We present a critical assessment of the force-field model of NASICON solid electrolytes, benchmarking it against ab initio molecular dynamics (AIMD) and nudged elastic band (NEB) calculations. While the classical molecular dynamics (CMD) employing the force field slightly overestimates the rigidity of the Na3Zr2Si2PO12 polyhedral skeleton, it yields remarkable agreement with AIMD in predicting Na+ migration channels and overall mobility. Our results validate the accuracy of CMD-derived predictions on the pivotal role of Si/P ordering in governing ion transport. Furthermore, AIMD trajectories reveal that ion transport involves a cooperative mechanism between Na+ and the polyhedral skeleton, as evidenced by the significant drop in ion mobility upon freezing the framework dynamics, consistent with the earlier CMD predictions. These findings suggest that the model offers a cost-efficient platform for exploring aliovalent substitutions of chemically simple dopants, in search of highly conducting compositions within the NASICON-family.

Aqueous alkali metal carbonates (M2CO3, where M = Li+, Na+, and K+) have attracted significant attention due to their critical role in CO2 capture, energy storage, and various industrial processes. Despite extensive experimental progress in understanding their macroscopic properties, the molecular-level mechanisms governing solvation and ion-association behavior in these systems with varying concentrations remain largely unexplored. To address this gap, we present a comprehensive molecular dynamics (MD) investigation of aqueous Li2CO3, Na2CO3, and K2CO3 using a refined force field model, validated against the experimental solubility limits of the respective salts. The study highlights how the local environment of carbonate ions (CO2-3) evolves with solute concentration, providing fresh insights into the role of alkali metal cations in promoting the self-aggregation of carbonates. The trajectory analysis predicts that the mobility of all the solute and solvent species decreases with concentration, leading to the co-movement of cations and carbonate ions near their solubility limit. Notably, Li+ exhibits anomalously slow diffusion compared to the larger Na+ and K+ ions, owing to its stronger association with carbonate ions, which is consistent with the experimentally observed low solubility of Li2CO3 in water. This behavior is attributed to the size-sensitive "hardness" of the hydration shells of cations, corroborated in terms of the water residence times in their first hydration shells. Together, these results facilitate a fresh molecular-level understanding of the diverse physical properties of aqueous alkali metal carbonates and have significant implications for the development of carbon capture strategies.

Extensive molecular dynamics investigations of the NASICON solid solutions, Na1+x Zr2Si x P3-x O12, where 0 <= x <= 3, are carried out employing classical molecular dynamics (MD) simulations in the isothermal-isobaric (NPT) ensemble at 600 K. These calculations are complimented with additional sets of constrained MD simulations, which restrict the dynamics of the framework ions and/or the unit cell volume with composition. A comparison of Na+ mobility across these series of simulations establishes the significance of the dynamical correlation between Na+ ions with framework-oxygens, which gets compounded by the anomalous volume expansion of the unit cell with composition. Further, it is demonstrated that there are two competing factors sensitive to composition (x), namely, the Na+ interaction with the Si4+/P5+ ions of the framework and the mutual repulsion between the Na+ ions. However, they impact differently at the two dominant Na+ sites in the migration channel, resulting in a near barrier-free transport for Na+ around x = 2.5, which leads to the observed anomalous ionic conductivity of NASICON with composition. The results present insights into enhancing the ionic conductivity in similar covalent framework solids through targeted aliovalent substitutions.

Molecular dynamics simulations of divalent cation-doped anti-perovskites, Li3-2xMxOCl, with x = 0.1 and for M= Mg, Ca, Sr and Ba, are carried out at 450 K to investigate the microscopic structural and dynamical properties. It is predicted that for larger dopants, Ca, Sr and Ba, these materials are thermodynamically unstable at 450 K, limiting their utility as solid electrolytes for battery applications. On the other hand, the Mg-doped anti-perovskites Li2.8Mg0.1OCl are quite stable in their cubic phase and produce high ionic conductivity of about 1 mS/cm at 450 K. These simulations are supplemented with nudged elastic band calculations on Li2.8Mg0.1OCl. The study unravels microscopic insights on the nature of the vacancy-driven Li+ transport in the system, and the cooperativity of the anionic sub-lattice in promoting the ionic conductivity of the system.

