Ab initio nonequilibrium molecular dynamics in the solid superionic conductor LiBH4

The color-diffusion algorithm is applied to ab initio molecular dynamics simulation of hexagonal LiBH(4) to determine the lithium diffusion coefficient and diffusion mechanisms. Even in the best solid lithium ion conductors, the time scale of ion diffusion is too long to be readily accessible by ab initio molecular dynamics at a reasonable computational cost. In our nonequilibrium method, rare events are accelerated by the application of an artificial external field acting on the mobile species; the system response to this perturbation is accurately described in the framework of linear response theory and is directly related to the diffusion coefficient, thus resulting in a controllable approximation. The calculated lithium ionic conductivity of LiBH(4) closely matches published measurements, and the diffusion mechanism can be elucidated directly from the generated trajectory.     

Structural stability of complex hydrides: LiBH4 revisited

A systematic approach to study the phase stability of LiBH4 based on ab initio calculations is presented. Three thermodynamically stable phases are identified and a new phase of Cc symmetry is proposed for the first time for a complex hydride. The x-ray diffraction pattern and vibrational spectra of the Cc structure agree well with recently reported experimental data on LiBH4. Calculations of the free energy at finite temperatures suggest that the experimentally proposed P6(3)mc phase is unstable at T>0 K.     

Destabilization of LiBH4 by mixing with LiNH2

It was revealed that LiBH₄ is destabilized by mixing with LiNH₂ and the mixture desorbs a large amount of hydrogen. First-principles calculations predicted that the enthalpies of dehydrogenation for LiBH₄ alone and the mixture of LiBH₄+2LiNH₂ are 75 kJ/molH₂ and 23 kJ/molH₂, respectively. Motivated by this prediction, we experimentally examined the dehydrogenation properties for LiBH₄ and the mixture under hydrogen pressure. The amounts of desorbed hydrogen from LiBH₄ and the mixture at 703 K and 522 K were 10.6 mass% and 7.8 mass%, respectively. The dehydrogenation pressure of the mixture was much higher than that of LiBH₄ alone, although the mixture was measured at approximately 180 K lower temperature. This result suggests that the mixture is much unstable as compared with LiBH₄ alone.PACS 81.05.Zx; 71.20.Ps; 82.30.-b     

High pressure phase transition in super-cell LiBH4: An ab initio prediction

LiBH₄ which have attracted considerable attention from researchers due to the crystal structure characteristics is a metal hydride that can bind four hydrogen atoms. LiBH₄ which has high gravimetric and volumetric hydrogen density shows phase transitions at high pressures. In this regard, we created and analyzed LiBH₄ structure based on the first principles calculations, and then obtained the super-cell LiBH₄ structure. We achieved the phase transitions up to 20 GPa pressure with 2 GPa regular intervals for the super-cell LiBH₄. We observed the Pnma to Pnma*, Pnma* to P2₁/c, and P2₁/c to C2/c phase transitions and calculated the volume contractions accompanying these phase transitions. According to the obtained volumetric values, one can conclude that LiBH₄ can minimize the volumetric requirements of the hydrogen storage for systems that can be used at high pressures. Thus, the hydrogen storage capacity of LiBH₄ may increase at particular phases.     

Ionic conductivity in three crystalline phases of LiBH4 under pressure

The AC electrical conductivity of LiBH₄ was investigated below 2 GPa between 1 Hz and 1.6 MHz. The high-temperature phase has an ionic conductivity of up to 0.01 S cm^?1, while the low-temperature phases have conductivities two orders of magnitude lower. All phases show an Arrhenius behaviour with activation energies E _a between 0.5 and 0.7 eV, in good agreement with earlier data except for phase III, which is found to have the highest activation energy of the phases studied. The high-temperature phase has a minimum in E _a near 1 GPa, close to the triple point, correlated with a sudden change in activation volume. These features may indicate an isostructural phase transition. The conductivities of the ambient temperature phases increase temporarily by an order of magnitude after transitions between these phases, probably due to new diffusion channels via structural defects. The phase diagram agrees well with earlier results.     

Stabilization of LiBH4 superionic phase by halide doping and H disordering

We performed first-principles density-functional theory calculations to investigate the structural properties and the effect of halide ion doping to the superionic-conducting, high-temperature phase and the effect of halide doping on the phase of LiBH₄. It is computationally demonstrated that the superionic phase is stabilized owing to the halide doping with the large ions which fill the interlayer space of the superionic phase. The H-disordered phase is observed in the structure and is found to contribute to the stabilization of the superionic phase.     

The formation of positronium in molecular substances

The theories regarding the formation of positronium in molecular substances are discussed. A modified theory based upon the “spur” theory is described. Theoretical derivation and experimental results suggest that the fraction of the long lifetime component due too-positronium annihilation has a complex origin. Initially, the electron of the positron-electron pair is likely to be separated from the positron by medium molecules even when the total kinetic energy of the pair is less than the potential energy between them. The formation of positronium depends on the mobilities of the positron and electron, the slowing down process and the potential energy between them. Positronium is formed in less than 10 ps. Then the positronium produced may react with the radicals created in the “spur” due to the positron. Only the fraction ofo-positronium able to escape the “spur” will have a long lifetime. Therefore, positronium formation is highly related to the fast reactions in the “spur” during the first 100 ps.     

