Sodium's elusive tI50 phase predicted to be a body-centered tetragonal electride.

Sodium exhibits an anomalous melting curve with a minimum at high pressure, where several complex crystalline phases coinciding with this minimum have been observed but remain unresolved. Solving these structures would help better understand sodium's unusual melting behavior. In this study, we determine the crystal structure of one such phase, the tI50 phase. Using a data-derived potential-assisted structure search, we identify the tI50 phase as a body-centered tetragonal structure with 50 atoms per unit cell, crystallizing in the I4/m space group. The predicted lattice parameters deviate by less than 0.38% from experimental values, and enthalpy calculation confirms this structure as the ground state for sodium across a ~20 GPa pressure range. The structure features a cage-like polyhedral network with interstitial electride states and remains weakly metallic, unlike the insulating hP4 phase. Resolving this unique structure advances understanding of sodium's diverse structural behavior under high pressure.