Perovskite-based protonic conductors are promising electrolyte materials for solid oxide fuel cells. Although numerous studies have been conducted in these systems, many microscopic aspects related to proton transport are still unknown. We present ab initio molecular dynamics studies of proton transport in Sc-doped CaZrO3 to gain insights into proton conduction mechanisms. It is noted that the proton transfer along the two primary channels, namely, intraoctahedral and interoctahedral, is sensitive to the local cationic environments. While the proton hops in the Zr-O(H)-Zr environments are predominantly interoctahedral hops, the Zr-O(H)-Sc environments promote both channels alike. However, the interoctahedral proton hops from the Zr-O(H)-Sc environments are followed by persistent jump-reversals leading to the localization of the protons, with negligible contribution to the ionic conductivity. Exploiting the van-Hove correlation functions, it is demonstrated that the nature of the localization essentially involves proton scrambling over the ScO6 octahedra, unlike earlier perceptions. These results are complemented with proton binding energies and interoctahedral migration barriers, estimated respectively from geometry optimization and nudged elastic band calculations.

Classical molecular dynamics simulation is employed to study La1-xSrxFeO3-x/2 at 1200 K over a range of dopant concentrations, x = 0.1 to 0.6. Sr ions tend to improve the energetics of the oxide ion sites, resulting in a higher fraction of vacancies in the vicinity of La. The oxide ion migration in the system involves predominantly intra-octahedral hops along the edges of the FeO6 octahedra. This ion migration is controlled by a triangular bottleneck of cations, formed by two La/Sr ions and one Fe ion, appearing midway between two neighboring oxide ion sites. It is noted that these bottlenecks pose higher barriers as more Sr ions are introduced. This increase in the microscopic energy barriers for oxide ion migration in the system corroborates the observed slowdown of ion transport with dopant concentration. The study also elucidates a dynamic correlation between the mobile species and the cationic framework, wherein as an oxide ion approaches the bottleneck, the cations move apart, increasing the cross-sectional area of the bottleneck.

Classical molecular dynamics simulations are carried out on cationically ordered yttria-doped zirconia, YxZr1-xO2-x/2, at the dopant (Y3+) concentration of x = 12.5%. A variety of Zr4+/Y3+ ordered structures are examined for local migration pathways and microscopic energetics governing oxide ion transport in the system. Starting from a layer of cubic Y2O3 spanning the basal plane, the number of Y3+ layers in the simulation cell is multiplied systematically, at the expense of their coverage per layer. The study reveals that cationic ordering in YxZr1-xO2-x/2 can produce a profound impact on the oxide ion transport in the framework, wherein with the maximal dispersion of the dopant a four-fold enhancement in the ionic conductivity is observed relative to the cationically disordered matrix. We demonstrate that this improvement in ion mobility is due to the homogenization of oxide ion vacancies across the matrix. This study thus provides valuable insights for the enhancement of the electrochemical performance of solid oxide fuel cells.

Yttria-doped ceria (YDC) is one of the most promising solid electrolytes for solid oxide fuel cells (SOFC) operating at intermediate temperatures (500–750 $$^\circ$$ C) owing to its high oxygen ion conductivity, among other favorable properties. Employing classical molecular dynamics (MD) technique, yttria-doped ceria, $$\text{Y}_{x}\text{Ce}_{1-x}\text{O}_{2-x/2}$$ , is investigated over the dopant concentrations, x =4 to 40 mol% at 1300 K. The gross structural and dynamical features of the system, such as the variation of lattice parameters and ionic conductivity, from the present study are in good agreement with previous experimental and theoretical reports. The oxygen ion conductivity shows a pronounced maximum around x= 14 mol% of yttrium doping and drops off thereafter. The observed variation in the ionic conductivity is found to be associated with a non-monotonic variation of the residence time of the oxygen ions at their tetrahedral locations in the fluorite lattice. It is noted that the energy of oxygen ions increases systematically with the number of yttrium in their local environment. Further, the microscopic energy barrier for ion hops along the various channels connecting the distinct oxygen environments is also sensitive to the yttrium presence. The statistically averaged barriers along these channels produce a minimum for the high conducting composition, thereby explaining the observed conductivity variation.

