Daily Overview: Today’s highlighted work focuses on an in-depth understanding of the electronic structure of mixed Ruddlesden–Popper nickelates. Addressing the controversy over the crystal structure of the pressurized superconducting state of La3Ni2O7, [1] used high-pressure Raman spectroscopy and polarization analysis at temperatures as low as 3 K and pressures up to 32.7 GPa to track its pressure-driven structural evolution. It found that at about 14.5 GPa a transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase occurs, accompanied by pronounced phonon reconstruction, while D2h symmetry persists before and after the transition, thereby ruling out the tetragonal I4/mmm phase as a possible structure of the superconducting state. The onset of bulk superconductivity precisely coincides with this structural transition, indicating that below 19.45 GPa the intrinsic superconducting main phase of La3Ni2O7 is the orthorhombic Fmmm structure. These results settle the long-standing structural controversy and provide a key foundation for understanding the superconductivity mechanism of bilayer nickelates and their structure–electronic-state correlation. arXiv submission processing window: 2026-09-29 00:00 to 2026-09-29 00:00 UTC.

1. Identifying the structure of La3Ni2O7 in the pressurized superconducting state

Summary: This study used high-pressure Raman spectroscopy at temperatures as low as 3 K and pressures up to 32.7 GPa, combined with polarization analysis, to track the structural evolution of La3Ni2O7 under pressure and resolve the controversy over the crystal structure of its superconducting state. Based on symmetry selection rules, the authors identified a structural transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase at approximately 14.5 GPa, accompanied by pronounced phonon reconstruction; D2h symmetry persisted across the transition, thereby ruling out the tetragonal I4/mmm phase as the structure of this superconducting state. The onset of bulk superconductivity coincided precisely with this structural transition. The results indicate that below 19.45 GPa the intrinsic primary superconducting phase of La3Ni2O7 is orthorhombic Fmmm, settling a long-standing structural controversy and providing a key foundation for understanding the superconductivity mechanism in bilayer nickelates.