Pressure-Dependent “Insulator–Metal–Insulator” Behavior in Sr-Doped La3Ni2O7

Recently, superconductivity at high temperatures is observed in bulk La₃Ni₂O_7−δ under high pressure. However, the attainment of high-purity La₃Ni₂O_7−δ single crystals remains a formidable challenge. Here, the crystal structure and physical properties of single crystals of Sr-doped La₃Ni₂O₇ synthesized at high pressure (20 GPa) and high temperature (1400 °C) are reported. Through single crystal X-ray diffraction, it is shown that high-pressure-synthesized paramagnetic Sr-doped La₃Ni₂O₇ 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 Ni²⁺/Ni³⁺ in Sr-doped La₃Ni₂O₇, aligning them with those of “La₃Ni₂O_7.05”, thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La₃Ni₂O₇ 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 La₃Ni₂O₇ 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 La₃Ni₂O_7−δ, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.