Emergent superconductivity in clathrate Sr(B,C)9 at low pressures

The experimental synthesis of superconducting clathrate SrB₃C₃ has attracted considerable attention to strontium B-C compounds. We conducted a systematic prediction of the crystal structures for the SrB_xC_9-x system under low-pressure conditions, using an effective unbiased structure search method. By evaluating the formation enthalpies and phonon dispersions, we established the thermodynamic and dynamic stability of two new compounds, SrB₆C₃ and SrB₇C₂. These compounds exhibit remarkable mechanical properties, characterized by the calculated Vickers hardness values ranging from 21.4 to 34.0 GPa. Both SrB₆C₃ and SrB₇C₂ are metallic, as shown by the crossing of the bonding and antibonding states of the B-p orbitals at the Fermi level. Using the Migdal-Eliashberg theory to evaluate the electron–phonon coupling, we found that SrB₆C₃ lacks superconductivity, whereas clathrate SrB₇C₂ is calculated to exhibit superconductivity of 5.7 K under ambient pressure conditions. This discovery provides valuable insights into the exploration of boron–carbon clathrate superconducting materials with exceptional mechanical properties.