  A systematic ab initio search for low enthalpy phases of disilane (Si_2H_6)
at high pressures was performed based on the minima hopping method. We found a
novel metallic phase of disilane with Cmcm symmetry, which is enthalpically
more favorable than the recently proposed structures of disilane up to 280 GPa,
but revealing compositional instability below 190 GPa. The Cmcm phase has a
moderate electron-phonon coupling yielding a superconducting transition
temperature T_c of around 20 K at 100 GPa, decreasing to 13 K at 220 GPa. These
values are an order of magnitude smaller than previously predicted T_c for
disilane, and cast strong doubts on the possibility of high-T_c
superconductivity in these systems as well as in other hydrogen-rich compounds
under moderate pressure.
