  Prior to the superconducting transition at Tc = 2.3 K, Mo3Sb7 undergoes a
symmetry-lowering, cubic-to-tetragonal structural transition at Ts = 53 K. We
have monitored the pressure dependence of these two transitions by measuring
the resistivity of Mo3Sb7 single crystals under various hydrostatic pressures
up to 15 GPa. The application of external pressure enhances Tc but suppresses
Ts until Pc ~ 10 GPa, above which a pressure-induced first order structural
transition takes place and is manifested by the phase coexistence in the
pressure range 8 < P < 12 GPa. The cubic phase above 12 GPa is also found to be
superconducting with a higher Tc =6 K that decreases slightly with further
increasing pressure. The variations with pressure of Tc and Ts satisfy the
Bilbro-McMillan equation, i.e. Tc^nTs^(1-n) = constant, thus suggesting the
competition of superconductivity with the structural transition that has been
proposed to be accompanied with a spin-gap formation at Ts. This scenario is
supported by our first-principles calculations which imply the plausible
importance of magnetism that competes with the superconductivity in Mo3Sb7.
