Pressure-tunable structural instabilities in single-layer-trilayer La₃Ni₂O₇.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Layered nickelates are candidates for superconductivity, analogous to cuprates.
• The exact crystal structure of superconducting nickelates remains unclear.
• La3Ni2O7 exhibits superconductivity, but its structural properties under pressure need clarification.

Purpose of the Study:

• Investigate pressure-dependent structural instabilities in single-layer-trilayer La3Ni2O7.
• Clarify the crystal structure of superconducting layered nickelates.
• Compare theoretical predictions with experimental findings.

Main Methods:

• First principles calculations
• Phonon band structure analysis
• Group-theoretical analysis of instabilities
• Structural relaxation simulations

Main Results:

• Identified a nearly dispersionless, unstable phonon branch in the parent P4/mmm phase of La3Ni2O7.
• Observed additional doubly-degenerate instabilities at lower pressures.
• Theoretical models predict low-energy structures involving both nondegenerate and doubly-degenerate instabilities, contrasting with experimental data.
• Found structural distortions energetically favorable at 20 GPa, contrary to experimental observations.

Conclusions:

• The study provides a theoretical framework for understanding structural instabilities in La3Ni2O7 under pressure.
• Discrepancies between theoretical predictions and experimental results highlight areas for further investigation.
• The findings contribute to clarifying the complex relationship between structure and superconductivity in layered nickelates.