18 research outputs found
Liquid Polymorphism and Double Criticality in a Lattice Gas Model
We analyze the possible phase diagrams of a simple model for an associating
liquid proposed previously. Our two-dimensional lattice model combines
oreintati onal ice-like interactions and \"{}Van der Waals\"{} interactions
which may be repulsive, and in this case represent a penalty for distortion of
hydrogen bonds in the presence of extra molecules. These interactions can be
interpreted in terms of two competing distances, but not necessarily soft-core.
We present mean -field calculations and an exhaustive simulation study for
different parameters which represent relative strength of the bonding
interaction to the energy penalty for its distortion. As this ratio decreases,
a smooth disappearance of the doubl e criticality occurs. Possible connections
to liquid-liquid transitions of molecul ar liquids are suggested
Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys
We report the results of ab initio molecular dynamics simulations of liquid
Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600
K, just above the melting point of GaAs. The liquid is predicted to be metallic
at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity
maximum near x = 0.5, consistent with the Faber-Ziman expression. The
electronic density of states is finite at the Fermi energy for all
concentrations; there is, however, a significant pseudogap especially in the
As-rich samples. The Ga-rich density of states more closely resembles that of a
free-electron metal. The partial structure factors show only a weak indication
of chemical short-range order. There is also some residue of the covalent
bonding found in the solid, which shows up in the bond-angle distribution
functions of the liquid state. Finally, the atomic diffusion coefficients at
1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in
Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in
Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.
Dynamics of liquid acetonitrile at high frequencies
Quasielastic neutron scattering spectra were measured for pure acetonitrile at 25°C. In the framework of a simple model of translation and rotation, it was found that the short-time self-diffusion coeff. of liq. acetonitrile is similar to the long-time one measured by NMR. As far as the rotational motion is concerned, the characteristic time was found to be close to the typical value of the mol. spinning motion. The complete inelastic and quasielastic spectra may be further used to check the results of mol. dynamics simulations of acetonitrile
Dynamics and spatial correlations of tetrapentylammonium ions in acetonitrile
Quasielastic neutron scattering (QENS) spectra were measured for a 0.43 M solution of n‐tetrapentylammonium bromide in deuterated acetonitrile at 25 °C, 5 °C, and −15 °C. Values of the translational diffusion coefficient of the cations were inferred from these data using a simple model of translation and rotation. These values are significantly higher than the ones obtained by nuclear magnetic resonance (NMR) spin–echo measurements. The difference can be explained by the different time scales covered by QENS and NMR. QENS shows essentially the contribution of a second order electrophoretic effect to the diffusion coefficient whereas NMR encompasses both electrophoretic and relaxation effects. Consequently, the combination of both techniques allows the two effects to be separated. The relaxation contribution to the diffusion coefficient was calculated by brownian dynamics simulation and compared to the experimental results. The solvent‐averaged ion pair potentials used for this computation were simultaneously adjusted to the thermodynamic and to the small‐angle neutron scattering data by means of hypernetted chain (HNC) calculations
Lithium bromide in acetonitrile and water: a neutron scattering study
The structure around lithium ions in solutions of lithium bromide in acetonitrile and water has been studied by neutron diffraction. For this purpose the isotopic first-order difference method has been applied to the lithium ion. For a 0.58 M acetonitrile solution it has been found that the bromide anion enters into the first solvation shell around the lithium ion, whereas in the case of a 1.88 M aqueous solution the first hydration shell of the cation is not disturbed by the anion. The solvation number is approximately 3 in the case of acetonitrile and approximately 4.5 in the case of water
Neutron scattering experiments on nonaqueous electrolyte solutions
In this paper we have shown the usefulness of neutron scattering experiments for the study of time-averaged correlations in electrolyte solutions. Special emphasis was laid on nonaqueous solutions which offer a vast field of research for interesting phenomena. It is hoped that such kind of experiments contribute to a better understanding of how charged particles behave in liquid phase
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Spherical momentum distribution of the protons in hexagonal ice from modeling of inelastic neutron scattering data
The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic
Gaussian model, with a statistical accuracy comparable to that of the
model independent scheme used previously, but providing more detailed
information on the three dimensional potential energy surface
experienced by the proton. A recently proposed theoretical concept is
also employed to directly calculate the mean force from the experimental
neutron Compton profile, and to evaluate the accuracy required to
unambiguously resolve and extract the effective proton potential from the experimental dat