Triply bonded C≡C characteristics between planar hypercoordinate carbons in a two-dimensional Be4C3 monolayer

We report the discovery of two 2D Be4C3 monolayers-o-Be4C3 and m-Be4C3-as the lowest-energy structures, each exhibiting unique planar hypercoordinate carbon bonding characteristics. Both monolayers feature a C≡C triply bonded motif formed between planar tetracoordinate carbon (ptC) and planar pentacoordinate carbon (ppC) centers, coordinating with three Be atoms in o-Be4C3 and four Be atoms in m-Be4C3. Bonding analysis reveals dual behavior: the p-orbital electrons in the C≡C bonds remain primarily localized, while other ptC atoms coordinated with four Be atoms exhibit fully delocalized electron density across the Be4C framework. This mainly localized π-bonding makes Be4C3 the first 2D material to feature multiply bonded planar hypercoordinate carbon motifs. Both monolayers demonstrate good thermodynamic and kinetic stability. Notably, o-Be4C3 possesses a small indirect bandgap, while m-Be4C3 exhibits 2D phonon-mediated superconductivity with a transition temperature (Tc) of 4.5 K, connecting "anti-van't Hoff/Le Bel" structures to promising applications in electronics and optoelectronics.