Ab initio determination of melting and sound velocity of neon up to the deep interior of the Earth.

The thermophysical properties and elemental abundances of the noble gases in terrestrial materials can provide unique insights into the Earth's evolution and mantle dynamics. Here, we perform extensive ab initio molecular dynamics simulations to determine the melting temperature and sound velocity of neon up to 370 GPa and 7500 K to constrain its physical state and storage capacity, together with to reveal its implications for the deep interior of the Earth. It is found that solid neon can exist stably under the lower mantle and inner core conditions, and the abnormal melting of neon is not observed under the entire temperature (T) and pressure (P) region inside the Earth owing to its peculiar electronic structure, which is substantially distinct from other heavier noble gases. An inspection of the reduction for sound velocity along the Earth's geotherm evidences that neon can be used as a light element to account for the low-velocity anomaly and density deficit in the deep Earth. A comparison of the pair distribution functions and mean square displacements of MgSiO3-Ne and Fe-Ne alloys further reveals that MgSiO3 has a larger neon storage capacity than the liquid iron under the deep Earth condition, indicating that the lower mantle may be a natural deep noble gas storage reservoir. Our results provide valuable information for studying the fundamental behavior and phase transition of neon in a higher T-P regime, and further enhance our understanding for the interior structure and evolution processes inside the Earth.

Multishock to Quasi-Isentropic Compression of Dense Gaseous Deuterium-Helium Mixtures up to 120 GPa: Probing the Sound Velocities Relevant to Planetary Interiors.

Shock reverberation compression experiments on dense gaseous deuterium-helium mixtures are carried out to provide thermodynamic parameters relevant to the conditions in planetary interiors. The multishock pressures are determined up to 120 GPa and reshock temperatures to 7400 K. Furthermore, the unique compression path from shock-adiabatic to quasi-isentropic compressions enables a direct estimation of the high-pressure sound velocities in the unexplored range of 50-120 GPa. The equation of state and sound velocity provide particular dual perspectives to validate the theoretical models. Our experimental data are found to agree with several equation of state models widely used in astrophysics within the probed pressure range. The current data improve the experimental constraints on sound velocities in the Jovian insulating-to-metallic transition layer.

Transport properties of a quasisymmetric binary nitrogen-oxygen mixture in the warm dense regime.

Transport properties of mixtures in the warm dense matter (WDM) regime play an important role in natural astrophysics. However, a physical understanding of ionic transport properties in quasisymmetric liquid mixtures has remained elusive. Here, we present extensive ab initio molecular dynamics (AIMD) simulations on the ionic diffusion and viscosity of a quasisymmetric binary nitrogen-oxygen (N-O) mixture in a wide warm dense regime of 8-120 kK and 4.5-8.0 g/cm^{3}. Diffusion and viscosity of N-O mixtures with different compositions are obtained by using the Green-Kubo formula. Unlike asymmetric mixtures, the change of proportions in N-O mixtures slightly affects the viscosity and diffusion in the strong-coupling region. Furthermore, the AIMD results are used to build and verify a global pseudo-ion in jellium (PIJ) model for ionic transport calculations. The PIJ model succeeds in reproducing the transport properties of N-O mixtures where ionization has occurred, and provides a promising alternative approach to obtaining comparable results to AIMD simulations with relatively small computational costs. Our current results highlight the characteristic features of the quasisymmetric binary mixtures and demonstrate the applicability of the PIJ model in the WDM regime.

New possible candidate structure for phase IV of solid hydrogen.

