X-Ray Diffraction of Ramp-Compressed Sodium*

Extreme compression can alter the free-electron behavior of “simple” metals such as sodium. At pressures greater than 200 GPa, Na is predicted to transform into a transparent electride structure where valence electrons are localized in interstitial regions. Laser-driven ramp compression is used to compress Na into this unexplored pressure regime to investigate the crystal structure, optical properties, and melting behavior of Na. X-ray diffraction is used to constrain the crystal structure and detect melting. Optical reflectivity measurements are used to detect metal-insulator transitions and simultaneous velocimetry is used to infer the Na pressure. We show the highest‑pressure solid x-ray diffraction and reflectivity data on Na to date.