P. Padma Kumar

Ionic conduction in the solid state

Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these solids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simulations on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed.

Hydrogen-bonding structure and dynamics of aqueous carbonate species from Car-Parrinello molecular dynamics simulations

A comprehensive Car-Parrinello molecular dynamics (CP-MD) study of aqueous solutions of carbonic acid (H(2)CO(3)), bicarbonate (HCO(3)(-)), carbonate (CO(3)(2-)), and carbon dioxide (CO(2)) provides new quantitative insight into the structural and dynamic aspects of the hydrogen-bonding environments for these important aqueous species and their effects on the structure, H-bonding, and dynamical behavior of the surrounding water molecules. The hydration structures of the different carbonate species depend on their ability to accept and donate H-bonds with H(2)O. The H-bonds donated by the C-O-H sites of the carbonate species to water molecules are generally stronger and longer-lived than those accepted by these sites from water molecules. The structural relaxation among the water molecules is dominated by diffusional (translational) motion of H(2)O, whereas the H-bond reorganization is dominated by the librational motion of the water molecules and the carbonate species. The rates of structural relaxation of the H(2)O molecules and the rates of H-bond reorganization among them are slower in systems containing carbonate species, consistent with previous studies of simple salt solutions. The strengths and lifetimes of H-bonds involving the carbonate species positively correlate with the total negative charge on the species. H-bond donation from H(2)O to CO(2) is weak, but the presence of CO(2) noticeably affects the structure and structural relaxation of the surrounding H-bonding network leading to generally stronger H-bonds and slower relaxation rates, the behavior typical of a hydrophobic solute.

Molecular dynamics computer simulations of the effects of hydrogen bonding on the properties of layered double hydroxides intercalated with organic acids

Anion exchange capabilities of layered double hydroxides (LDHs) make them uniquely suitable for the creation of bio-inorganic nanocomposites, with amino acids and DNA fragments occurring as negatively charged species at most pH values. To better understand molecular-level structural, thermodynamic and kinetic aspects of their interactions with LDHs, we have performed molecular dynamics (MD) computer simulations of glutamate(1−) and glutamate(2−) intercalated in Mg–Al LDH. The results are compared with previous simulations of the hydration and swelling behaviour of similar LDHs intercalated with other organic anions (formate, acetate, propanoate and citrate). The MD simulations provide important insight into the interpretation of NMR and X-ray diffraction data for the same systems. The organic species interact with the LDH layers principally via electrostatic and van der Waals forces, and the hydrated interlayer galleries are stabilised via the development of an integrated hydrogen-bonding network among the anions, water molecules and OH-groups of the LDH layers. Deprotonated carboxylate groups are the primary strong H-bond acceptors, whereas, for glutamate(2−), the amino groups are additional H-bond acceptors from the LDH surface. On the other hand, the protonated amine groups of glutamate(1−) serve as additional strong H-bond donors in the interlayer, responsible for up to 1/6 of all H-bonds formed by each carboxylic group in the interlayer. The organic species preferably accept H-bonds from H2O molecules rather than from surface OH-groups due to structural restrictions on the development of tetrahedrally coordinated H-bonding environments for the carboxylate groups at the surface.

Dissociation of carbonic acid: Gas phase energetics and mechanism from ab initio metadynamics simulations

A comprehensive metadynamics study of the energetics, stability, conformational changes, and mechanism of dissociation of gas phase carbonic acid, H2CO3, yields significant new insight into these reactions. The equilibrium geometries, vibrational frequencies, and conformer energies calculated using the density functional theory are in good agreement with the previous theoretical predictions. At 315 K, the cis-cis conformer has a very short life time and transforms easily to the cis-trans conformer through a change in the O=C-O-H dihedral angle. The energy difference between the trans-trans and cis-trans conformers is very small (approximately 1 kcal/mol), but the trans-trans conformer is resistant to dissociation to carbon dioxide and water. The cis-trans conformer has a relatively short path for one of its hydroxyl groups to accept the proton from the other end of the molecule, resulting in a lower activation barrier for dissociation. Comparison of the free and potential energies of dissociation shows that the entropic contribution to the dissociation energy is less than 10%. The potential energy barrier for dissociation of H2CO3 to CO2 and H2O from the metadynamics calculations is 5-6 kcal/mol lower than in previous 0 K studies, possibly due to a combination of a finite temperature and more efficient sampling of the energy landscape in the metadynamics calculations. Gas phase carbonic acid dissociation is triggered by the dehydroxylation of one of the hydroxyl groups, which reorients as it approaches the proton on the other end of the molecule, thus facilitating a favorable H-O-H angle for the formation of a product H2O molecule. The major atomic reorganization of the other part of the molecule is a gradual straightening of the O=C=O bond. The metadynamics results provide a basis for future simulation of the more challenging carbonic acid-water system.

