Crystal Structure and Reflectivity of Laser Ramp-Compressed Sodium*

Extreme compression can alter the free-electron behavior of “simple” metals such as sodium. At pressures exceeding 200 GPa, Na was observed to become transparent to visible light under static compression. First-principles calculations suggest this is caused by a transformation to an electride phase where electrons are localized in interstitial positions. Laser-driven ramp compression is used to compress Na into an unexplored pressure regime to investigate the crystalline structure, reflectivity, and melting behavior of Na. X-ray diffraction is used to constrain the crystalline structure and detect melting. Optical reflectivity measurements at 532 nm are used to detect a transition to the observed insulating electride phase. We show the highest-pressure solid x-ray diffraction and reflectivity data on Na to date. The results indicate the Na phase diagram is more complicated than predicted by zero-temperature density functional theory.