Ionization of highly compressed ammonia has previously been predicted by computation. Here, the authors provide experimental evidence for this autoionization process at high pressures, showing the transformation of molecular ammonia into ammonium amide.

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Ammonia as a case study for the spontaneous ionization of a simple hydrogen-bonded compound

We investigate, via quantum molecular dynamics simulations, the structural and transport properties of ammonia along the principal Hugoniot for temperatures up to 10 eV and densities up to 2.6 g/cm3. With the analysis of the molecular dynamics trajectories by use of the bond auto-correlation function, we identify three distinct pressure-temperature regions for local chemical structures of ammonia. We derive the diffusivity and viscosity of strong correlated ammonia with high accuracy through fitting the velocity and stress-tensor autocorrelation functions with complex functional form which includes structures and multiple time scales. The statistical error of the transport properties is estimated. It is shown that the diffusivity and viscosity behave in a distinctly different manner at these three regimes and thus present complex features. In the molecular fluid regime, the hydrogen atoms have almost the similar diffusivity as nitrogen and the viscosity is dominated by the kinetic contribution. When entering into the mixture regime, the transport behavior of the system remarkably changes due to the stronger ionic coupling, and the viscosity is determined to decrease gradually and achieve minimum at about 2.0 g/cm3 on the Hugoniot. In the plasma regime, the hydrogen atoms diffuse at least twice as fast as the nitrogen atoms.

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Structural and transport properties of ammonia along the principal Hugoniot.

▪ Abstract We know that giant planets played a crucial role in the making of our Solar System. The discovery of giant planets orbiting other stars is a formidable opportunity to learn more about these objects, what their composition is, how various processes influence their structure and evolution, and most importantly how they form. Jupiter, Saturn, Uranus, and Neptune can be studied in detail, mostly from close spacecraft flybys. We can infer that they are all enriched in heavy elements compared to the Sun, with the relative global enrichments increasing with distance to the Sun. We can also infer that they possess dense cores of varied masses. The intercomparison of presently characterized extrasolar giant planets shows that they are also mainly made of hydrogen and helium, but that they either have significantly different amounts of heavy elements, have had different orbital evolutions, or both. Hence, many questions remain and need to be answered to make significant progress on the origins of planets...

THE INTERIORS OF GIANT PLANETS: Models and Outstanding Questions

1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

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The Exoplanet Handbook

In the search for new functional materials, quantum mechanics is an exciting starting point. The fundamental laws that govern the behaviour of electrons have the possibility, at the other end of the scale, to predict the performance of a material for a targeted application. In some cases, this is achievable using density functional theory (DFT). In this Review, we highlight DFT studies predicting energy-related materials that were subsequently confirmed experimentally. The attributes and limitations of DFT for the computational design of materials for lithium-ion batteries, hydrogen production and storage materials, superconductors, photovoltaics and thermoelectric materials are discussed. In the future, we expect that the accuracy of DFT-based methods will continue to improve and that growth in computing power will enable millions of materials to be virtually screened for specific applications. Thus, these examples represent a first glimpse of what may become a routine and integral step in materials discovery. Density functional theory has become an indispensable tool in the design of new materials. This Review details the principles of computational materials design, highlighting examples of the successful prediction and subsequent experimental verification of materials for energy harvesting, conversion and storage.

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Computational predictions of energy materials using density functional theory

The melting curves of argon, helium 4, ice $({\mathrm{H}}_{2}\mathrm{O}),$ and hydrogen $({\mathrm{H}}_{2})$ have been measured from room temperature up to a maximum temperature of 750 K. This extends the previous determination of the melting lines of ${\mathrm{H}}_{2}$ and He by nearly a factor of 2 in pressure. The experiments were carried out with a resistively heated diamond anvil cell. Improved accuracy with respect to previous determinations, when existing, was achieved by the use of an optical metrology which gives an in situ measurement of both the pressure and temperature of the sample. The melting lines of argon and ${\mathrm{H}}_{2}\mathrm{O}$ are found to be well represented by the following Simon-Glatzel equations: $P=2.172\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{T}^{1.556}\ensuremath{-}0.21$ (argon) and $P=2.17+1.253[{(T/354.8)}^{3.0}\ensuremath{-}1]$ $({\mathrm{H}}_{2}\mathrm{O}).$ But the Simon-Glatzel form was found inadequate to reproduce the melting data of ${}^{4}\mathrm{He}$ and ${\mathrm{H}}_{2}$ over the whole temperature range. In the case of ${}^{4}\mathrm{He},$ this deviation from a Simon law is explained by the softening of the pair interaction with density. A Kechin equation is proposed for ${\mathrm{H}}_{2}:$ ${T=14.025(1+P/0.0286)}^{0.589}\mathrm{exp}(\ensuremath{-}4.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P).$ This form is in excellent agreement with all published experimental data for ${\mathrm{H}}_{2}$ and interestingly predicts a maximum on the melting curve at 128 GPa and 1100 K.

