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.
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