  Recently BCS superconductivity at 203 K has been discovery in a highly
compressed hydrogen sulfide. We use first-principles calculations to
systematically examine the effects of partially substituting the chalcogen
atoms on the superconductivity of hydrogen chalcogenides under high pressures.
We find detailed trends of how the critical temperature changes with increasing
the V-, VI- or VII-substitution rate, which highlight the key roles played by
low atomic mass and by strong covalent metallicity. In particular, a possible
record high critical temperature of 280 K is predicted in a stable
H3S0.925P0.075 with the Im-3m structure under 250 GPa.
