Superconductivity in monolayer-trilayer phase of La 3 Ni 2 O 7 under high pressure

The discovery of 80 K superconductivity in pressurized bilayer Ruddlesden-Popper (RP) nickelate La 3 Ni 2 O 7 has established a new high-temperature superconductor family. The quest to understand the governing principles of RP nickelate superconductivity has become a central focus in condensed matter physics. Here, we report a critical advance by synthesizing and investigating a distinct structural polymorph of the same compound: the monolayer-trilayer (1313) hybrid phase of La 3 Ni 2 O 7. Under high pressure, synchrotron X-ray diffraction and Raman spectroscopy reveal a structural transition from the orthorhombic Cmmm to the tetragonal P4/mmm space group at 13 GPa. Above 19 GPa, the phase exhibits a clear superconducting transition, confirmed by a zero-resistance state, albeit at a significantly reduced temperature of 3.6 K. The stark contrast with the 80 K transition in the bilayer phase provides a uniquely clean experimental comparison. Our results demonstrate that the superconducting transition temperature is directly governed by the nature of the interlayer coupling, and the bilayer NiO 6 block as the essential structural motif for achieving high- T c superconductivity in the RP nickelates.