Path to high-Tc superconductivity via Rb substitution of guest metal atoms in the SrB3C3 clathrate

Recently, a host-guest clathrate SrB3C3 with sp(3)-bonded boron-carbon framework was synthesized at similar to 50 GPa. Based on electron count, the structure is understood as guest Sr2+ cations intercalated in the (B3C3)(3-) framework. Previous calculations suggest that SrB3C3 is a hole conductor with an estimated superconducting critical temperature (T-c) of 42 K at ambient pressure. If atoms with similar radius, such as Rb, can substitute Sr2+ in the lattice, the electronic as well as superconductivity properties of this material will be modified significantly. Here, we perform extensive simulations on the stability and physical properties of the Rb-Sr-B3C3 system using first-principles density functional calculation in combination with cluster expansion and the CALYPSO structure prediction method. We predict a phonon-mediated superconductor Rb0.5Sr0.5B3C3 with a remarkably high T-c of 75 K at ambient pressure, which is a significant improvement from the estimated value (42 K) in SrB3C3. The current results suggest that substitution of alkali atoms in synthesized clathrate SrB3C3 is a viable route toward high-T-c compounds.