Structure Responsible for the Superconducting State in La₃Ni₂O₇ at High-Pressure and Low-Temperature Conditions

Very recently, a new superconductor with T_c = 80 K has been reported in nickelate (La₃Ni₂O₇) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with T_c above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high T_c state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La₃Ni₂O₇. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the e_g orbitals (3d and 3d) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between T_c and this structural evolution, especially Ni-O octahedra regularity and the in-plane Ni-O-Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.