On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth's liquid outer core consists of mainly liquid iron alloyed with about ten per cent (by weight) of light elements. Although the concentrations of the light elements are small, they nevertheless affect the Earth's core: its rate of cooling, the growth of the inner core, the dynamics of core convection, and the evolution of the geodynamo. Several light elements-including sulphur, oxygen, silicon, carbon and hydrogen-have been suggested, but the precise identity of the light elements in the Earth's core is still unclear. Oxygen has been proposed as a major light element in the core on the basis of cosmochemical arguments and chemical reactions during accretion. Its presence in the core has direct implications for Earth accretion conditions of oxidation state, pressure and temperature. Here we report new shockwave data in the Fe-S-O system that are directly applicable to the outer core. The data include both density and sound velocity measurements, which we compare with the observed density and velocity profiles of the liquid outer core. The results show that we can rule out oxygen as a major light element in the liquid outer core because adding oxygen into liquid iron would not reproduce simultaneously the observed density and sound velocity profiles of the outer core. An oxygen-depleted core would imply a more reduced environment during early Earth accretion.

Thermal equation of state and thermodynamic Grüneisen parameter of beryllium metalWe investigate the phase transition, elastic constants, phonon dispersion curves, and thermal properties of beryllium (Be) at high pressures and high temperatures using density functional theory. By comparing the Gibbs free energy, in the quasiharmonic approximation (QHA), of hexagonal-closed-packed (hcp) with those of the face-centered cubic (fcc) and body-centered-cubic (bcc) we find that the hcp Be is stable up to 390 GPa, and then transforms to the bcc Be. The calculated phonon dispersion curves are in excellent agreement with experiments. Under compression, the phonon dispersion curves of hcp Be do not show any anomaly or instability. At low pressure the phonon dispersion of bcc Be display imaginary along C-N in the T 1 branches. Within the quasiharmonic approximation, we predict the thermal equation of state and other properties including the thermal expansion coefficient, Hugoniot curves, heat capacity, Grüneisen parameter, and Debye temperature.

We have performed coexistence phase molecular dynamics ͑MD͒ simulations to investigate the melting curve of tantalum over a wide range of pressures. To ensure faithful MD simulations, three types of potentials, including the extended Finnis-Sinclair ͑EFS͒ potential, the long-range empirical potential ͑LREP͒, and the force-matching ͑FM͒ potential, are fully tested. Through a series of tests, such as equation of states, thermal expansion, and other thermodynamic properties for liquid Ta, we have found that the EFS potential is the reliable potential for simulating both solid and liquid Ta. The EFS potential can also produce a satisfying melting curve, consistent well with both experiments of ambient pressure and shock melting at high pressure. However, the other two melting curves from the LREP and the FM potential have not so satisfying agreement with shocking melting at high pressure. Hence we recommend that the EFS should be the reliable potential for simulating melting properties of Ta as well as other properties of solid and liquid Ta.

Resonant frequency analysis of Timoshenko nanowires with surface stress for different boundary conditions J. Appl. Phys. 112, 074322 (2012) Giant domain wall response of highly twinned ferroelastic materials Appl. Phys. Lett. 101, 141913 (2012) Electromechanical instabilities of thermoplastics: Theory and in situ observation Appl. Phys. Lett. 101, 141911 (2012) Mechanical properties and local mobility of atactic-polystyrene films under constant-shear deformation J. Chem. Phys. 137, 124902 (2012) Effect of macroscopic relaxation on residual stress analysis by diffraction methodsWe report a detailed first-principles calculation to investigate the structures, elastic constants, and phase transition of Ti. The axial ratios of both ␣-Ti and -Ti are nearly constant under hydrostatic compression, which confirms the latest experimental results. From the high pressure elastic constants, we find that the ␣-Ti is unstable when the applied pressures are larger than 24.2 GPa, but the -Ti is mechanically stable at all range of calculated pressure. The calculated phonon dispersion curves agree well with experiments. Under compression, we captured a large softening around ⌫ point of ␣-Ti. When the pressure is raised to 35.9 GPa, the frequencies around the ⌫ point along ⌫-M-K and ⌫-A in transverse acoustical branches become imaginary, indicating a structural instability. Within quasiharmonic approximation, we obtained the full phase diagram and accurate thermal equations of state of Ti. The phase transition -Ti→ ␣-Ti→ ␤-Ti at zero pressure occurs at 146 K and 1143 K, respectively. The predicted triple point is at 9.78 GPa, 931 K, which is close to the experimental data. Our thermal equations of state confirm the available experimental results and are extended to a wider pressure and temperature range.

We report a detailed ab initio study for body-centered-cubic (bcc) Ta within the framework of the quasiharmonic approximation (QHA) to refine its thermal equation of state and thermodynamic properties. Based on the excellent agreement of our calculated phonon dispersion curve with experiment, the accurate thermal equations of state and thermodynamic properties are well reproduced. The thermal equation of state (EOS) and EOS parameters are considerably improved in our work compared with previous results by others. Furthermore, at high temperatures, the excellent agreement of our obtained thermal expansion and Hugoniot curves with experiments greatly verifies the validity of the quasiharmonic approximation at higher temperatures. It is known that pressure suppresses the vibrations of atoms from their equilibrium positions, i.e. the bondings among atoms are strengthened by pressure; for the same temperature, anharmonicity becomes less important at high pressure. Thus the highest valid temperature of the QHA can be reasonably extended to the larger range.