Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.

Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km/s. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD 2) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 30-70 GPa. The results of these experiments disagree with previously reported Hugoniot measurements of LD 2 in the pressure range above ~40 GPa, but are in good agreement with first principles, ab-initio models for hydrogen and its isotopes.

Evaluation of models and theory of high-pressure material response is largely made through comparison with shock wave data, which rely on impedance match standards. The recent use of quartz as a shock wave standard has prompted a need for improved data. We report here on measurements of the quartz Hugoniot curve from 0.1-1.6 TPa. The new data, in agreement with our ab initio calculations, reveal substantial errors in the standard and have immediate ramifications for the equations of state of deuterium, helium, and carbon at pressures relevant to giant planets and other high-energy density conditions.

it is furthermore crucial to note that a distinct part of the nucleation driving force given by the change in Gibbs energy is already released by stable cluster formation (SOM section 2.6, fig. S14, and Fig. 2).Prenucleation-stage cluster formation on the basis of equilibrium thermodynamics can be qualitatively shown also for the biominerals calcium phosphate and calcium oxalate (SOM section 2.7 and fig. S15) and suggests a similar nucleation mechanism for these minerals. The clusterformation mechanism on the basis of equilibrium thermodynamics can be speculatively explained by entropic solvent effects. The probable release of water molecules from the hydration layer of ions caused by cluster formation may result in an increased number of degrees of freedom of the system. In classical nucleation theories, only enthalpic effects (interaction potentials) are taken into account, and entropic solvent effects are neglected. In the end, a pH-dependent change of ionic hydration layers may explain the pH dependency of cluster-formation thermodynamics. The high-energy density behavior of carbon, particularly in the vicinity of the melt boundary, is of broad scientific interest and of particular interest to those studying planetary astrophysics and inertial confinement fusion. Previous experimental data in the several hundred gigapascal pressure range, particularly near the melt boundary, have only been able to provide data with accuracy capable of qualitative comparison with theory. Here we present shock-wave experiments on carbon (using a magnetically driven flyer-plate technique with an order of magnitude improvement in accuracy) that enable quantitative comparison with theory. This work provides evidence for the existence of a diamond-bc8-liquid triple point on the melt boundary.

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