  We predict superconductivity for the carbon-boron clathrate SrB3C3 at 27-43 K
for Coulomb pseudopotential (mu*) values between 0.17 and 0.10 using
first-principles calculations with conventional electron-phonon coupling.
Electrical transport measurements, facilitated by a novel in situ experimental
design compatible with extreme synthesis conditions (>3000 K at 50 GPa), show
non-hysteretic resistivity drops that track the calculated magnitude and
pressure dependence of superconductivity for mu*=0.15, and transport
measurements collected under applied magnetic fields confirm superconductivity
with an onset Tc of approximately 20 K at 40 GPa. Carbon-based clathrates thus
represent a new class of superconductors similar to other covalent metals like
MgB2 and doped fullerenes. Carbon clathrates share structures similar to
superconducting superhydrides, but covalent C-B bonds allow metastable
persistence at ambient conditions.
