  Crystal structure searching and ab initio calculations have been used here to
explore low-energy structures of boron carbides under high pressure. Under
pressures of 85-110 GPa, a metastable B6C with R-3m symmetry is found to be
energetically more stable than the mixture of previous B4C and elemental boron.
This B6C is a rhombohedral structure and contains mooncake-like B24 clusters
with stuffing of C2 pairs. The mechanical and dynamical stabilities of this
structure at ambient pressure are confirmed by its elastic constants and phonon
dispersions. The bulk modulus and shear modulus of this newly predicted B6C at
ambient pressure reach high values of 291 GPa and 272 GPa, respectively. The
Vickers hardness is calculated to be around 48 GPa, and its melting temperature
at ambient pressure is estimated to be around 2500 K, indicating that this
metastable B6C is a potential superhard material with very good thermal
stability. Interestingly, this superhard B6C structure is also found to be
superconducting and its superconducting critical temperature (Tc) is evaluated
to be around 12.5 K at ambient pressure by electron-phonon coupling.
