Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇

摘要

The discovery of high-temperature superconductivity in La₃⁢Ni₂⁢O₇ at pressures above 14 GPa has spurred extensive research efforts. Yet, fundamental aspects of the superconducting phase, including the possibility of a filamentary character, are currently subjects of controversial debates. Conversely, a crystal structure with NiO₆ octahedral bilayers stacked along the 𝑐-axis direction was consistently posited in initial studies on La₃⁢Ni₂⁢O₇. Here, we reassess this structure in optical floating zone-grown La₃⁢Ni₂⁢O₇ single crystals that show signs of filamentary superconductivity. Employing scanning transmission electron microscopy and single-crystal x-ray diffraction under high pressures, we observe multiple crystallographic phases in these crystals, with the majority phase exhibiting alternating monolayers and trilayers of NiO₆ octahedra, signifying a profound deviation from the previously suggested bilayer structure. Using density functional theory, we disentangle the individual contributions of the monolayer and trilayer structural units to the electronic band structure of La₃⁢Ni₂⁢O₇, providing a firm basis for advanced theoretical modeling and future evaluations of the potential of the monolayer-trilayer structure for hosting superconductivity.在高于 14 GPa 的压力下， La₃⁢Ni₂⁢O₇ 中高温超导性的发现激发了广泛的研究。然而，超导相的基本性质，包括其丝状特征的可能性，目前仍存在争议。相反，在对 La₃⁢Ni₂⁢O₇ 的早期研究中，人们一直假设其晶体结构为沿 𝑐 轴方向堆叠的 NiO₆ 八面体双层。本文中，我们重新评估了光学浮区法生长的 La₃⁢Ni₂⁢O₇ 单晶中的这种结构，这些单晶表现出丝状超导的迹象。利用扫描透射电子显微镜和高压下的单晶 X 射线衍射技术，我们观察到这些晶体中存在多种晶相，其中主要相由 NiO₆ 八面体的单层和三层交替构成，这表明其与先前提出的双层结构存在显著偏差。利用密度泛函理论，我们分离出单层和三层结构单元对 La₃⁢Ni₂⁢O₇ 电子能带结构的各个贡献，为高级理论建模和未来评估单层-三层结构承载超导性的潜力提供了坚实的基础。

材料

• La₃Ni₂O₇

方法

• STEM
• single-crystal X-ray diffraction under high pressures
• DFT

关键词

• filamentary superconductivity
• alternating monolayers and trilayers
• crystallographic phases
• nio6 octahedra

亮点

• Discovery of a monolayer-trilayer structural polymorph in La₃Ni₂O₇.
• Profound deviation from the proposed bilayer structure.

结论

• The majority phase in La₃Ni₂O₇ single crystals exhibits alternating monolayers and trilayers of NiO₆ octahedra, not the previously suggested bilayer structure.

主要论断

• The majority crystal phase in OFZ-grown La₃Ni₂O₇ single crystals is not the conventional bilayer RP structure but an alternating monolayer-trilayer (ML-TL) stacking.
  • 证据: Full text: STEM images and XRD refinements show alternating ML and TL NiO₆ octahedra.,Abstract: 'the majority phase exhibiting alternating monolayers and trilayers of NiO₆ octahedra, signifying a profound deviation from the previously suggested bilayer structure'
• Superconductivity in this material is filamentary, not bulk, with non-Ohmic behavior and finite residual resistance.
  • 证据: Full text: 'a current-dependent resistance below the superconducting onset is a strong indicator for filamentary superconductivity',Abstract: 'these crystals that show signs of filamentary superconductivity'
• Oxygen deficiency is detrimental to superconductivity; as-grown oxygen-deficient crystals do not show superconductivity up to 17 GPa.
  • 证据: Full text: 'oxygen deficiency is detrimental for the occurrence of superconductivity in this system',Full text: 'Resistance drop remains absent in our high-pressure transport measurements up to 17 GPa on an as-grown crystal'

研究流程

• Crystal Growth and Characterization — Annealed crystals show consistent metallic behavior.
  • 材料: La₃Ni₂O₇ single crystals grown by optical floating zone (OFZ) technique; High oxygen pressure (15 bar)
  • 方法: OFZ synthesis; Annealing at 600°C in 600 bar O₂
  • 观察: Composition La₂.98(1)Ni₁.99(1)O₆.83(7) for as-grown; improved metallicity after annealing
• High-Pressure Transport Measurements — Filamentary superconductivity is observed with strong pressure and field dependence.
  • 材料: Diamond anvil cell (DAC) with CsI pressure medium; Four-point measurement
  • 方法: Electrical resistance vs. temperature and pressure; Field and current dependence
  • 观察: Superconducting onset above 8 GPa with T_c up to 80K; Non-Ohmic behavior below T_drop indicating filamentary superconductivity; Hysteretic behavior between compression and decompression
• Structural Characterization (STEM and XRD) — The crystal structure of La₃Ni₂O₇ is composed of alternating ML and TL units, not bilayer RP stacking.
  • 材料: Annealed and as-grown La₃Ni₂O₇ crystals
  • 方法: Scanning transmission electron microscopy (STEM-HAADF and ABF); Single-crystal X-ray diffraction under high pressure (synchrotron); Membrane-driven DAC with He pressure medium
  • 观察: Majority phase exhibits alternating monolayer (ML) and trilayer (TL) NiO₆ octahedra stacking; Orthorhombic Fmmm at low pressure, tetragonal P₄/mmm at >12.3 GPa; No conventional bilayer stacking detected
• DFT Calculations of Electronic Structure — The electronic structure of ML-TL La₃Ni₂O₇ is unique and distinct from conventional RP bilayer stacking.
  • 材料: DFT for ML-TL structure (P₄/mmm phase at 12.3 GPa)
  • 方法: Density functional theory; Band structure and Fermi surface analysis
  • 观察: Bands near Fermi level mainlyNi 3d character (dx2-y2 and dz2); Fermi surface sheets: α, β, γ, ε and duplications; Distinguishing ε sheet with pure dz2 character from TL unit