Structural stability, electronic structure, and magnetism of the d9 double infinite-layer La₃Ni₂O₅F under chemical pressure and epitaxial strain

Summary

Using first-principles calculations, this study investigates the structural stability, electronic structure, and magnetism of the newly synthesized double-infinite-layer oxyfluoride La₃Ni₂O₅F, along with the effects of chemical pressure and epitaxial strain. The phonon spectrum confirms the dynamical stability of the crystal structure, while the electronic structure exhibits a highly two-dimensional, cuprate-like Fermi surface dominated by Ni-d_(x²−y²) states, with an effective filling of approximately d1.2 resulting from self-doping by rare-earth ions, and a charge-transfer energy close to the cuprate regime. These electronic features remain remarkably robust under chemical pressure and epitaxial strain, with only moderate changes in the Ni-d_(x²−y²) filling. Spin-polarized calculations reveal multiple near-degenerate magnetic configurations with various in-plane and out-of-plane spin arrangements, and compressive strain further enhances magnetic frustration without significantly altering the electronic structure. Consequently, La₃Ni₂O₅F is established as a promising cuprate analog, and lattice engineering can effectively provide fine-tuning of its electronic and magnetic properties.

Materials

• La₃Ni₂O₅F
• La₂SmNi₂O₅F

Methods

• DFT
• Phonon calculations
• Electronic susceptibility
• Spin-polarized calculations
• LDA+U
• PBE+U

Keywords

• cuprate like
• self doping
• magnetic frustration
• charge transfer energy
• two dimensional fermi surface
• chemical pressure
• epitaxial strain

Highlights

• The double infinite-layer oxyfluoride La₃Ni₂O₅F bridges single-layer T′ oxyfluorides and infinite-layer nickelates, preserving a clean cuprate-like Fermi surface with moderate self-doping.
• The charge-transfer energy is reduced compared to other nickelates, bringing the system closer to the cuprate regime.
• Despite the bilayer architecture, negligible bonding-antibonding splitting of the Ni-dx2-y2 bands is observed, retaining a canonical two-dimensional fermiology.
• An extended manifold of nearly degenerate collinear magnetic states is discovered, reminiscent of the magnetic frustration in bulk FeSe.
• Lattice engineering via epitaxial strain and chemical pressure enables fine-tuning of self-doping and magnetic competition without degrading the cuprate-like electronic properties.

Conclusions

• The reported double infinite-layer crystal structure of La₃Ni₂O₅F is dynamically stable, as confirmed by the absence of imaginary phonon frequencies.
• La₃Ni₂O₅F exhibits a strongly cuprate-like electronic structure with a highly two-dimensional, weakly hybridized Ni-dx2-y2 Fermi surface and moderate self-doping from rare-earth-derived states.
• These electronic features are remarkably robust under chemical pressure and epitaxial strain, which provide controlled means to fine-tune the carrier balance.
• La₃Ni₂O₅F shows a strong tendency toward magnetism with an extended manifold of nearly degenerate magnetic configurations, forming a frustrated quasi-two-dimensional magnetic landscape.
• Epitaxial strain further enhances the magnetic near-degeneracy and frustration without significantly altering the local moments or the underlying electronic structure.

