Maxwell-Stefan Diffusion And Dynamical Correlation In Molten LiF-KF: A Molecular Dynamics Study

In this work our main objective is to compute Dynamical correlations, Onsager coefficients and Maxwell-Stefan (MS) diffusivities for molten salt LiF-KF mixture at various thermodynamic states through Green-Kubo formalism for the first time. The equilibrium molecular dynamics (MD) simulations were performed using BHM potential for LiF-KF mixture. The velocity autocorrelations functions involving Li ions reflect the endurance of cage dynamics or backscattering with temperature. The magnitude of Onsager coefficients for all pairs increases with increase in temperature. Interestingly most of the Onsager coefficients has almost maximum magnitude at the eutectic composition indicating the most dynamic character of the eutectic mixture. MS diffusivity hence diffusion for all ion pairs increases in the system with increasing temperature. Smooth variation of the diffusivity values denies any network formation in the mixture. Also, the striking feature is the noticeable concentration dependence of MS diffusivity between cation-cation pair, DLi-K which remains negative for most of the concentration range but changes sign to become positive for higher LiF concentration. The negative MS diffusivity is acceptable as it satisfies the non-negative entropy constraint governed by 2nd law of thermodynamics. This high diffusivity also vouches the candidature of molten salt as a coolant

First principles DFT investigation Of Yttrium-decorated Boron-Nitride Nanotube: Electronic Structure and Hydrogen Storage

The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0 mu(B). Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium

Unraveling the Nature of Pressure-Induced Phases of MAPbBr3 by ab initio Molecular Dynamics

Pressure-induced phases of hybrid perovskite MAPbBr3 are investigated at room temperature in a pressure range 0-2.8 GPa by ab initio molecular dynamics. We find two structural transitions at 0.7 and 1.1 GPa involving confinement of MA orientational fluctuations to a crystal plane - one (cubic to cubic) involving dynamic disordering over the plane and another (cubic to tetragonal) corresponding to a static disordering of MA dipoles along two crystal axes on the same plane. This is similar to isotropic to isotropic and isotropic to oblate transition from the perspective of nematic transitions of liquid crystal. In the latter phase, both local anti-polar and polar domains, consisting of at least two units, are formed. The two transitions are primarily driven by octahedral tilting modes of the host lattice involving a displacive character in the first and an ordering of layer-wise tilts in the second transition. Coupling between the MA (guest) orientations/translations and octahedral tilting/lattice scissoring in the inorganic host are also altered along the transitions. H-bonding interactions, which primarily mediate host/guest coupling, facilitate the static disordering of MA dipoles along two crystal axes. Unlike temperature-driven transitions in the system, high pressures suppress CH3 torsional motion emphasizing the role of C-H· · · Br bonds in driving the transitions

First-principles determination of the relative stability of the α and Cmcm structures of AlPO<SUB>4</SUB>

Berlinite, &#945;-AlPO<SUB>4</SUB>, belongs to a class of isostructural compounds MXO<SUB>4</SUB> (where MX refers to IV-IV or III-V elements), whose high-pressure behavior is of considerable interest due to their geophysical importance. We present here an ab initio study, based on density-functional theory, of the equation of state of AlPO<SUB>4</SUB> in two phases, &#945; and Cmcm, and their relative stabilities as a function of pressure. Our total-energy calculations show that the &#945; to Cmcm phase transformation is energetically favorable beyond &#8776;9.5GPa, consistent with recent synchrotron X-ray-diffraction studies which showed that the &#945;-AlPO<SUB>4</SUB> transforms to another crystalline phase (identified as the Cmcm phase) at a pressure of &#8776;13GPa. For both &#945; and Cmcm phases the fractional atomic coordinates, optimized through minimization of forces, are predicted and, whenever a comparison is possible, are found to be in good agreement with available experimental results

Trapping of Li⁺ Ions by [ThF<i>_n</i>]^4–<i>n</i> Clusters Leading to Oscillating Maxwell–Stefan Diffusivity in the Molten Salt LiF–ThF₄