Polarization‐dependent Raman spectroscopy of LiBH4 single crystals and Mg(BH4)2 powders

The phonons and the crystal structure of the complex hydride LiBH₄ are studied on single crystals using micro‐Raman spectroscopy. The symmetry of the modes is determined by polarization‐dependent measurements at liquid helium temperature, allowing a better comparison and a more reliable assignment to the computed phonon wavenumbers. This has led to the revision of some former assignments made from Raman measurements on polycrystalline samples. In addition, a higher integration time allowed the detection of very weak lines, so that 35 out of 36 predicted Raman lines have been identified. We have also performed explorative Raman measurements on Mg(BH₄)₂ powders. In contrast to LiBH₄, the very poor crystallinity of this material inhibits the exploitation of the full potential of Raman spectroscopy. Only broad lines are observed, which we compare to phonon wavenumbers calculated for various possible structures using density functional theory. Copyright © 2011 John Wiley & Sons, Ltd.     

Predicting experimental yields as an index to rank synthesis routes II: application to the Curtius rearrangement

Synthesis yields of organic reactions are one of the most important factors in ranking synthesis routes created by synthesis route design systems such as Transform‐Oriented Synthesis Planning and Knowledge base‐Oriented Synthesis Planning. If it is possible to predict the yields of synthesis reactions before starting experiments, one can easily determine an order of synthesis routes for experimental works. In the present study, the reaction profiles of the Curtius rearrangement with different substituents were calculated to generate an equation predicting experimental yields of this reaction. Reactions followed by the formation of isocyanates were also analyzed to consider the relationship between reaction times and experimental yields. A partial least squares (PLS) regression was used to correlate the experimental yields with the calculated activation energies, E_a(calc), together with experimental conditions such as dielectric constants of solvents, reaction times, and reaction temperatures as explanatory variables. Although the PLS regression using all the data gave very poor results, we succeeded in making a model equation with R² = 0.887 using a modified data set. However, there is a conflict between the predictability and the interpretability on the reaction time. This discrepancy mainly comes from unnecessarily long reaction times in the experiments for azides with calculated E_a values of less than 33 kcal mol^–1. To construct a good model equation for the experimental yields of the Curtius reaction, we have to use data sets obtained from within 90 min of the reaction for the PLS regression. Copyright © 2011 John Wiley & Sons, Ltd.     

Crystal structure analysis of novel complex hydrides formed by the combination of LiBH4 and LiNH2

Combination of LiBH₄ and LiNH₂ by ball milling forms the series of novel complex hydrides Li₂BNH₆, Li₃BN₂H₈ and Li₄BN₃H₁₀, depending on the combination ratios. The crystal structure of Li₄BN₃H₁₀ analyzed by synchrotron X-raydiffraction measurements is determined to be a cubic system (space group: I2₁3) with the lattice constant of a=10.673(2)Å. It should be emphasized that Li₄BN₃H₁₀ is an ionic crystal which is composed of a lithium cation Li⁺ and two different kinds of the complex anion [BH₄]^- and [NH₂]^-. These anions are located in the vertex and face-center of the cubic sub-lattice, and the lithium cation Li⁺ in the interstitial site between the anions, respectively. The other series of complex hydrides, Li₂BNH₆ and Li₃BN₂H₈, are also predicted to possess similar structures composed of a lithium cation Li⁺ and two different kinds of the complex anion [BH₄]^- and [NH₂]^-.     

Efficient measurement of linear susceptibilities in molecular simulations: application to aging supercooled liquids

We propose a novel method to measure time-dependent linear susceptibilities in molecular simulations, which does not require the use of nonequilibrium simulations, subtraction techniques, or fluctuation-dissipation theorems. The main idea is an exact reformulation of linearly perturbed quantities in terms of observables accessible in a single unperturbed trajectory. We apply these ideas to supercooled liquids in a nonequilibrium aging regime. We show that previous work had underestimated deviations from fluctuation-dissipation relations in the case of a Lennard-Jones system, while our results for silica are in qualitative disagreement with earlier results.     

Predicting secondary ion formation in molecular dynamics simulations of sputtering

We present a model to incorporate the excitation and ionization of sputtered particles into molecular dynamics simulations of ion bombardment-induced collision cascades in metals. The kinetic excitation of the solid is described by electronic friction experienced by all moving particles and electron promotion in close atomic collisions. Transport of the resulting excitation energy is treated by a nonlinear diffusion equation. The resulting space- and time-dependent electron temperature is then introduced into a rate equation model describing the ionization of ejected particles. This way, secondary ion formation is described by assigning an individual ionization probability to every sputtered atom. Averaging over the entire flux, this allows the prediction of measurable quantities like integral or spectral ionization probabilities as well as velocity spectra of secondary ions.     

Adsorption and molecular dynamics of low-molecular substances on modified montmorillonite nanoparticles

The effect of the structure of stable nitroxyl radicals on their adsorption from organic solvents on montmorillonite modified with cationic surfactants is examined. The influence of the nature and content of the modifier and solvent, as well as additional modification with cationic dyes, on the adsorption is considered. The kinetic parameters of the adsorption of decane, toluene, and ethanol from the gas phase are determined. The spin probe method is used to determine the phase state and distribution of adsorbed additives, as well as the influence of an additional adsorption of hydrocarbons on the molecular dynamics at the surface of nanoparticles of montmorillonite and nanopigments based on cationic dyes.     

An application of cell theory to molecular models of n-alkane solids

Solid phase properties for hard sphere chain molecular models of n-alkanes are calculated using the cell theory, and a numerical method for implementation of cell theory for chain molecules is described. Good agreement with Monte Carlo simulations for solid phase properties is obtained from the theory. By using cell theory for the solid phase and an equation of state for the fluid phase, solid-phase equilibrium can be calculated. The predictions are in quite good agreement with Monte Carlo simulation results. Cell theory is used to assess the impact of an approximate treatment used in earlier work for the effect of the temperature dependence of the molecular flexibility upon the solid phase properties of a hard chain model with a realistic torsional potential.