Molecular dynamics of liquid alkaline-earth metals near the melting point

Results of the studies of the properties like binding energy, the pair distribution function g(r), the structure factor S(q), specific heat at constant volume, velocity autocorrelation function (VACF), radial distribution function, self-diffusion coefficient and coordination number of alkaline-earth metals (Be, Mg, Ca, Sr and Ba) near melting point using molecular dynamics (MD) simulation technique using a pseudopotential proposed by us are presented in this article. Good agreement with the experiment is achieved for the binding energy, pair distribution function and structure factor, and these results compare favourably with the results obtained by other such calculations, showing the transferability of the pseudopotential used from solid to liquid environment in the case of alkaline-earth metals.     

Molecular dynamics simulations of a fluid near its critical point

We present computer simulations for the static and dynamic behavior of a fluid near its consolute critical point. We study the Widom-Rowlinson mixture, which is a two component fluid where like species do not interact and unlike species interact via a hard core repulsion. At high enough densities this fluid exhibits a second order demixing transition that is in the Ising universality class. We find that the mutual diffusion coefficient DAB vanishes as DAB approximately xi(-1.26 +/- 0.08), where xi is the correlation length. This is different from renormalization-group and mode coupling theory predictions for model H, which are DAB approximately xi(-1.065) and DAB approximately xi(-1), respectively.     

Structure changes in gallium near its melting point

The electrical resistivity as a function of temperature was measured for gallium samples annealed above the melting point and cooled down to −100°C. The observed phase transition temperatures depend on the temperature of the annealing in the liquid state. The complete phase diagram in (T_ann, T_cr) coordinates was constructed. Neutron diffraction measurements are consistent with the phase diagram obtained from the electrical resistivity data.     

Amorphization of ice near the melting point

In addition to reflections of the hexagonal phase of ice I _h, the intense diffuse scattering of X-rays mainly due to the amorphization of ice is revealed on the X-ray diffraction patterns of water ice samples prepared at liquid nitrogen (studied by the authors earlier) and samples prepared at T = ?10°C (this work). The measurements are performed in the temperature range from ?25 to 0°C. The existence of reflections of the crystalline phase and intense diffuse scattering on the X-ray diffraction patterns makes it possible make a conclusion about the coexistence of crystalline and amorphous structures of ice. Splitting of the first maximum on the electron-density radial distribution function is detected on the basis of an X-ray diffraction pattern recorded at T = ?3°C. This splitting is explained by an increase in the interatomic distances between the nearest-neighbor atoms located at different levels. Similar splitting was also detected on a radial distribution function constructed using an X-ray diffraction pattern recorded at ?10°C.     

Molecular dynamics simulation of zirconia melting

The melting point for the tetragonal and cubic phases of zirconia (ZrO₂) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.     

Spin dynamics near the Lifshitz point

Spin dynamics in the vicinity of Lifshitz points associated with Heisenberg and planar ferromagnets and antiferromagnets is discussed. Mode coupling arguments are used to obtain the dynamic exponents and the temperature dependence of the diffusion constants.     

Raman scattering from potassium iodide near the melting point

We have measured the Raman spectrum of potassium iodide in the crystalline phase at high temperatures, and in the melt. As temperature increases the various second order features in the low temperature spectra coalesce into a broad feature, which persists into the melt. A smooth, quasi-exponential wing also appears in the melt. Thus, the spectrum is structurally similar to those found in solid electrolytes and silver halide melts. Possible interpretations of the spectrum are indicated.     

Optical absorption near the melting point of LiNO3 crystal

Temperature dependence of the absorption spectrum in the ultraviolet region below 4.2 eV is measured in the temperature range from room temperature to just below the melting point T_M of LiNO₃ crystal. Anomalous behaviors in the observed spectra are found on approaching to T_M. The low energy tails of the spectra are analyzed by using Urbach rule. The steepness parameter σ(T), derived from the experimental results, also shows anomalous temperature change near T_M.     

Molecular dynamics of NaCl melting under pressure

We have performed one-phase molecular dynamics (MD) simulations to investigate the melting curve of NaCl over a wide range of pressures. To ensure faithful MD simulations, two types of potentials, the shell-model (SM) and the two-body rigid-ion Born-Mayer-Huggins-Fumi-Tosi (BMHFT) potentials, are fully tested. Compared with SM potential, the MD simulation with BMHFT potential is very successful in reproducing accurately the measured volumes of NaCl. The BMHFT potential can also produce a satisfactory melting curve, consistent with both experiments and two-phase simulations. Hence we recommend that the BMHFT should be the reliable potential for simulating high-pressure properties of NaCl.     

The molecular dynamics of a naphthalene crystal near melting

It is suggested that atom-atom potential functions become more appropriate for molecular dynamics calculations for systems as they approach melting. A simulation for naphthalene shows the molecular rate of reorientation about the axis of greatest inertia to be approaching 100 MHz within 20 K from melting. Self-correlations for this motion are highly significant, though neighbour correlations appear to be random in this case. Such behaviour is contrasted with plastic crystal behaviour where neighbour correlations are high, and it is suggested that this characterises the difference between a true crystal and a plastic crystal. For naphthalene this result contrasts with an experimental conclusion where the motion about the axis of least inertia is though to be responsible for the onset of melting.     

Structure of liquid solutions near the critical point

Specific features of the structure of the critical state of binary liquid solutions leading to an anomalous behavior of the Rayleigh line due to a dramatic increase in concentration and density fluctuations are considered. It is shown that an experimental treatment must deal with two fluctuation regions near the critical point of solvent vaporization. In the first region, one can achieve a sufficient degree of accuracy by using theories like self-consistent field theory. In the second region, which is closer to the critical point than the first region, scaling theory of second-order phase transitions may be applied. It is found that the anomalous behavior of the Rayleigh line associated with kinetic coefficients is determined by the equilibrium thermodynamic properties and by the radius of fluctuation correlation (r_c). A general theory is developed for calculating thermodynamic potentials, especially the chemical potential and its concentration derivative in the fluctuation region. The results of these calculations are compared with the experimental data briefly described in the paper. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 655–668, July–August, 1998.     

Molecular dynamics studies of ultrafast laser-induced nonthermal melting

Molecular Dynamics (MD) is employed to investigate nonthermal melting triggered by coherent phonon excitation in bismuth telluride, which has Peierls distortion in the lattice structure. Results showed that the structural distortion caused by coherent phonons appears as early as 80 fs, while it takes several picoseconds for the whole phonon-excited area to evolve into a liquid state. It was also found that the temperature in the phonon-excited area rises quickly within tens of femtoseconds, while the rest of the lattice remains at the initial temperature even after several picoseconds, which is separated from the high temperature region across a thin transition area. This phenomenon is analogous to the heat transfer across a solid–liquid interface, even though in our case there is no abrupt solid-liquid interface between the cold lattice and the quasiliquid.     

纳米团簇熔化过程的分子动力学模拟

本文采用分子动力学结合嵌入原子多体势,模拟了不同半径的Ni纳米团簇的升温熔化过程,研究团簇尺寸对熔点和表面能的影响.模拟结果表明:团簇的熔点显著低于体材料的熔点.团簇熔化的过程首先是在团簇的表面出现预熔,然后向团簇内部扩展,直到整个团簇完全熔为液态.在模拟的纳米尺度范围内,团簇的熔点与团簇尺寸基本成线性关系.团簇的表面能随着团簇尺寸的增大而减小,而且表面能均高于体材料的表面能.     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.