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

Very recently, a new superconductor with = 80 K has been reported in nickelate (LaNiO) at around 15-40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with 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 state. In addition to the phase transition from the orthorhombic structure to the orthorhombic structure, a tetragonal phase with the space group of 4/ was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in LaNiO. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the e orbitals (3d and 3d) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between 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.