Crystal and Electronic Structure Studies of La₄Ni₃O_10-δ under High-Pressure and Low-Temperature Conditions

Recent discoveries of pressure-induced high-temperature superconductivity in bilayer and trilayer Ruddlesden-Popper nickelates have sparked global research interest in these unconventional superconducting systems. Understanding the crystal and electronic structures is essential to uncover the underlining mechanism of superconductivity. For trilayer La₄Ni₃O_10-δ, structural studies have only been conducted at ambient temperature, leaving a vast P-T space unexplored. Here we report our in situ investigations using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), covering pressures from 0 to 51.3 GPa and temperatures from 10 to 300 K, to explore the critical P-T phase diagram of La₄Ni₃O_10-δ. Our results show that La₄Ni₃O_10-δ undergoes a phase transition from monoclinic (P2₁/a) to tetragonal (I4/mmm) under pressure and is further transformed to an orthorhombic (Bmab) phase as it enters the superconducting state upon cooling at pressures above 48 GPa. Additionally, the Ni valence state increases with the pressure and remains stable at low temperatures. The p and d orbitals of Ni and the enhanced hybridization between Ni 3d and O 2p orbitals play a significant role in the superconducting state due to the distortion of the NiO₆ octahedra. Our study provides important insights into the mechanisms driving high-T_c superconductivity in nickelates and establishes a basis for further investigation into related superconducting systems.