Main claims

• The crystal structure of La₃Ni₂O₅F is dynamically stable.
  • Evidence: The calculated phonon spectrum confirms the dynamical stability of the reported double infinite-layer crystal structure.,No imaginary frequencies are observed across the entire Brillouin zone, confirming that the structure is stable.
• La₃Ni₂O₅F exhibits a strongly cuprate-like electronic structure: a highly two-dimensional Fermi surface dominated by Ni-d(x2-y2), moderate self-doping, and a charge-transfer energy close to the cuprate regime.
  • Evidence: We find a highly two-dimensional cuprate-like Fermi surface dominated by Ni-d(x2-y2) states, with a moderate rare-earth-derived self-doping yielding an effective ≈ d1.2 filling.,The charge-transfer energy Δ ≈ 2.5 eV is significantly lower than in infinite-layer nickelates (≈4 eV) and closer to the cuprate regime (≈1.5 eV).
• These electronic features remain remarkably robust under both chemical pressure and epitaxial strain, providing a practical route to fine-tune carrier balance.
  • Evidence: These electronic features remain remarkably robust under both chemical pressure and epitaxial strain.,The rare-earth-derived conduction bands are substantially more strain-sensitive, making epitaxial strain an effective means of controlling the self-doping.
• La₃Ni₂O₅F hosts a highly frustrated magnetic landscape with near-degenerate collinear configurations, further enhanced by compressive strain.
  • Evidence: Spin-polarized calculations reveal an extended manifold of nearly degenerate magnetic configurations with different in-plane and out-of-plane spin arrangements.,Compressive strain further enhances this magnetic frustration while leaving the underlying electronic structure largely unchanged.
• Lattice engineering (chemical pressure and epitaxial strain) is an effective strategy for fine-tuning the electronic and magnetic properties of fluorinated nickelates.
  • Evidence: Our results thus identifyLa₃Ni₂O₅F as a promising cuprate analogue and establish lattice engineering as an effective strategy for fine tuning its electronic and magnetic properties.

Workflow

• Structural stability analysis — The reported double infinite-layer crystal structure of La₃Ni₂O₅F is dynamically stable.
  • Materials: Experimentally determined crystal structure of La₃Ni₂O₅F [13]
  • Methods: Phonon spectrum calculation using the finite-displacement method (phonopy); DFT structural relaxation (VASP)
  • Observations: No imaginary frequencies across the entire Brillouin zone; Structure remains dynamically stable under epitaxial strain
• Nonmagnetic electronic structure calculation — La₃Ni₂O₅F exhibits a cuprate-like electronic structure with a dominant Ni-d(x2-y2) 2D Fermi surface and moderate self-doping.
  • Materials: Optimized La₃Ni₂O₅F crystal structure
  • Methods: Non-magnetic DFT band structure and density of states (VASP); Fermi surface analysis using ifermi
  • Observations: Low-energy states dominated by Ni-d(x2-y2) character; Highly two-dimensional Fermi surface with Ni-d(x2-y2) cylinders and a La-5d pocket at Γ; Charge-transfer energy Δ ≈ 2.5 eV, lower than infinite-layer nickelates; Effective Ni-d(x2-y2) filling ≈ d1.2 from self-doping
• Chemical pressure and epitaxial strain effects — The electronic structure is remarkably robust against chemical pressure and epitaxial strain, allowing fine-tuning of self-doping without inducing structural instabilities.
  • Materials: La₂SmNi₂O₅F (Sm substitution for chemical pressure); Strain-modified structures: in-plane lattice parameters fixed from -2% to +2%
  • Methods: DFT band structure calculations under strain and chemical substitution; Static electronic susceptibility χ0(q) calculation
  • Observations: Ni-d(x2-y2) band structure remains robust under both perturbations; Rare-earth-derived bands shift, enabling continuous tuning of self-doping; χ0(q) peaks along Γ-M shift with strain, but no Kohn anomaly or structural instability appears
• Magnetic calculations — La₃Ni₂O₅F exhibits a frustrated quasi-two-dimensional magnetic landscape with an extended manifold of near-degenerate collinear states.
  • Materials: La₃Ni₂O₅F and La₂SmNi₂O₅F supercells
  • Methods: Spin-polarized DFT (LDA, PBE, LDA+U, PBE+U) comparing NM, FM, A-AFM, C-AFM, G-AFM configurations
  • Observations: Nearly degenerate magnetic configurations with different in-plane and out-of-plane spin arrangements; G-AFM ground state only meV/Ni below nonmagnetic state within PBE; Weak interlayer magnetic coupling; magnetic frustration enhanced by compressive strain