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Register a new userSeven-coordinated Silicon in a SiO2He Compound Formed under the Extreme Conditions of Planetary Interiors
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Changes in atomic coordination numbers at high pressures are fundamental to condensed-matter physics because they initiate the emergence of unexpected structures and phenomena. Silicon is capable of forming eight-, nine-, and ten-coordinated structures under compression,in addition to the usual six-coordinated structures. The missing seven-coordinated silicon remains an open question, but here our theoretical study provides evidence for its existence at high pressures. A combination of a crystal-structure prediction method and first-principles calculations allowed prediction of a stable SiO2He compound containing unique SiO7 polyhedrons, which is a configuration unknown in any proposed silica phase. Consequently, seven-coordinated SiO7 is a possible form of silica at high pressures. Further calculations indicate that the SiO2He phase remains energetically stable with a solid character over a wide range of pressures exceeding 607 GPa and temperatures of 0-9000 K, covering the extreme conditions of the core-mantle boundary in super-Earth exoplanets, or even the Solar Systems ice giant planets. Our results may provide theoretical guidance for the discovery of other silicides at high pressures, promote the exploration of materials at planetary core-mantle boundaries, and enable planetary models to be refined.
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We present a combined theoretical and experimental study of the high-pressure behavior of thallium. X-ray diffraction experiments have been carried out at room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the pressure range of previous experiments. We have confirmed the hcp-fcc transition at 3.5 GPa and determined that the fcc structure remains stable up to the highest pressure attained in the experiments. In addition, HP-HT experiments have been performed up to 8 GPa and 700 K by using a combination of x-ray diffraction and a resistively heated diamond-anvil cell. Information on the phase boundaries is obtained, as well as crystallographic information on the HT bcc phase. The equation of state for different phases is reported. Ab initio calculations have also been carried out considering several potential high-pressure structures. They are consistent with the experimental results and predict that, among the structures considered in the calculations, the fcc structure of thallium is stable up to 4.3 TPa. Calculations also predict the post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa.
Helium is the second most abundant element in the universe, and together with silica, they are major components of giant planets. Exploring the reactivity and state of helium and silica under high pressure is of fundamental importance for developing and understanding of the evolution and internal structure of giant planets. Here, using first-principles calculations and crystal structure predictions, we identify four stable phases of a helium-silica compound with seven/eight-coordinated silicon atoms at pressure range of 600-4000 GPa, corresponding to the interior condition of the outer planets in the solar system. The density of HeSiO2 agrees with current structure models of the planets. This helium-silica compound exhibits a superionic-like helium diffusive state at the high pressure and high temperature conditions along the isentropes of Saturn, a metallic fluid state in Jupiter, and a solid state in the deep interiors of Uranus and Neptune. The reaction of helium and silica may lead to the erosion of the rocky core of giant planets and form a diluted core region. These results highlight the reactivity of helium under high pressure to form new compounds, and also provides evidence to help build more sophisticated interior models of giant planets.
The theoretical understanding of plasmon behavior is crucial for an accurate interpretation of inelastic scattering diagnostics in many experiments. We highlight the utility of linear-response time-dependent density functional theory (LR-TDDFT) as a first-principles framework for consistently modeling plasmon properties. We provide a comprehensive analysis of plasmons in aluminum from ambient to warm dense matter conditions and assess typical properties such as the dynamical structure factor, the plasmon dispersion, and the plasmon lifetime. We compare our results with scattering measurements and with other TDDFT results as well as models such as the random phase approximation, the Mermin approach, and the dielectric function obtained using static local field corrections of the uniform electron gas parametrized from path integral Monte Carlo simulations. We conclude that results for the plasmon dispersion and lifetime are inconsistent between experiment and theories and that the common practice of extracting and studying plasmon dispersion relations is an insufficient procedure to capture the complicated physics contained in the dynamic structure factor in its full breadth.
Liquid metallic hydrogen (LMH) was recently produced under static compression and high temperatures in bench-top experiments. Here, we report a study of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the Drude free-electron model to determine its optical conductivity. We find static electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A substantial dissociation fraction is required to best fit the energy dependence of the observed reflectance. LMH at our experimental conditions is largely atomic and degenerate, not primarily molecular. We determine a plasma frequency and the optical conductivity. Properties are used to analyze planetary structure of hydrogen rich planets such as Jupiter.
We use density-functional theory to study the structure of AlSb(001) and GaSb(001) surfaces. Based on a variety of reconstruction models, we construct surface stability diagrams for AlSb and GaSb under different growth conditions. For AlSb(001), the predictions are in excellent agreement with experimentally observed reconstructions. For GaSb(001), we show that previously proposed model accounts for the experimentally observed reconstructions under Ga-rich growth conditions, but fails to explain the experimental observations under Sb-rich conditions. We propose a new model that has a substantially lower surface energy than all (nx5)-like reconstructions proposed previously and that, in addition, leads to a simulated STM image in better agreement with experiment than existing models. However, this new model has higher surface energy than some of (4x3)-like reconstructions, models with periodicity that has not been observed. Hence we conclude that the experimentally observed (1x5) and (2x5) structures on GaSb(001) are kinetically limited rather than at the ground state.
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