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Abstract The properties of all materials at one atmosphere of pressure are controlled by the configurations of their valence electrons. At extreme pressures, neighboring atoms approach so close that core-electron orbitals overlap, and theory predicts the emergence of unusual quantum behavior. We ramp-compress monovalent elemental sodium, a prototypical metal at ambient conditions, to nearly 500 GPa (5 million atmospheres). The 7-fold increase of density brings the interatomic distance to 1.74 Å well within the initial 2.03 Å of the Na + ionic diameter, and squeezes the valence electrons into the interstitial voids suggesting the formation of an electride phase. The laser-driven compression results in pressure-driven melting and recrystallization in a billionth of a second. In situ x-ray diffraction reveals a series of unexpected phase transitions upon recrystallization, and optical reflectivity measurements show a precipitous decrease throughout the liquid and solid phases, where the liquid is predicted to have electronic localization. These data reveal the presence of a rich, temperature-driven polymorphism where core electron overlap is thought to stabilize the formation of peculiar electride states. 

The discovery of more than 4500 extrasolar planets has created a need for modeling their interior structure and dynamics. Given the prominence of iron in planetary interiors, we require accurate and precise physical properties at extreme pressure and temperature. A first-order property of iron is its melting point, which is still debated for the conditions of Earth’s interior. We used high-energy lasers at the National Ignition Facility and in situ x-ray diffraction to determine the melting point of iron up to 1000 gigapascals, three times the pressure of Earth’s inner core. We used this melting curve to determine the length of dynamo action during core solidification to the hexagonal close-packed (hcp) structure. We find that terrestrial exoplanets with four to six times Earth’s mass have the longest dynamos, which provide important shielding against cosmic radiation. 

The response of forsterite, Mg₂SiO₄, under dynamic compression is of fundamental importance for understanding its phase transformations and high‐pressure behavior. Here, we have carried out an in situ X‐ray diffraction study of laser‐shocked polycrystalline and single‐crystal forsterite (a‐,b‐, andc‐orientations) from 19 to 122 GPa using the Matter in Extreme Conditions end‐station of the Linac Coherent Light Source. Under laser‐based shock loading, forsterite does not transform to the high‐pressure equilibrium assemblage of MgSiO₃bridgmanite and MgO periclase, as has been suggested previously. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg₂SiO₄, coexisting in a mixed‐phase region from 33 to 75 GPa for both polycrystalline and single‐crystal samples. Densities inferred from X‐ray diffraction data are consistent with earlier gas‐gun shock data. At higher stress, the response is sample‐dependent. Polycrystalline samples undergo amorphization above 79 GPa. For [010]‐ and [001]‐oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the [100]‐oriented forsterite adopts an unknown phase at 122 GPa. The first two sharp diffraction peaks of amorphous Mg₂SiO₄show a similar trend with compression as those observed for MgSiO₃in both recent static‐ and laser‐driven shock experiments. Upon release to ambient pressure, all samples retain or revert to forsterite with evidence for amorphous material also present in some cases. This study demonstrates the utility of femtosecond free‐electron laser X‐ray sources for probing the temporal evolution of high‐pressure silicate structures through the nanosecond‐scale events of shock compression and release.