Daily Overview: Today’s highlight work focuses on the in-depth understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. In Ref. [1], the researchers employed single-crystal synchrotron X-ray diffraction with helium as the pressure-transmitting medium to map for the first time the intrinsic pressure–temperature structural phase diagram of the stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure. The experiment reveals a clear phase-transition pathway: the charge ordering and octahedral tilting in the ambient-pressure polar orthorhombic Am2m phase gradually weaken upon compression, and the structure transforms directly into the tetragonal I4mm structure at about 10 GPa without passing through the previously disputed intermediate Amam phase. In this tetragonal phase, the interlayer Ni–O–Ni bonds become completely straightened and the orthorhombic twinning disappears; moreover, its stable temperature–pressure region coincides exactly with the region where superconductivity at approximately 68 K emerges in resistance measurements, thus establishing the precise structural framework that hosts the superconducting state. This discovery provides crucial experimental evidence for understanding the structural basis and microscopic origin of superconductivity in this nickelate system. arXiv submission processing window: 2026-07-31 00:00 to 2026-07-31 00:00 UTC.
1. Single-crystal structural phase diagram of stoichiometric bilayer nickelate La3Ni2O7 under hydrostatic pressure
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5.8501 - Authors: Misaki Sasaki, Zhehong Liu, Takeshi Hara, Shunsuke Kitou, Markus Kriener, Haruto Yoshimochi, Shion Yamada, Chieko Terakura, Naohisa Hirao, Hirokazu Kadobayashi, Yusuke Wakabayashi, Yoshinori Tokura, Yasujiro Taguchi, Taka-hisa Arima, Yukako Fujishiro
- Affiliations: Tohoku University, RIKEN, Japan Synchrotron Radiation Research Institute, The University of Tokyo
- Link: https://arxiv.org/abs/2607.27607
- Paper page: Single-crystal structural phase diagram of stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure
Summary: Using helium as a pressure-transmitting medium, this study constructs the intrinsic pressure–temperature structural phase diagram of stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure via synchrotron single-crystal X-ray diffraction. At ambient pressure, the material adopts a polar orthorhombic Am2m structure, accompanied by charge ordering between inequivalent Ni sites and tilting of NiO₆ octahedra. With increasing pressure, the orthorhombic lattice distortion gradually decreases, while the intensity of superstructure reflections indicative of charge ordering diminishes linearly. At approximately 10 GPa, a direct transition occurs from the charge-ordered Am2m phase to a tetragonal I4/mmm phase, with no intervening Amam phase; orthorhombic twinning splitting disappears, and unit cell parameters approach tetragonal symmetry. This tetragonal phase is characterized by the complete elimination of octahedral tilting, resulting in linear interlayer Ni–O–Ni bonds, and it persists in the pressure–temperature region where superconductivity emerges. This structural transition coincides precisely with the onset of bulk superconductivity at ~68 K as detected in resistance measurements, indicating that the superconducting state resides within the I4/mmm tetragonal framework. These findings resolve controversies over the structural identity of the superconducting phase, establish the intrinsic structural evolution pathway of La₃Ni₂O₇, and provide a critical structural basis for understanding the microscopic origin of high-temperature superconductivity in nickelates.