The recently discovered anti-perovskites of the formula, Li3OCl1−xBrx, where x=0 to 1, have been garnering interest as potential candidates for solid electrolytes in battery applications. It is observed that the Li ion transport in these solids is rather slow, and unaccessible within typical molecular dynamics time scales. However, by judicially augmenting it with the accelerated sampling technique of metadynamics, the present study derives fresh insights on the nature of Li ion transport in these systems. The microscopic of ion migration along intra- and inter-octahedral channels, suggests that intra-octahedral channels account for over 95% Li transport in the system. The bromide end member offers lower free energy barriers, due to entropic factors resulting from the expansion of the lattice. However, the large free energy barriers estimated in the present work suggest that the undoped Li3OCl1−xBrx anti-perovskites are unlikely to exhibit superionic behavior.

A comprehensive molecular dynamics investigation of yttria stabilized zirconia, YxZr1-xO2-x/2, is carried out for a wide range of composition, x = 4 to 40 mol%, and over the temperature spanning 800-2200 K. The lattice parameter of the fluorite cell shows a monotonic increase with concentration, while the self-diffusivity of the oxide ion as well as the resulting ionic conductivity shows an optimum value around x = 10 mol%. These gross structural and transport properties of the system from the present study are in good agreement with previous experimental and theoretical investigations. It is noted that oxygen migration occurs along straight channels parallel to the crystallographic axes, connecting the tetrahedral holes of the fluorite lattice occupied by them. A microscopic investigation of distinct oxygen environments, variably coordinated to Y3+ and Zr4+ cations, and of the channels connecting them is carried out. Analysis of these local channels for their energetics and their contribution to overall oxygen transport, resolved in terms of the cationic edges connecting them, provides fresh insights into the oxygen migration mechanism in the system.

A comprehensive study of aqueous arsenic acid (H3AsO3) and its deprotonated derivatives, namely H2AsO3- and HAsO32-, is carried out employing Car-Parrinello Molecular Dynamics (CPMD). Calculations are carried out with and without the incorporation of the dispersion correction, and critical comparison of its role on various structural and dynamical properties of the solute-solvent system are discussed. Aqueous H3AsO3 and H2AsO3- species solvated by 60 water molecules (which closely emulates normal pH conditions) are found to be stable, that is, without undergoing proton transfer with the solvent over the 50 ps of MD simulations performed. However, HAsO is found to be stable only in the basic media, emulated through the introduction of a hydroxyl ion (OH-) in lieu of one water molecule. The AsO33- species is not stable, and protonates quickly to HAsO32-, even under the basic environment of a hydroxyl ion (OH-) among 59 other water molecules simulated. Detailed analysis of the hydrogen bonding and vibrational spectra provide fresh microscopic insights on the hydration structure and dynamical characteristics of these species in aqueous environment. The spectroscopic insights derived, through comparison with their parent gas-phase species (H3AsO3), would be useful in the detection as well as quantification these species in various aqueous environments, for developing water remediation strategies of these environmentally hazardous species. (C) 2020 Elsevier B.V. All rights reserved.

Molecular dynamics (MD) simulations have been carried out on fast-ion-conducting LiM2IV(PO4)3, where MIV = Zr, Hf, Sn, and Ti, having a NASICON structure. The structural parameters and variation in...

Molecular dynamics (MD) is a powerful tool to investigate microscopic transport of atoms and molecules in condensed matter. However, there lies a large class of systems wherein atomic diffusion is too slow a process relative to the feasible time scales of typical atomistic simulations. Here, we demonstrate that with judicial implementation of a metadynamics (MTD) technique, the microscopic mechanism of atomic transport in solids can be accessed within a reasonable computational time. The calculations are carried out on the two end members of the true NASICON solid solutions, namely NaZr2(PO4)3 and Na4Zr2(SiO4)3, wherein Na+ diffusion is too slow to be accessed through standard MD simulations. The study also provides fresh insights on correlated ion hops and their implications on the effective diffusion barrier. The results are compared with climbing image nudged elastic band (CI-NEB) calculation, and available experimental results.

