Isao Okada

MD study of the mixed alkali effect in a lithium&.zsbnd;potassium metasilicate glass

A molecular dynamics simulation of (Li,K)SiO3 has been carried out to determine the mixed alkali effect. A decrease in dynamic property by mixing is reproduced by the simulation. The distinct part of the van Hove function reveals that the jump of alkali ions into a site previously occupied by a different type of alkali ion has a low probability in a mixed alkali glass. The result means that an interception of the mutual diffusion pathways occurs in the glassy state.

DENSITY DEPENDENCE OF STRUCTURE OF SUPERCRITICAL CARBON DIOXIDE ALONG AN ISOTHERM

Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 A−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short‐ranged structure and long‐ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction‐predominant gaslike structure to the repulsion‐predominant liquidlike one with increasing fluid density. With respect to the long‐ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also be...

Molecular Dynamics Simulation of Alkali Silicates Based on the Quantum Mechanical Potential Surfaces

Abstract Two potential parameter sets for alkali silicates were derived on the basis of ab-initio MO calculations. One is a model containing completely ionic alkali (model I), and another is that derived from cluster calculations (model II). These sets were tested against the crystal, glass, and liquid of metasilicates. The model II can reproduce these structures well under constant pressure conditions, and is found to be better than model I as a whole.

MD study of the mixed alkali effect in terms of the potential surface in the lithium-potassium metasilicate glass

Interception of jump path among the unlike alkali ion sites has been found in a previous molecular dynamics study of lithium-potassium metasilicate ((Li,K)SiO3) glass. The cause for the interception leading to the mixed alkali effect has been studied in terms of the potential surfaces. A site mismatch between the unlike ions with respect to the potential energies has been clearly observed. The activation energies of the jumps have been evaluated. The jump path of each kind of alkali ions has been visualized and the jump motions in such a path have been described from a standpoint of the mixed alkali effect.

X-ray and neutron diffraction and molecular dynamics simulation of molten lithium and rubidium nitrates

Molecular dynamics simulations have been performed for lithium and rubidium nitrate melts at 550 and 600 K, respectively, together with X-ray and neutron diffraction experiments. Simple Coulomb pair potentials with Born-type repulsions have been adopted in the simulations with a rigid body model for the nitrate ion. Structure functions derived from the X-ray and neutron experiments are well reproduced by the simulations, from which the three-dimensional cation distribution around the nitrate ion has been revealed. The self-diffusion coefficients, the velocity autocorrelation functions and the self-exchange velocities of lithium, rubidium and nitrate ions have been calculated. Anisotropic motion of nitrate ions has been found and is discussed on the basis of the structure of the melts.

Molecular dynamics simulation of the dipalmitoylphosphatidylcholine (DPPC) lipid bilayer in the fluid phase using the Nosé-Parrinello-Rahman NPT ensemble

Abstract A dipalmitoylphosphatidylcholine (DPPC) bilayer could successfully be simulated in the Nose-Parrinello-Rahman NPT ensemble from an arbitrarily generated crystal-like initial configuration. The initial condition dependence may be small and various artefacts, which were found in molecular dynamics calculations with rectangular cells, could be avoided.

A neutron scattering study of the structure of supercritical carbon dioxide

Abstract The structure factor S(Q) of supercritical carbon dioxide has been measured over a wide range of the scattering vector Q (0.018–30 A−1) using two neutron apparatuses. The measured S(Q) has been Fourier transformed into the neutron weighted radial distribution function, GN(r). This measurement enables us to analyze the short-range and long-range structures simultaneously in the supercritical fluid.

Raman spectroscopic study on the vibrational and rotational relaxation of OH− ion in molten LiOH

Raman scattering spectra have been measured for molten LiOH at 773 K. The highly corrosive melt was held as a drop under a silver wire ring. While the Rayleigh wing spread over more than 1000 cm−1, only one distinct peak for the OH− stretching mode appeared at 3607 cm−1, the band shape being asymmetric owing to inhomogeneous broadening. The vibrational and rotational autocorrelation functions for the OH− ion, Gv(t) and Gr(t), respectively, have been evaluated from the Fourier transformation of the observed polarized and depolarized band contours. The Gv(t) is virtually of an exponential form and its decay is faster than that of the NO−3 ion in molten LiNO3. The Gr(t) can be obtained with good accuracy, whereas inhomogeneous broadening is obviously observed. The relaxation rate of Gr (t) is extremely high as compared with that for the NO−3 ion in molten LiNO3 . The difference is explained in terms of the difference mainly in the moments of inertia and partly in the temperatures. The Gr (t) has oscillatory ...

DENSITY DEPENDENCE OF ROTATIONAL RELAXATION OF SUPERCRITICAL CF3H

Polarized and depolarized Raman scattering measurement and molecular dynamics (MD) calculations have been performed for supercritical CF3H at various densities along an isotherm higher than Tc by about 6 K in order to investigate the density dependence of rotational relaxation. The rotational autocorrelation functions obtained from both methods, which are in satisfactory agreement with each other, showed liquid‐like diffusional decay for the fluid at densities higher than ρc. The function changed in shape continuously to a nearly free‐rotor‐like one at the lowest density going through the oscillatory ones at intermediate densities. The detailed analysis based upon the MD trajectories has been done in order to clarify the relaxation mechanism at each density. Applicability of the J‐extended diffusion model was also examined. They showed that the density dependence of the rotational relaxation may be explained in terms of the states of molecular aggregation in the fluid.

Ionic mobilities of monovalent cations in molten salt mixtures

Abstract For a better understanding of the mechanism of electrical conductivity of ionic melts, additive binary systems yield more heuristic information than pure melts do. Thus, mobility isotherms of monovalent cations with a common anion are discussed. The difference in external and internal mobilities is explained. The methods for determining transport numbers of individual ionic species in mixtures are briefly mentioned. The profiles of the isotherms of the internal mobilities of monovalent cations in common anion binary systems may be classified into two types. In most of the binary melts a crossing of the isotherms occurs. Interpretation of the ionic mobilities reversal is given based on molecular dynamics simulation as well as experimental data, which is a clue for elucidation of the mechanism of ionic transport in molten salts.