Daily Overview: Today’s highlighted work focuses on the structural identification of La₃Ni₂O₇ in its pressurized superconducting state. In [1], the authors employed high-pressure, variable-temperature Raman spectroscopy combined with polarization analysis to systematically track the structural evolution of La₃Ni₂O₇ single crystals under conditions down to 3 K and up to 32.7 GPa. They discovered a first-order structural phase transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase at approximately 14.5 GPa, precisely where bulk superconductivity emerges. Polarization measurements further ruled out the possibility of a tetragonal I4/mmm phase, clearly establishing that the intrinsic structure of the pressurized superconducting state is orthorhombic Fmmm. This study reveals that a Ni–O–Ni bond angle of 180° is a key structural prerequisite for achieving a high superconducting transition temperature, laying an important foundation for understanding the superconducting mechanism of bilayer nickelates. arXiv submission processing window: 2026-07-08 00:00 to 2026-07-08 00:00 UTC.

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

Summary: Using high-pressure variable-temperature Raman spectroscopy and polarization analysis, this work systematically tracks the structural evolution of a La₃Ni₂O₇ single crystal down to 3 K and up to 32.7 GPa. Based on rigorous symmetry selection rules, the disappearance and renormalization of multiple phonon modes in the spectra indicate a first-order structural phase transition from the orthorhombic Amam phase to the orthorhombic Fmmm phase at approximately 14.5 GPa, precisely coinciding with the emergence of bulk superconductivity. Polarized Raman measurements further reveal that above 1.92 GPa the sample recovers its intrinsic D₂h symmetry through detwinning, and in the superconducting state (3 K, 19.45 GPa) phonon modes are still observed in polarization channels, directly ruling out the tetragonal I4/mmm phase. These results confirm that the intrinsic crystal structure of the pressurized superconducting state below 19.45 GPa is orthorhombic Fmmm, rather than the previously disputed tetragonal phase, and disclose that the 180° Ni–O–Ni bond angle along the c-axis is a key structural prerequisite for achieving a high superconducting transition temperature, thereby establishing a vital structural foundation for understanding the superconducting mechanism of bilayer nickelates.