Prediction of a thermodynamically stable carbon-based clathrate*

Carbon-based clathrates are predicted to exhibit a range of exceptional properties that are of fundamental and technological interest. Although several candidate structures have been proposed, the experimental synthesis of carbon clathrates remains unrealized due large formation energies, even under high-pressure conditions. Here, we predict, through first-principles calculations and unbiased automatic structure searching methods, a carbon-based clathrate in the bipartite sodalite structure, SrB₃C₃, that is thermodynamically stable at high pressure and dynamically stable at ambient pressure. Strikingly, SrB₃C₃ clathrate is predicted to be a superconductor with an estimated T_c of 42 K at ambient pressure. Calculated stress-strain relations for SrB₃C₃ clathrate demonstrate its intrinsic hard nature with Vickers hardness of 24-31 GPa. Our results reveal that boron substitution aids in the stabilization of SrB₃C₃ clathrate, and offers valuable insights into design guidelines for various carbon-based materials.