  Recently the discovery of superconductivity with a critical temperature Tc up
to 80 K in Ruddlesden-Popper phases Lan+1NinO3n+1 (n = 2) under pressure has
garnered considerable attention. Up to now, the superconductivity was only
observed in La3Ni2O7 single crystal grown with the optical-image floating zone
furnace under oxygen pressure. It remains to be understood the effect of
chemical doping on superconducting La3Ni2O7 as well as other Ruddlesden-Popper
phases. Here, we systematically investigate the effect of external pressure and
chemical doping on polycrystalline Ruddlesden-Popper phases. Our results
demonstrate the application of pressure and doping effectively tunes the
transport properties of Ruddlesden-Popper phases. We find pressure-induced
superconductivity up to 86 K in La3Ni2O7 polycrystalline sample, while no
signatures of superconductivity are observed in La2NiO4 and La4Ni3O10 systems
under high pressure up to 50 GPa. Our study sheds light on the exploration of
high-Tc superconductivity in nickelates.
