Pressure and strain tuning of the alternating bilayer-trilayer Ruddlesden-Popper nickelate: crystal and electronic structure

Summary

Through first-principles calculations, this work investigates the crystal and electronic structures of the mixed bilayer-trilayer Ruddlesden-Popper nickelate La₇Ni₅O₁₇ under hydrostatic pressure and biaxial compressive strain. B_y analyzing the irreducible representations of dynamically unstable phonon modes in the high-symmetryP₄/mmm structure, the authors identify a dynamically stable low-symmetryC₂/c structure characterized by octahedral tilting. Both applied pressure and compressive strain suppress the octahedral tilting, leading to structural tetragonalization, a behavior akin to conventional Ruddlesden-Popper phases. In terms of electronic structure, the overall features under hydrostatic pressure and strain are similar, but a key difference lies in the position of the d_z² bonding band within the trilayer block: at 30 GPa pressure, this band crosses the Fermi level, whereas any magnitude of compressive strain keeps it below the Fermi level. This strain-induced electronic structure variation aligns with observations in conventional bilayer nickelates, offering critical insights into the distinct effects of pressure and strain on superconductivity in this class of materials.

Materials

• La₇Ni₅O₁₇

Methods

• First-principles calculations (DFT)
• Phonon calculations (ALAMODE)
• Group theory analysis

Keywords

• octahedral tilts
• tetragonalization
• dz2 bonding band
• bilayer trilayer hybrid
• fermi surface pocket

Highlights

• First systematic study of pressure and strain effects on the hybrid bilayer-trilayer RP nickelate La₇Ni₅O₁₇.
• Identification of a dynamically stable C₂/c structure with octahedral tilts.
• The dz2 pocket emerges under pressure but is absent under compressive strain, providing a test for the role of this pocket in superconductivity.

Conclusions

• Both pressure and compressive strain suppress octahedral tilts, tetragonalizing the structure.
• The dz2 bonding band from the trilayer block crosses the Fermi level at 30 GPa but remains below for compressive strain.
• The electronic structure under strain mimics that at 15 GPa more than at 30 GPa.
• The extra corner pocket from the trilayer block is fragile and can be shifted by on-site Coulomb repulsion.

Main claims

• Under hydrostatic pressure, the structure becomes tetragonal with octahedral tilts suppressed above 25 GPa.
  • Evidence: Abstract,Full text: Under pressure, the structure becomes tetragonal, with the tilts of oxygen octahedra being completely suppressed at ≈25 GPa.
• The dz2 bonding band from the trilayer block crosses the Fermi level at 30 GPa, but remains below it under compressive strain.
  • Evidence: Abstract,Full text: This band crosses the Fermi level at a pressure of 30 GPa, but it remains below it for any level of compressive strain.
• The electronic structure under strain is similar to that at 15 GPa, not 30 GPa, due to differences in lattice parameters.
  • Evidence: Full text,Section III C: The electronic structure at this strain mimics better that obtained at 15 GPa.

Workflow

• structural_optimization
  • Materials: La₇Ni₅O₁₇
  • Methods: DFT with Quantum ESPRESSO
  • Observations: C₂/c structure with octahedral tilts at ambient; tetragonalization under pressure and strain
• phonon_analysis
  • Methods: frozen-phonon method (ALAMODE); group theory analysis
  • Observations: dynamically unstable modes at R point; mode hardening with pressure
• electronic_structure_calculation
  • Methods: DFT band structure; Fermi surface analysis
  • Observations: dz2 bonding band from trilayer crosses Fermi level at 30 GPa but not under strain
• comparison
  • Methods: hopping parameter extraction
  • Observations: hoppings similar to isolated bilayer and trilayer