Pressure‐Dependent “insulator–metal–insulator” Behavior in Sr‐Doped La3Ni2O7

Recently, superconductivity at high temperatures has been observed in bulk La_3Ni_2O_7-δ under high pressure. However, the attainment of high-purity La_3Ni_2O_7-δ single crystals, exhibiting controlled and homogeneous stoichiometry through the post-annealing process in an oxygen-rich floating zone furnace, remains a formidable challenge. Here, we report the crystal structure and physical properties of single crystals of Sr-doped La_3Ni_2O_7 synthesized at high pressure (20 GPa) and high temperature (1400 C). Through single crystal X-ray diffraction, we showed that high-pressure-synthesized paramagnetic Sr-doped La_3Ni_2O_7 crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2)out-of-plane and 175.0(2)and 176.7(2)in plane. The substitution of Sr alters in band filling and the ratio of Ni^2+/Ni^3+ in Sr-doped La_3Ni_2O_7, aligning them with those of "La_3Ni_2O_7.05", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La_3Ni_2O_7 exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr-doped La_3Ni_2O_7 transits back from a metal-like behavior to an insulator. The insulator-metal-insulator trend under high pressure dramatically differs from the behavior of the parent compound La_3Ni_2O_7-δ, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.