properties of amorphous SiC: First-principles molecular dynamics simulations

The structural, mechanical and electronic structure properties of amorphous silicon carbide and silicon carbonitride alloys were studied depending on chemical ordering, and nitrogen content, respectively, by using first-principles molecular dynamics simulations. The structure, chemical bonding, electronic structure, frequency-dependent dielectric functions, and mechanical properties (elastic moduli, Poisson ratio, bulk modulus to shear modulus ratio, ideal shear strength, Vickers hardness, Debye temperature and fracture toughness) were studied. It was established that an increase of nitrogen content in carbonitride alloys and a decrease of chemical ordering in carbide alloys cause the weakening of the Si–C network, strengthening of the C–C network, decrease in four-fold coordination and increase in three-fold coordination of Si and C atoms. All these factors lead to a deterioration of mechanical properties. The highly nitrided alloys are supposed to be wide-gap semiconductors with estimated mobility gaps in the range of 2.9–4.2 eV. The calculated data were used to elucidate available experimental results.