A molten salt mixture of lithium fluoride and thorium fluoride (LiF–ThF₄) serves as a fuel as well as a coolant in the most sophisticated molten salt reactor (MSR). Here, we report for the first time dynamic correlations, Onsager coefficients, Maxwell–Stefan (MS) diffusivities, and the concentration dependence of density and enthalpy of the molten salt mixture LiF–ThF₄ at 1200 K in the composition range of 2–45% ThF₄ and also at eutectic composition in the temperature range of 1123–1600 K using Green–Kubo formalism and equilibrium molecular dynamics simulations. We have observed an interesting oscillating pattern for the MS diffusivity for the cation–cation pair, in which <i>Đ</i>_Li–Th oscillates between positive and negative values with the amplitude of the oscillation reducing as the system becomes rich in ThF₄. Through the velocity autocorrelation function, vibrational density of states, radial distribution function analysis, and structural snapshots, we establish an interplay between the local structure and multicomponent dynamics and predict that formation of negatively charged [ThF<i>_n</i>]^4–<i>n</i> clusters at a higher ThF₄ mole % makes positively charged Li⁺ ions oscillate between different clusters, with their range of motion reducing as the number of [ThF<i>_n</i>]^4–<i>n</i> clusters increases, and finally Li⁺ ions almost get trapped at a higher ThF₄% when the electrostatic force on Li⁺ exerted by various surrounding clusters gets balanced. Although reports on variations of density and enthalpy with temperature exist in the literature, for the first time we report variations of the density and enthalpy of LiF–ThF₄ with the concentration of ThF₄ (mole %) and fit them with the square root function of ThF₄ concentration, which will be very useful for experimentalists to obtain data over a range of concentrations from fitting the formula for design purposes. The formation of [ThF<i>_n</i>]^4–<i>n</i> clusters and the reduction in the diffusivity of the ions at a higher ThF₄% may limit the percentage of ThF₄ that can be used in the MSR to optimize the neutron economy

Electronic and Magnetic Properties of Yttrium-doped Silicon Carbide Nanotubes: Density Functional Theory Investigations

The electronic structure of yttrium-doped Silicon Carbide Nanotubes has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom is bonded strongly on the surface of the nanotube with a binding energy of 2.37 eV and prefers to stay on the hollow site at a distance of around 2.25 angstrom from the tube. The semi-conducting nanotube with chirality (4, 4) becomes half mettalic with a magnetic moment of 1.0 mu(B) due to influence of Y atom on the surface. There is strong hybridization between d orbital of Y with p orbital of Si and C causing a charge transfer from d orbital of the Y atom to the tube. The Fermi level is shifted towards higher energy with finite Density of States for only upspin channel making the system half metallic and magnetic which may have application in spintronic devices

Investigating the role of compression rates in pressure induced polymerization of crystalline acrylamide using ab initio molecular dynamics

Varying the rate at which pressure is applied on a crystal is experimentally known to yield different pressure induced polymorphic structures. Herein, we explore the effect of pressure increase rate on pressure induced polymerization in crystalline acrylamide, using a density functional theory based approach. While quasi-static compression at 0 K stabilizes a 3-dimensional topochemical polymer, Pol-I, at 23 GPa, rapid compression optimizations suggest the presence of multiple polymeric intermediates in the system. Room temperature ab initio molecular dynamics performed with two different compression rates - 0.4 GPa/ps and 2 GPa/ps - revealed very different structural evolution of the system. While both rates ultimately yielded a metastable 1-dimensional polymer at pressures beyond 64 GPa, rapid compression resulted in many disordered polymers at lower pressures with unanticipated linkages. The mechanisms leading to polymerization as well as the structure and electronic properties of the various polymer polymorphs obtained in the two compression routes are described. While large kinetic barriers delay the formation of the thermodynamically favored polymer Pol- I, our simulations suggest a hierarchical route for the pressure induced polymerization of solid acrylamide towards the thermodynamically favorable Pol-I

Pressure Induced Topochemical Polymerizationof Solid Acryalmide Facilitated by Anisotropic Response of Hydrogen Bond Network

The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) based study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. Our calculations predict a polymerization pressure of 94 GPa, in contrast to experimental estimates of 17 GPa, while being able to reproduce the experimentally measured pressure dependent spectroscopic features. Interestingly, we find that the two-dimensional hydrogen bond network templates a topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both NH... O and C-H...O bonds.</p