The electride nature of ramp-compressed sodium in the terapascal regime

At high-energy-density conditions, a new realm of quantum behavior emerges including electron localization, structural complexity, and core-electron chemistry. Sodium (Na) behaves unusually at these conditions because of its high compressibility and lone valence electron. Normally a shiny ideal metal, Na transforms to a topological insulator at 200 GPa. This topologically insulating phase (hP4) is due to the valence electrons occupying interstitial positions of its crystalline lattice rather than the orbitals centered on ionic cores. Recent experiments on ramp-compressed sodium at the National Ignition Facility achieve pressures greater than 1 TPa and x-ray diffraction measurements constrain the structure where Na is expected to be in an insulating electride phase. These data provide an experimental basis for understanding electron localization in traditionally simple metals at significant compressions.