Metallic crossover through the tilt-free transition in La₃Ni₂O₇ at high pressure and temperature

Summary

This study systematically investigates the structural phase transitions and electronic property evolution of the bilayer nickelate La₃Ni₂O₇ under pressure and temperature using high-pressure high-temperature Raman spectroscopy and synchrotron infrared reflectance spectroscopy. Raman measurements confirm a pressure-driven structural phase transition from the tilted Amam phase to the untilted Fmmm or I₄/mmm phase, with the emergence of Fano line shapes indicating enhanced electron-phonon coupling. High-temperature data reveal an upper temperature limit of 544 K for this transition at ambient pressure, refining the temperature-pressure phase diagram. Infrared reflectivity measurements show that the phase transition is accompanied by an increase in carrier density by nearly two orders of magnitude, marking a crossover from a bad metal to a good metal. The experiments establish a unified picture where the structural phase transition is strongly coupled to electronic properties, with superconductivity emerging at approximately 6-7 GPa closely associated with the appearance of the untilted phase; however, high symmetry and metallicity alone are insufficient to induce superconductivity, suggesting that strong electronic correlation effects such as density wave fluctuations may also need to be considered.

Materials

• La₃Ni₂O₇

Methods

• High-pressure Raman spectroscopy
• Synchrotron infrared reflectivity
• High-temperature Raman spectroscopy

Keywords

• electron phonon coupling
• bad metal to good metal crossover
• fano line shape
• carrier density enhancement
• structural phase transition

Highlights

• First report of the upper temperature limit (544 K) for the structural transition at ambient pressure in La₃Ni₂O₇.
• Two-order-of-magnitude increase in free carrier density across the structural transition, demonstrated by infrared reflectivity.
• Unified T-P phase diagram constructed from combined high-pressure and high-temperature Raman and infrared measurements.
• Fano line shapes provide direct evidence of enhanced electron-phonon coupling in the tilt-free phase.

Conclusions

• The structural transition from tilted Amam to untilted phase is accompanied by a metallic crossover from a bad metal to a good metal with a two-order-of-magnitude increase in carrier density.
• Superconductivity emerges near the boundary of the coexistence region around 6 GPa, suggesting a filamentary onset that becomes bulk near 15 GPa.
• High symmetry and metallicity alone are insufficient to induce superconductivity; additional electronic correlations such as density-wave fluctuations may be required.
• The upper temperature limit of the Amam phase at ambient pressure is 544 K, establishing a new boundary in the T-P phase diagram.

Main claims

• The structural transition from tilted Amam to tilt-free phase is accompanied by a crossover from bad metal to good metal with two orders of magnitude increase in carrier density.
  • Evidence: Raman shows phonon disappearance and Fano asymmetry; infrared shows plasma frequency increase from 3450 to 32000 cm^-1
• The Amam phase has an upper temperature limit of 544 K at ambient pressure.
  • Evidence: High-temperature Raman shows phonon disappearance at 544K
• Superconductivity emerges near 6-7 GPa at the onset of the tilt-free phase, but the high-symmetry structure and metallicity alone are insufficient for superconductivity.
  • Evidence: Phase diagram shows superconductivity appears at boundary of coexistence region; O₂-annealed samples with tilt-free phase at ambient pressure do not superconduct

Workflow

• sample_preparation — High-quality La₃Ni₂O₇ single crystals were synthesized.
  • Materials: La₂O₃; NiO; single crystal La₃Ni₂O₇
  • Methods: high-pressure optical floating-zone growth; characterization by XRD and SQUID
  • Observations: high-quality single crystals with sharp magnetic transition at 151K
• high_pressure_raman_spectroscopy — Pressure-driven structural transition from tilted Amam to tilt-free phase occurs near 15 GPa.
  • Materials: La₃Ni₂O₇ single crystal; diamond anvil cell
  • Methods: Raman spectroscopy with 532 nm laser; pressure calibration
  • Observations: blueshift of phonon modes; Fano line shape above 5-6 GPa; disappearance of phonons (1) and (3) at 15.25 GPa
• high_temperature_raman_spectroscopy — Upper temperature limit of Amam phase is 544 K at ambient pressure.
  • Materials: La₃Ni₂O₇ single crystal; heating stage
  • Methods: Raman spectroscopy with laser power variation; Stokes/anti-Stokes ratio for temperature determination
  • Observations: redshift of phonon modes; disappearance of phonons (1) and (3) at 544K
• synchrotron_infrared_reflectivity_measurements — Metallic crossover from bad metal to good metal occurs with increased carrier density.
  • Materials: La₃Ni₂O₇ single crystal; diamond anvil cell
  • Methods: synchrotron infrared reflectivity; Drude-Lorentz fitting
  • Observations: plasma frequency increases from 3450 cm^-1 to 32000 cm^-1; carrier density increases by two orders of magnitude
• phase_diagram_construction — Unified T-P phase diagram with tilted Amam, coexistence, and tilt-free phases.
  • Materials: experimental data from Raman and infrared
  • Methods: integration of phonon intensity as proxy for volume fraction; plotting boundaries
  • Observations: coexistence region between 6-15.25 GPa at 300 K and 400-544 K at 0 GPa