Crystal Structures and Chemical Bonding of Magnesium Carbide at High Pressure

Recent studies of the magnesium carbide (Mg–C) system under pressure were motivated by the successful high-pressure and high-temperature synthesis of Mg2C and Mg2C3. Here, we systematically investigate the high-pressure structures and chemical bonding of the Mg2C, Mg2C3, and MgC2 system using the swarm optimization technique in combination with first-principles electronic structure methodology. The structural evolution with pressure of the Mg–C systems clearly shows a systematic trend with a progressive increase of electron donation from the Mg to C. To accommodate the electrons, the C valence sp orbitals rebybridized continually and adopted different modes of chemical bonding. We demonstrated that the evolution of the electronic and crystal structures can be explained from a Zintl–Klemen charge-transfer concept. Therefore, at sufficiently high pressure metallic MgC2 and Mg2C transformed to semiconductors, while Mg2C3 undergoes an insulator–metal transition. The present results established the richness of carbon bonding of different stoichiometries under high pressure.