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.

No abstract

Recently, there has been a tremendous increase in the number of identified extrasolar planetary systems. Our understanding of their formation is tied to exoplanet internal structure models, which rely upon equations of state of light elements and compounds such as water. Here, we present shock compression data for water with unprecedented accuracy that show that water equations of state commonly used in planetary modeling significantly overestimate the compressibility at conditions relevant to planetary interiors. Furthermore, we show that its behavior at these conditions, including reflectivity and isentropic response, is well-described by a recent first-principles based equation of state. These findings advocate that this water model be used as the standard for modeling Neptune, Uranus, and "hot Neptune" exoplanets and should improve our understanding of these types of planets.

The transformation of liquid water to solid ice is typically a slow process. To cool a sample below the melting point requires some time, as does nucleation from the metastable liquid 1 , so freezing usually occurs over many seconds 2. Freezing conditions can be created much more quickly using isentropic compression techniques, which provide insight into the limiting timescales of the phase transition. Here, we show that water rapidly freezes without a nucleator under sufficient compression, establishing a practical limit for the metastable liquid phase. Above 7 GPa, compressed water completely transforms to a high-pressure phase within a few nanoseconds. The consistent observation of freezing with different samples and container materials suggests that the transition nucleates homogeneously. The observation of complete freezing on these timescales further implies that the liquid reaches a hypercooled state 3. Computational studies suggest that freezing can occur on 0.1-1 ns timescales, although for water such simulations require a highly confined geometry 4 and/or strong electric fields 5,6. Unconfined simulations of supercooled water 7 indicate that freezing is possible on 100 ns timescales, many times faster than experimental observations. Simply cooling a liquid on that timescale is challenging: 10 7 −10 10 K s −1 cooling rates can be achieved by spraying droplets into a cryogen 8 , but it is difficult to carry out real-time measurements. The fastest real-time observation of freezing in expansion-cooled water clusters occurred on 10−30 μs timescales 9 , leaving a 2-3 decade gap between the experimental and computational studies of freezing. Adiabatic compression is an alternative route to solidification, even though liquids become hotter in the process. Temperature increase can be mitigated by using isentropic (rather than single shock wave) compression techniques, yielding the coldest possible adiabatic state. As shown in Fig. 1, isentropic compression of liquid water crosses the melt line between 2 and 3 GPa (T ≈ 400 K). Although compression freezing involves a different portion of the phase diagram than cooling (ice VII (ref. 10) rather than ice Ih), freezing conditions are created very quickly, providing insight into the limiting phase-transition timescales. When liquid water is isentropically compressed above 2 GPa in the presence of a quartz or fused-silica window, freezing will be observed over 10-100 ns timescales 11,12. The phase transition quickens with increasing pressure, but only in the presence of a silica window. Even at 5 GPa, where the liquid is nearly 70 K below the equilibrium melt line, no freezing is observed during compression within sapphire windows (≈800 ns experiment duration). Solidification is characterized by two basic events: the onset and the completion of freezing. The onset of freezing is defined by the time needed to create freezing conditions (whether by cooling

A novel approach was developed to probe density compression of liquid deuterium (L-D2) along the principal Hugoniot. Relative transit times of shock waves reverberating within the sample are shown to be sensitive to the compression due to the first shock. This technique has proven to be more sensitive than the conventional method of inferring density from the shock and mass velocity, at least in this high-pressure regime. Results in the range of 22-75 GPa indicate an approximately fourfold density compression, and provide data to differentiate between proposed theories for hydrogen and its isotopes.