It has been proved in experiments that there are at least five phases of solid hydrogen at high pressure, however, only the structure of phase I has been absolutely determined. We revisited the phase space of solid hydrogen in the pressure range of 200-500 GPa using the particle swarm optimization technique combined with first-principles simulations. A novel orthorhombic structure named Ama2 is proposed as a possible candidate structure for phase IV. The Ama2 structure is a 'mixed structure' with two different types of layers and is distinctly different from the previously reported Pc structure. Enthalpies and Gibbs free energies show that Ama2 and Pc are competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies of Ama2 calculated by using density functional perturbation theory based on first-principles lattice dynamics show a better agreement with the experimental measurements than those of the Pc structure. And the pressure dependence of these low-frequency Raman vibrons of Ama2 obtained from the first-principles molecular dynamics simulation shows a steeper slope, which resolves the long-standing issue of large discrepancies between the calculated Raman frequencies and the experimental ν 1 [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101 and C. S. Zha, R. E. Cohen, H. K. Mao and R. J. Hemley, Proc. Natl. Acad. Sci. U.S.A., 2014, 111, 4792]. Structural and vibrational analyses show that the hydrogen molecules in the weakly bonded molecular layer of Ama2 form distorted hexagonal patterns, and their vibration can be used to explain the experimental ν 1 vibron. It is found that the weakly bonded layer is almost the same as the layers in the C2/c structure. This confirms the experimental conclusion [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101] that the ordering of hydrogen molecules in the weakly bonded molecular layers of the 'mixed structure' for phase IV is similar to that in the layers of the C2/c structure.

Benchmarking the effective one-component plasma model for warm dense neon and krypton within quantum molecular dynamics simulation.

The effective one-component plasma (EOCP) model has provided an efficient approach to obtaining many important thermophysical parameters of hot dense matter [J. Clérouin, et al., Phys. Rev. Lett. 116, 115003 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.115003]. In this paper, we perform extensive quantum molecular dynamics (QMD) simulations to determine the equations of state, ionic structures, and ionic transport properties of neon and krypton within the warm dense matter (WDM) regime where the density (ρ) is up to 12 g/cm^{3} and the temperature (T) is up to 100 kK. The simulated data are then used as a benchmark to explicitly evaluate the EOCP and Yukawa models. It is found that, within present ρ-T regime, the EOCP model can excellently reproduce the diffusion and viscosity coefficients of neon and krypton due to the fact that this model defines a system which nearly reproduces the actual physical states of WDM. Therefore, the EOCP model may be a promising alternative approach to reasonably predicting the transport behaviors of matter in WDM regime at lower QMD computational cost. The evaluation of Yukawa model shows that the consideration of the energy level broadening effect in the average atom model is necessary. Finally, with the help of EOCP model, the Stokes-Einstein relationships about neon and krypton are discussed, and fruitful plasma parameters as well as a practical ρ-T-dependent formula of the effective coupling parameter are obtained. These results not only provide valuable information for future theoretical and experimental studies on dense neon and krypton but also reveal the applicability of the EOCP model and the limitation of the Yukawa model in WDM regime and further support the continuing search for a unified description of ionic transport in dense plasma.

Equation of state, ionic structure, and phase diagram of warm dense krypton.

Extensive quantum molecular dynamics (QMD) simulations are performed to determine the equation of state, sound velocity, and phase diagram of middle-Z krypton in a warm dense regime where the pressure (P) is up to 300 GPa and the temperature is up to 60 kK. The shock wave experimental data are used to validate the present theoretical models. It is found that, within the regime of the current density (ρ) and temperature (T), sound velocity can effectively discriminate differences between different theoretical models, and therefore it is more suitable as a benchmark to verify the practicability of models. The QMD-simulated results of the ionic structures and electronic properties imply the occurrence of two kinds of phase transitions, including transition from a solidlike to fluid state and that from an insulator to conductive fluid in this T-P regime. The calculated electrical conductivities confirm that the metallization transition occurs at about 60 GPa and 17.5 kK along the principal Hugoniot. With the help of simulation results and experimental data, a comprehensive phase diagram for krypton is constructed by using the solid-fluid and insulator-metal fluid phase boundaries, which fills the gap of the experimental work [Proc. Natl. Acad. Sci. USA 112, 7925 (2015)PNASA60027-842410.1073/pnas.1421801112]. These results will provide an instructive basis for the experimental investigations of rare gases over a wide T-P range.