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

摘要

本文通过第一性原理计算研究了新合成的双无限层氧氟化物La₃Ni₂O₅F的结构稳定性、电子结构与磁性，并探讨了化学压力与外延应变的影响。声子谱证实了该晶体结构的动力学稳定性，其电子结构呈现高度二维的类铜氧化物费米面，主要由Ni-d_(x²-y²)态主导，伴随稀土离子自掺杂导致的~d^(1.2)有效填充，电荷转移能接近铜氧化物区间。化学压力和外延应变下，这类电子特征保持显著稳健，镍-d_(x²-y²)填充仅发生适度变化。自旋极化计算揭示出多种面内与面间自旋排列的近简并磁构型，压应变进一步增强磁阻挫，却不显著改变电子结构。因此，La₃Ni₂O₅F被确立为有前景的铜氧化物类似物，晶格工程可有效精细调控其电子与磁学性质。

材料

• La₃Ni₂O₅F
• La₂SmNi₂O₅F

方法

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

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• The crystal structure of La₃Ni₂O₅F is dynamically stable.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.

研究流程

• Structural stability analysis — The reported double infinite-layer crystal structure of La₃Ni₂O₅F is dynamically stable.
  • 材料: Experimentally determined crystal structure of La₃Ni₂O₅F [13]
  • 方法: Phonon spectrum calculation using the finite-displacement method (phonopy); DFT structural relaxation (VASP)
  • 观察: 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.
  • 材料: Optimized La₃Ni₂O₅F crystal structure
  • 方法: Non-magnetic DFT band structure and density of states (VASP); Fermi surface analysis using ifermi
  • 观察: 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.
  • 材料: La₂SmNi₂O₅F (Sm substitution for chemical pressure); Strain-modified structures: in-plane lattice parameters fixed from -2% to +2%
  • 方法: DFT band structure calculations under strain and chemical substitution; Static electronic susceptibility χ0(q) calculation
  • 观察: 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.
  • 材料: La₃Ni₂O₅F and La₂SmNi₂O₅F supercells
  • 方法: Spin-polarized DFT (LDA, PBE, LDA+U, PBE+U) comparing NM, FM, A-AFM, C-AFM, G-AFM configurations
  • 观察: 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