Superconductive Sodium Carbides with Pentagon Carbon at High Pressures

The design of metal-bearing carbon-based materials with unique structures and intriguing properties is highly desirable in the fields of physics, chemistry, and materials science. Here, within swarm-intelligence structure search and first-principles computations, we uncovered several hitherto unknown sodium carbides (i.e., Na₄C, Na₃C₂, NaC, Na₂C₃, and NaC₂) under high pressure. Intriguingly, the C atom arrangement reveals multiple structure evolution behavior with increased carbon content, from isolated anions in Na₄C, tetramers in Na₃C₂, extended chains in NaC, pentagonal rings in Na₂C₃, to eventually hexagonal rings in NaC₂. Among predicted phases, the superconducting critical temperature T_c of NaC₂ could approach ∼42 K at 80 GPa, which is slightly higher than the T_c of 39 K in the highest phonon-mediated superconductivity of MgB₂ at ambient pressure. This work offers insights into the reaction of carbides containing alkali metals and paves the way for the future investigation of high superconductivity in metal carbide systems.