Identifying the structure of La₃Ni₂O₇ in the pressurized superconducting state

摘要

该工作利用高压变温拉曼光谱和偏振分析，在低至3 K、高达32.7 GPa的条件下系统追踪了La₃Ni₂O₇单晶的结构演化。基于严格的对称性选择定则，光谱中多个声子模式的消失与重整化在约14.5 GPa处指示了一个从正交Amam相到正交Fmmm相的一级结构相变，且体超导电性恰好在这一压力点出现。偏振拉曼测量进一步表明，在1.92 GPa以上，样品因退孪晶而恢复本征D₂h对称性；在超导态（3 K, 19.45 GPa）下，极化通道中仍可观测到声子模式，直接排除了四方I₄/mmm相。研究结果确认，低于19.45 GPa的加压超导态本征晶体结构为正交Fmmm，而非此前争议的四方相，并揭示沿c轴呈180°的Ni–O–Ni键角是实现高超导转变温度的关键结构前提，为理解双层镍氧化物超导机制奠定了重要的结构基础。

材料

• La₃Ni₂O₇

方法

• high-pressure Raman spectroscopy
• polarization analysis

关键词

• structural phase transition
• superconductivity
• bilayer nickelates
• ni o ni bond angle
• d2h symmetry
• phonon renormalization
• detwinning
• fano lineshapes

亮点

• Polarized Raman spectroscopy reveals that pressure induces detwinning, restoring the intrinsic D₂h symmetry of the orthorhombic lattice.
• Enhanced electron-phonon coupling and asymmetric Fano lineshapes are observed as the Ni–O–Ni bond angle approaches 180°.
• The combination of mode-count reduction and symmetry analysis overcomes the limitations of high-pressure synchrotron XRD in distinguishing Fmmm from I₄/mmm.
• The precise coincidence of the Amam-to-Fmmm transition with the emergence of superconductivity establishes the structural prerequisite.
• The study provides a definitive structural identification for La₃Ni₂O₇ in the superconducting state, settling conflicting reports.

结论

• The orthorhombic Fmmm structure is the intrinsic host of superconductivity in La₃Ni₂O₇ below 19.45 GPa.
• The 180° Ni–O–Ni bond angle along the c-axis is a key structural prerequisite for high-T_c superconductivity in bilayer RP nickelates.
• A single structural transition from the orthorhombic Amam phase to the Fmmm phase occurs at ≈14.5 GPa, coinciding with the emergence of bulk superconductivity.
• Polarized Raman spectroscopy confirms D₂h symmetry persists in the superconducting state, ruling out the tetragonal I₄/mmm phase.
• The structural transition resolves the controversy regarding the high-pressure phase of La₃Ni₂O₇ and provides a foundation for understanding the superconducting mechanism.

主要论断

• La₃Ni₂O₇ undergoes a structural phase transition from orthorhombic Amam to orthorhombic Fmmm at approximately 14.5 GPa.
  • 证据: Raman spectra show profound phonon renormalization and disappearance of multiple phonon modes at 14.5 GPa,factor group analysis and polarization data confirm D₂h symmetry of the high-pressure phase
• The superconducting state of La₃Ni₂O₇ below 19.45 GPa adopts the orthorhombic Fmmm structure, not tetragonal I₄/mmm.
  • 证据: phonon modes observed in polarization channel at 19.45 GPa (3 K) and 20.48 GPa (300 K), indicating D₂h symmetry,polarized Raman measurements rule out tetragonal I₄/mmm in the superconducting state
• The 180° Ni-O-Ni bond angle along the c-axis is a key structural prerequisite for high-T_c superconductivity in bilayer nickelates.
  • 证据: transition coincides with emergence of bulk superconductivity,enhanced electron-phonon coupling and Fano lineshapes with pressure indicate bond straightening,both Amam and Fmmm phases with 180° bond angle host superconductivity

研究流程

• sample_preparation — sample ready for high-pressure Raman spectroscopy
  • 材料: La₃Ni₂O₇ single crystals; neon; helium; diamond anvil cell (DAC)
  • 方法: optical floating-zone crystal growth; cleaving along the ab-plane; loading into DAC with pressure-transmitting medium (neon or helium)
  • 观察: single crystals cleaved and loaded in DAC
• measurement — observed structural evolution and a phase transition at ≈14.5 GPa coinciding with superconductivity
  • 材料: DAC; optical magnetic cryostat; 633 nm and 532 nm lasers
  • 方法: high-pressure Raman spectroscopy with polarization analysis; backscattering geometry with light propagating along c-axis; in-situ pressure calibration using ruby and diamond; temperature control (3 K and 300 K)
  • 观察: Raman peaks shift to higher wavenumbers with pressure; new phonon mode emerges at 284 cm-1 above 3.1 GPa; pronounced phonon renormalization and disappearance of several peaks at 14.5 GPa; polarization channel shows phonon modes in superconducting state
• analysis — the high-pressure superconducting phase has orthorhombic Fmmm structure, not tetragonal I₄/mmm
  • 方法: factor group analysis; Raman tensor and polarization selection rules; mass-frequency relation
  • 观察: number of observable Raman modes matches D₂h symmetry in high-pressure phase; persistence of phonon modes in polarization channel at 19.45 GPa (3 K) and 20.48 GPa (300 K) indicates D₂h symmetry; disappearance of octahedral tilting mode above 14 GPa
• interpretation — the 180° Ni-O-Ni bond angle along the c-axis is a key structural prerequisite for high-T_c superconductivity in bilayer nickelates
  • 方法: integration with prior high-pressure XRD and transport data
  • 观察: phase transition coincides precisely with emergence of bulk superconductivity; enhanced electron-phonon coupling and Fano lineshapes indicate Ni-O-Ni bond straightening; both Amam and Fmmm phases (with 180° bond angle) host superconductivity