Structural stability, electronic structure, and magnetic properties of the single-layer trilayer La₃Ni₂O₇ polymorph

Summary

This study systematically investigates the structural stability, electronic structure, and magnetic properties of the alternating monolayer-trilayer (1313) stacked La₃Ni₂O₇ polymorphs using first-principles calculations and group theory analysis. At ambient pressure, the highest-symmetry Cmmm structure exhibits multiple unstable phonon branches at high-symmetry points in the Brillouin zone, and the corresponding distortions can lead to another experimentally reported space group Imma, which features NiO₆ octahedral tilting. Magnetic analysis indicates that the electronic structure of this material at ambient pressure is predominantly governed by the trilayer block, whereas the monolayer block is in a Mott insulating state. Under pressure, the tetragonal P₄/mmm structure becomes stable, consistent with experimental observations. The study reveals that octahedral tilting is not a prerequisite for superconductivity and elucidates the symmetry relationships among different space groups as well as the pressure-driven structural phase transition mechanism.

Materials

• La₃Ni₂O₇

Methods

• DFT
• Density functional perturbation theory (DFPT)
• Group theory
• LDA+U

Keywords

• structural stability
• octahedral tilting
• phonon instabilities
• mott insulator
• pressure driven phase transition

Highlights

• The study reveals that octahedral tilting is not a prerequisite for superconductivity.
• Elucidates the symmetry relationships among different space groups and the pressure-driven structural phase transition mechanism.

Conclusions

• The Cmmm structure of La₃Ni₂O₇-1313 exhibits multiple unstable phonon branches; distortions can lead to the Imma space group with octahedral tilts.
• The Imma structure is the most feasible for La₃Ni₂O₇-1313 at ambient pressure, containing octahedral tilts.
• The electronic structure is dominated by the trilayer block, with the single-layer block in a Mott insulating state.
• Under pressure, the tetragonal P₄/mmm structure becomes stable.

Main claims

• The highest-symmetry Cmmm structure at ambient pressure has multiple unstable phonon branches; distortions can lead to the experimentally reported Imma space group with octahedral tilts.
  • Evidence: Abstract: 'At ambient pressure, the highest-symmetry Cmmm structure exhibits multiple unstable phonon branches at high-symmetry points in the Brillouin zone, and the corresponding distortions can lead to another experimentally reported space group Imma'
• Under pressure, the tetragonal P₄/mmm structure becomes stable; octahedral tilting is not a prerequisite for superconductivity.
  • Evidence: Abstract: 'Under pressure, the tetragonal P₄/mmm structure becomes stable… Octahedral tilting is not a prerequisite for superconductivity'

Workflow

• Structural stability analysis — Imma is the most stable ambient-pressure structure
  • Materials: La₃Ni₂O₇-1313 polymorph
  • Methods: DFT structural optimization; Phonon dispersion (DFPT); Group theory (ISODISTORT)
  • Observations: Cmmm structure has multiple unstable phonon branches; Distortions can lead to Imma structure with octahedral tilts
• Electronic and magnetic calculations — Electronic structure at ambient is dominated by trilayer block
  • Materials: Optimized structures
  • Methods: LDA+U calculations for various magnetic configurations
  • Observations: Magnetic ground state: SL: AFM; TL: FM; Single-layer block is Mott insulating; Trilayer block dominates electronic structure near Fermi level
• Pressure evolution — Octahedral tilting is not a prerequisite for superconductivity
  • Materials: Structures at various pressures
  • Methods: Enthalpy calculations; Phonon dispersion
  • Observations: Tetragonal P₄/mmm becomes stable under pressure