Phase Diagram of Pressure-Induced High Temperature Superconductor La3Ni2O7+δ

We successfully synthesized samples of La$_{3}$Ni$_{2}$O$_{7+\delta}$ ($\delta = -0.50$, $-0.16$, $0.00$, $+0.01$, and $+0.12$) and measured the resistance under extremely high pressures using a diamond anvil cell to establish the electronic phase diagram. A Mott insulating state appears at $\delta = -0.50$, where all Ni ions are divalent. With increasing oxygen content, superconductivity appears at $\delta = 0.00$ and higher, above approximately 25 GPa, passing through Anderson localization at $\delta = -0.16$. The superconducting transition temperature $T_{\mathrm{c}}$ decreases with increasing pressures for both $\delta = 0.00$ and $+0.12$, with the pressure dependence of $T_{\mathrm{c}}$ being much stronger in the latter than in the former.