Evolution of crystal and electronic structures of magnesium dicarbide at high pressure

Carbon-based compounds exhibit unexpected structures and electronic behavior at high pressure arising from various bonding features of carbon (e.g., sp, sp(2) and sp(3) C-C bonds). Here we report evolution of crystal structures of MgC(2) in a wide pressure range of 0–200 GPa as predicted through ab-initio calculations in combination with an unbiased swarm structure search. Three pressure-induced structural transformations are unraveled, following the phase sequence of ambient-pressure P4(2)/mnm (α-phase) → Cmcm (β-phase) → C2m (γ-phase) → EuGe(2)-type P-3m1 (δ-phase), where significant C-C bonding modifications from C-C dimer to quasi 1-dimensionzigzag chain, to polymerized ribbon and then to winkled quasi 2- dimension graphite sheet are evident. The predicted β- and γ-phases with sp(2) C-C hybridization are metals, while the δ- phase characterized by a sp(3)C-C hybridization is a narrow-gap semiconductor with a band gap of 0.667 eV. Strong electron-phonon couplings in the compressed β- and γ- phases arepredicted with β-phase showing a high superconducting critical temperature of 11.2 K. The current results indicate that pressure is effective in tuning the crystal and electronic structures of MgC(2), which is expected to have impact on physical properties for potential applications.