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

Abstract

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 SrBxC9-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.