Pressure studies on the electrical properties in R_2-xSr_xNi_1-yCu_yO_4+δ(R=La, Nd) and La₃Ni₂O_7+δ

The Ruddlesden-Popper series of compounds Lan+1NinO3n+1 are one of the strongly correlated electron systems. Among these compounds, La2NiO4 (n=1) is a well-known two-dimensional antiferromagnetic insulator. Electrical resistivity under high pressure have been measured on R2-xSrxNi1-yCuyO4+delta (R=La, Nd) up to 2 GPa and La3Ni2O7+delta up to similar to 20 GPa. Elecrical resistivty of La1.92Sr1.08NiO4 under high pressure reveals an enhancement of the electronic localization under pressure. On the other hand, in case of Nd1.6Sr0.4Ni0.5Cu0.5O4+delta, a metal-insulator transition is observed at similar to 240 K and a metallic state is favorable under pressure, in contrast to La1.92Sr1.08NiO4. InLa3Ni2O7+delta pressure also enhances a metallic state up to 2 GPa.