Melting of cryocrystals at high pressures. Computer simulation (original) (raw)
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The melting curve of hydrogen was computed for pressures up to 200 GPa, using molecular dynamics. The inter-and intramolecular interactions were described by the reactive force field ͑ReaxFF͒ model. The model describes the pressure-volume equation of state solid hydrogen in good agreement with experiment up to pressures over 150 GPa, however the corresponding equation of state for liquid deviates considerably from density functional theory calculations. Due to this, the computed melting curve, although shares most of the known features, yields considerably lower melting temperatures compared to extrapolations of the available diamond anvil cell data. This failure of the ReaxFF model, which can reproduce many physical and chemical properties ͑including chemical reactions in hydrocarbons͒ of solid hydrogen, hints at an important change in the mechanism of interaction of hydrogen molecules in the liquid state.
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Materials
A second melting temperature occurs at a temperature Tn+ higher than Tm in glass-forming melts after heating them from their glassy state. The melting entropy is reduced or increased depending on the thermal history and on the presence of antibonds or bonds up to Tn+. Recent MD simulations show full melting at Tn+ = 1.119Tm for Zr, 1.126Tm for Ag, 1.219Tm for Fe and 1.354Tm for Cu. The non-classical homogeneous nucleation model applied to liquid elements is based on the increase of the Lindemann coefficient with the heating rate. The glass transition at Tg and the nucleation temperatures TnG of glacial phases are successfully predicted below and above Tm. The glass transition temperature Tg increases with the heating rate up to Tn+. Melting and crystallization of glacial phases occur with entropy and enthalpy reductions. A universal law relating Tn+ and TnG around Tm shows that TnG cannot be higher than 1.293Tm for Tn+= 1.47Tm. The enthalpies and entropies of glacial phases have sin...
Simple Molecular Systems at Very High Pressures: Computer simulation studies
THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY, 1998
We discuss the application of ab initio molecular dynamics simulations results to a variety of simple molecular systems under pressure. In particular we consider the polymerization and subsequent amorphization of C,H2 crystals upon compression up to 50 GPa, the determination of the ground state structure of the broken symmetry phase of H2 in the pressure range 100-150 GPa, the fate of methane and ammonia along the isentrope of the middle ice layers of Neptune. We also discuss preliminary applications to 02 and CO.
The Melting Line of Molecular Hydrogen at High Pressure
Bulletin of the American Physical Society, 2008
The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature versus pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure and temperature by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and find a peak in the melting line at P 64:7 4 GPa and T 1055 20 K.
Ab initio simulation of hydrogen bonding in ices under ultra-high pressure
The Journal of Chemical Physics, 2012
In this article, as continuation of the previous publication (P. Zhang, L. Tian, Z. P. Zhang, G. Shao, and J. C. Li, J. Chem. Phys. 137, 044504 (2012)), we report a series of computational simulation results for ices using ab initio DFT methods. The results not only reproduced the main feature of inelastic neutron scattering spectra for ice Ih, but also other phases of ice such as VII and VIII. Furthermore, pressure dependent simulations for ice I and VIII have led us to obtain the spectra for the symmetrical structure of ice X. The transition from normal ice to the symmetrical form shows an extraordinary behaviour of H-bonding in term of vibrations associated with inter-and intra-molecular bonds, revealing a range of phenomena which was not seen before.