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.

Materials

• La₃Ni₂O₇

Methods

• high-pressure Raman spectroscopy
• polarization analysis

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• La₃Ni₂O₇ undergoes a structural phase transition from orthorhombic Amam to orthorhombic Fmmm at approximately 14.5 GPa.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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

Workflow

• sample_preparation — sample ready for high-pressure Raman spectroscopy
  • Materials: La₃Ni₂O₇ single crystals; neon; helium; diamond anvil cell (DAC)
  • Methods: optical floating-zone crystal growth; cleaving along the ab-plane; loading into DAC with pressure-transmitting medium (neon or helium)
  • Observations: single crystals cleaved and loaded in DAC
• measurement — observed structural evolution and a phase transition at ≈14.5 GPa coinciding with superconductivity
  • Materials: DAC; optical magnetic cryostat; 633 nm and 532 nm lasers
  • Methods: 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)
  • Observations: 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
  • Methods: factor group analysis; Raman tensor and polarization selection rules; mass-frequency relation
  • Observations: 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
  • Methods: integration with prior high-pressure XRD and transport data
  • Observations: 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