Predicting a metallic carbon allotrope: Pop-graphite via Na–C compounds

Carbon allotropes continue to captivate research interest due to their structural diversity and remarkable properties. While diamond-like carbon structures have been extensively studied, bulk graphite-like, layer-structured allotropes remain relatively unexplored and experimentally elusive. Here, we proposed a hitherto unknown layered carbon structure, designated as pop-graphite, synthesizable from high-pressure prepared sodium–carbon compounds. Through ab initio evolutionary structure searches and first-principles calculations, we identify a thermodynamically stable layered NaC3 intercalation compound at 14.0 GPa, featuring a distinctive edge-sharing penta-octa-penta (pop) carbon framework with intercalated sodium atoms. Upon sodium removal, the resultant pop-graphite structure remains stable under ambient conditions, exhibiting superior electrical conductivity and mechanical flexibility compared to conventional graphite. Our calculations reveal its intrinsic metallic nature and single-gap superconductivity, driven by electron–phonon coupling involving C pz states. These findings expand the frontiers of carbon allotrope design and their fundamental properties.