  Ab initio evolutionary structure searches coupled with quasiharmonic
calculations predict that the insulating Na hP4 phase transitions to a novel
P63/m phase between 200 GPa at 150 K, and 350 GPa at 1900 K. P63/m Na is a
topological semimetal with a Dirac nodal surface that is protected by a
non-symmorphic symmetry, S2z . It is characterized by localized non-nuclear
charge within 1D honeycomb channels and 0D cages rendering it an electride.
These results highlight the complexity of warm dense sodiums electronic
structure and free energy landscape that emerges at conditions where ionic
cores overlap.
