Prediction of novel metallic carbon and silicon allotropes using an inverse material design method

Carbon has a rich variety of structural allotropes due to its ability of forming sp, sp², and sp³ hybridized bonds. Graphene, a single layer of graphite, consists of all-sp² bonds and exhibits a semimetallic band structure with Dirac points. The cubic diamond phase of Si is semiconducting and this material is widely used as the key element in the semiconductor technology. Although many metastable forms of Si have been observed, no metallic phase at ambient conditions has been reported to date. In this work, we report novel metallic carbon and silicon allotropes using an inverse material design method based on evolutionary global optimization and first-principles DFT calculations. The new carbon allotrope, termed m-C₈, consists of five-membered rings with sp³ bonding interconnected by sp²-bonded graphitic carbon networks. Analyzing the electronic band structure, we identify that m-C₈ belongs to the class of topological nodal line semimetals. The new Si allotrope, termed P6/m-Si₆, contains open channels embedded in a simple hexagonal lattice. We find that the P6/m-Si₆ clathrate is superconducting with the critical temperature of about 12 K at zero pressure.