  Recently, a host/guest clathrate SrB3C3 with sp3-bonded boron-carbon
framework was synthesized at around 50 GPa. On the basis of 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 (Tc) 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 Rb-Sr-B3C3 system using
first-principles density functional calculation in combination with cluster
expansion and CALYPSO structure prediction method. We predict a phonon-mediated
superconductor Rb0.5Sr0.5B3C3 with a remarkably high Tc of 78 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 atom in
synthesized clathrate SrB3C3 is a viable route toward high-Tc compounds.