/pdf/extended-and-accurate-determination-of-the-melting-curves-of-4ffl3xax17.pdf

Extended and accurate determination of the melting curves of argon, helium, ice ( H 2 O ) , and hydrogen ( H 2 )

The high-pressure local structure of zinc oxide has been studied at room temperature using combined energy-dispersive x-ray-diffraction and x-ray-absorption spectroscopy experiments. The structural parameter u and the lattice-parameter ratio $c/a$ of the wurtzite phase is given as a function of pressure and compared with results from ab initio calculations based on a plane-wave pseudopotential method within the density-functional theory. It is shown that an accurate study of ZnO requires the explicit treatment of the d electrons of Zn as valence electrons. In good agreement with present calculations, our experimental data do not show any variation of $u(P)$ in the low-pressure wurtzite phase between 0 and 9 GPa, pressure at which the phase transition to the rocksalt phase occurs. Moreover, no dramatic modification of the r-phase K-edge position up to $\ensuremath{\sim}20\mathrm{GPa}$ is observed, indicating the absence of metallization. In view of all these results, theoretical models identifying the wurtzite-to-rocksalt transition as an homogeneous path are discussed.

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Local structure of condensed zinc oxide

We report accurate measurements of the equation of state of diamond and neon, measured by x-ray diffraction in a resistively heated diamond-anvil cell. The atomic volume of single crystals of diamond embedded in neon pressure-transmitting medium has been measured between 0 and $80\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and from $300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}900\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The atomic volume of neon is reported in the same $P\text{\ensuremath{-}}T$ range and also up to $208\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. The crystal structure of neon remains face-centered cubic over the domain of investigation. We show that a Mie--Gr\"uneisen--Debye formalism reproduces very well the present $P\text{\ensuremath{-}}V\text{\ensuremath{-}}T$ data for neon as well as low pressure--low temperature data available in the literature. This makes neon a well calibrated x-ray pressure gauge, suitable for high pressure--high temperature studies. The thermal behavior of diamond is more complex and cannot be completely described by a Mie--Gr\"uneisen--Debye model. Its thermal expansion decreases faster with increasing pressure than the predictions of simple thermodynamic models.

High pressure-high temperature equations of state of neon and diamond

We review the various optical pressure sensors that are suitable for high-pressure and high-temperature studies in a diamond anvil cell. Two different kinds of sensors are considered: those based on the pressure shift of a fluorescence line (ruby, SrB4O7:Sm2+) and those based on the pressure shift of a Raman line (c-BN, diamond). The calibration of those sensors are presented in detail, and discussion is made on their useful pressure and temperature ranges.

Optical pressure sensors for high-pressure-high-temperature studies in a diamond anvil cell

The structural transformations occurring to water from the low- to the high-density regimes have been studied by classical molecular dynamics calculations. The local structure is analyzed through a proper choice of the relevant orientational distribution functions. This approach sheds light on the key role played by the interstitial molecules in the second coordination shell and identifies a clear structural fingerprint of high-density water. As a consequence, the analogy between the structure of high-density water and those of high-density ices is evidenced.

Frédéric Datchi

Papers

Extended and accurate determination of the melting curves of argon, helium, ice ( H 2 O ) , and hydrogen ( H 2 )

Local structure of condensed zinc oxide

High pressure-high temperature equations of state of neon and diamond

Optical pressure sensors for high-pressure-high-temperature studies in a diamond anvil cell

Structure and phase diagram of high-density water: the role of interstitial molecules.