Probing a new regime of chemistry at HED conditions: Na as a prototypical example*

At high-energy-density (HED) conditions, a new realm of quantum behavior emerges. Examples at extreme HED compression include electron localization, structural complexity, core electron chemistry, and more. Sodium is an ideal prototype material to explore such behavior as it is so compressible, we can squeeze sodium to the point where core orbitals overlap at modest HED pressures. At 200 GPa, Na transforms from a simple free-electron metal to a structurally complex topological insulator. This phase is due to the electrons, which typically fill the Fermi Sea, instead being driven into interstitial positions due to the density-driven quantum mechanical constraints on the electronic wave functions. We report the structural and electronic properties of Na at the most extreme compressions yet studied, where the interatomic spacing approaches the Na⁺ ionic diameter and exceeds the 3s orbital diameter. Using lasers as the high-pressure driver, x-ray diffraction measurements to 480 GPa and 2000 K reveal unexpected new phases. Simultaneous reflectivity measurements suggest a dramatic drop in the conductivity of both the solid and dense liquid phases, where theory predicts the dense liquid undergoes a continuous transition from free electron to one where electrons are trapped in bubbles within the Na⁺ fluid. These data together with recent density functional theory calculations are consistent with the emergence of high-temperature “insulating plasma” states at extreme compression. I will discuss thoughts on how general this new phase of matter might be, as well as potential implications for such chemistry in the deep interiors of planets and stars throughout the universe.