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.

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

The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in Earth's mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories' Z Machine and extensive quantum calculations using density functional theory (DFT) and quantum Monte Carlo (QMC) methods. The combined data span from ambient conditions to 1.2 TPa and 42 000 K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate that under impact the solid and liquid phases coexist for more than 100 GPa, pushing complete melting to pressures in excess of 600 GPa. The high pressure required for complete shock melting has implications for a broad range of planetary collision events.

Symmetric, plate-impact experiments were performed to validate photon Doppler velocimetry (PDV) with established shock wave diagnostics. Impact velocity measurements using shorting pins demonstrated that the velocity accuracy of PDV can be 0.1% or better. Shock velocities and refractive indices were also measured with PDV (at 1550 nm) and velocity interferometer system for any reflector (VISAR) (at 532 nm) to obtain window corrections for single crystal LiF (100), c-cut sapphire, and z-cut quartz. Time-dependent, free-surface velocity histories for shocked LiF(100) provide a direct comparison between PDV and VISAR diagnostics and illustrate the benefits and shortcomings of the new diagnostic. Further implications of these results are presented.

While photonic Doppler velocimetry (PDV) is becoming a common diagnostic in dynamic compression research, its limiting accuracy and precision are not well understood. Velocity resolution is known to be inversely proportional to the time resolution, but resolution estimates differ by one to two orders of magnitude. Furthermore, resolution varies with the number of recorded signals and how these signals are analyzed. Numerical simulations reveal factors that affect accuracy and precision in PDV, and the results may be extended to a broad class of measurements. After systematic effects are eliminated, the limiting velocity uncertainty in a PDV measurement is governed by the sampling rate, the signal noise fraction, and the analysis time duration.