Topological Electride Phase of Sodium at High Pressures and Temperatures*

Ab initio evolutionary searches coupled with quasiharmonic calculations have solved the structure of a new phase of sodium whose X-ray diffraction pattern was measured during laser-driven ramp-compression experiments [Nat. Commun. 13, 2534 (2022)]. The predicted P6₃/m phase, a topological semimetal with a Dirac nodal surface that is protected by the non-symmorphic symmetry S_2z, is preferred over Na hP4 between 200 GPa at 150 K and 350 GPa at 1900 K. It is characterized by non-nuclear charge localized within 1D honeycomb channels and 0D cages, rendering it an electride. Our results yield new insight on the complexity of warm dense sodium’s electronic structure and rich polymorphism that emerges when ionic cores overlap, and show that the free energies are key for determining the phases that are created in dynamic compression experiments at extreme pressures and temperatures.