Molecular dynamics (MD) simulations have been carried out on fast-ion-conducting LiM₂ᴵⱽ(PO₄)₃, where Mᴵⱽ = Zr, Hf, Sn, and Ti, having a NASICON structure. The structural parameters and variation in Li⁺ conductivity across the series are in good qualitative agreement with earlier experimental reports. The salient feature of the present study is that it reveals a very interesting dynamical correlation between the mobile Li⁺ions and the [M₂ᴵⱽ(PO₄)₃]⁻ framework. It is demonstrated that this correlated motion contributes significantly to facilitate Li⁺ transport in the matrix.

The toxicity, mobility, and geochemical behaviors of arsenic are known to vary enormously with its speciation and oxidation states. The present work details results on the basis of ab initio molecular dynamics analysis of various waterborne As-V species, namely, H3AsO4, H2AsO4-, HAsO42-, and AsO43-. The nature of hydrogen bonding of these species with water and its influence on the solvent structure and relaxation behavior are discussed. Useful microscopic insights on the structural and spectroscopic signatures of the species in aqueous media are reported. Comparison of normal-mode frequencies of the species in gas phases to the vibrational density of states in solution provides insights on the influences of solvation and H bonding. The results are compared with the previous experimental and simulation studies, where available.

A comprehensive investigation of oxide ion transport in La1−x Sr x MnO3−0.5x (x = 0.2 and 0.5) over the temperature range of 800–2000 K is carried out employing long molecular dynamics simulations. The macroscopic structural and transport properties are in good agreement with previous experimental studies. The role of La/Sr ordering on oxide ion diffusivity is investigated. Fresh insights on diffusion pathways, microscopic energetics and mechanism of oxide ion transport are derived.

An ab initio molecular dynamics investigation is carried out on various water-borne Se(iv) species, H2SeO3, HSeO3- and SeO32-, in aqueous environment. Consistent with the reported acid dissociation constants, in neutral solution H2SeO3 exchanges protons with the surrounding water molecules establishing a dynamic equilibrium with HSeO3-. The SeO32- species is found to be stable only in basic environment, which is emulated in the present simulation through introducing a hydroxide ion, OH-, in the system. The hydration structure, hydrogen bonding and spectroscopic signatures of the species are comprehensively analyzed. The influence of the solute's hydration structure on the structural and dynamic response of the solvent is discussed. The correlation between the strength as well as the number of hydrogen bonds accepted by the solute on its vibrational properties are analyzed.

Microscopic investigation of solvation of selenic acid (H2SeO4) in the aqueous environment has been carried out using the Car-Parrinello molecular dynamics simulation technique. The species deprotonates to HSeO4(-) in a few picoseconds owing to its low pKa1 value of -3.0. A dynamic equilibrium between HSeO4(-) and SeO4(2-), is observed in qualitative agreement with the reported pKa2 value of 1.70. The governing deprotonation mechanism and the structural and dynamic evolutions of the system, particularly the nature of hydrogen bonding, their strengths and lifetimes are investigated comprehensively. A comparison of the vibrational spectra of the species recorded in the gas phase and in the aqueous environment provides further insights on the nature of the interaction between the solute species and water. The results are in good agreement with the available experimental data and other recent computational studies.

A series of molecular dynamics (MD) simulations are carried out, in which the Na+ content at the interlayers of Na2Ni2TeO6 is systematically varied, keeping the overall charge neutrality of the system, to identify the role of ion-ion correlation on Na+ diffusion in the system. It has been observed that interlayers having about 20% lower concentration of Na+ facilitate the highest conductivity that is one order of magnitude higher than those having normal Na+ concentration. The simulations predict a gradual crossover from energy-driven to entropy-driven transport of Na+ ions with the Na+ concentration at the interlayers. The transport mechanism and pathways of the mobile ions are also modified.

An interatomic potential is proposed for the recently discovered family of superionic solids of the formula Na2M2TeO6, where M = Ni, Zn, Co, or Mg. Molecular dynamics simulations demonstrating the quality of the potential in reproducing various structural and transport properties of this promising class of materials is presented. The study provides fresh insights on the microscopic energetics and Na+ migration pathways. Strong ionion correlations, resulting in a highly cooperative conduction mechanism, emerge from the study.