  First principle DFT simulations are employed to study structural and
mechanical properties of orthorhombic B12-based metal carbide-borides. The
simulations predict the existence of Ca- and Sr- based phases with the
structure similar to that of experimentally observed earlier compound
MeC$_2$B$_{12}$. Dynamical stability of both phases is demonstrated, and the
phase MeC$_2$B$_{12}$ is found to be thermodynamically stable. According to
simulations, Ca- and Sr- based phases have significantly enhanced mechanical
characteristics, which suggest their potential application as superhard
materials. Calculated shear and Young moduli of these phases are nearly 250 and
540 GPa, respectively, and estimated Vickers hardness is 45-55 GPa.
