Single-Crystal Structure Determination of Superconducting La₄Ni₃O_10-δ under High Pressure

Ruddlesden-Popper nickelates have attracted enormous attention since the discovery of high-temperature superconductivity in bilayer La₃Ni₂O₇ under high pressure. However, the crystal structure under high pressure remains elusive due to the lack of single crystal diffraction data. Here, high-pressure superconductivity with a superconducting onset temperature approaching 30K and a zero-resistance state at 7K at 53.8 GPa is reported in high-quality single crystals of trilayer La₄Ni₃O_10-δ grown at ambient pressure. A cascade of structural transitions from P2₁/a at 5.7 GPa to Bmab at 11.0 GPa and to I4/mmm at 16.6 GPa is unambiguously determined via synchrotron X-ray single crystal diffraction, revealing a correlation between the tetragonal I4/mmm and the emergence of superconductivity. These results not only demonstrate that ambient-pressure grown nickelate single crystals can superconduct under high pressure but also reveal an intermediate structure that was not reported under high pressure, providing new insights into the physics of nickelate superconductivity.