Framework flexibility of sodium zirconium phosphate: role of disorder, and polyhedral distortions from Monte Carlo investigation

A detailed Monte Carlo investigation of the structural changes of the framework of sodium zirconium phosphate, [Zr2P3O12]−,—NASICON (acronym for Na-SuperIonic CONductor)—accommodating alkali ions of varying sizes (Li+, Na+, K+, Rb+ and Cs+) is carried out over a range of temperatures. Simulation results are critically compared with the structural models proposed earlier and available experimental results. Anisotropic changes of the rhombohedral cell parameters—a contracts while c expands with the size of the alkali ion substituted—is observed in good agreement with previous experimental results. The mechanism of anisotropic variation of lattice parameters involves dominantly, coupled rotations of the polyhedra as proposed by Alamo and co-workers. It is, however, observed that the distortions of the PO4 tetrahedra and ZrO6 octahedra are significant, and accounts for nearly one-third of the total change in a and c—parameters as the size of the alkali ion increases. This suggests that ‘rigid’ polyhedral models, permitting only angular distortions of the polyhedra, are of limited quantitative applicability in these solids. The same mechanism is found to be responsible for the low/anisotropic thermal expansion of these solids. Evidence that the polyhedral rotations are dynamic, opposed to a static-frozen-in disorder, is provided.

Molecular dynamics investigation of Na+ in Na2Ni2TeO6

An inter-atomic potential for Na2Ni2TeO6 in the Parrinello- Rahman-Vashishta (PRV) model is parameterized empirically. The potential reproduces variety of structural and transport properties of that material in good agreement with recent experimental results. The study provides fresh insights on the migration channels and mechanism of Na+ in the system.An inter-atomic potential for Na2Ni2TeO6 in the Parrinello- Rahman-Vashishta (PRV) model is parameterized empirically. The potential reproduces variety of structural and transport properties of that material in good agreement with recent experimental results. The study provides fresh insights on the migration channels and mechanism of Na+ in the system.

First-Principle Molecular Dynamics Investigation of Waterborne As-V Species

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.

Ab initio molecular dynamics study of Se(IV) species in aqueous environment

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 solutes 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.

Monte Carlo Investigation of Structural Changes in NZP Skeleton on Alkali Ion Substitution

In order to understand the mechanism resulting in the high flexibility of the NZP skeleton in accommodating cations of varying sizes, we have carried out a series of NPT‐Monte Carlo simulations on MZr2P3O12, where M = Li+, Na+, K+, Rb+ and Cs+. Anisotropic changes in the lattice parameters–a contracts and c expands–with the size of the cation substituted, in agreement with previous experimental reports. It is observed that the building polyhedral blocks, ZrO6 octahedra and PO4 tetrahedra, of the framework do not suffer significant modifications, suggesting variation in lattice parameter are brought about largely by the way the polyhedra are interlinked.

Computer simulation studies of Ar clusters

Results of the solid-liquid transition of Ar13 cluster in a spherically symmetric external potential have been presented. The transition temperature is observed to show an elevation with pressure. The broadening of the heat capacity peaks indicate the transition becoming more diffused with pressure. The icosahedral structure of the cluster remains unaltered under pressure. Ar55 cluster has also been studied by similar approach. A possible connection between glass transition phenomenon and melting of clusters under pressure has been examined.