  We synthesized polycrystalline La3Ni2O7 samples by using the sol-gel method
without post-annealing under high oxygen pressure, and then measured
temperature-dependent resistivity under various hydrostatic pressures up to
14.5 GPa in a cubic anvil cell apparatus. We find that the density-wave-like
anomaly in resistivity is progressively suppressed with increasing pressure and
the resistivity drop corresponding to the onset of superconductivity emerges at
pressure as low as 7 GPa. Zero resistivity is achieved at 9 GPa below 6.6 K,
which increases quickly with pressure to 35.6 K at 14.5 GPa. The observation of
zero-resistance state in the polycrystalline La3Ni2O7 samples under high
pressures not only corroborates the recent report of superconductivity in the
pressurized La3Ni2O7 crystals but also facilitates further studies on this
emerging family of nickelate high-Tc superconductors.
