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

摘要

该研究通过第一性原理计算和群论分析，系统研究了交替单层-三层（1313）堆叠的La₃Ni₂O₇多晶型的结构稳定性、电子结构与磁性质。常压下，最高对称性Cmmm结构在布里渊区高对称点存在多个不稳定声子支，其对应的畸变可导致实验上报道的另一种空间群Imma，该结构显示出NiO₆八面体倾斜。磁性分析表明，常压下该材料的电子结构主要由三层块主导，单层块处于Mott绝缘态。压力下，四方P₄/mmm结构趋于稳定，与实验一致。研究揭示了八面体倾斜并非超导的必要条件，并阐明了不同空间群之间的对称关联及压力驱动的结构相变机制。

材料

• La₃Ni₂O₇

方法

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

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• 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.
  • 证据: 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.
  • 证据: Abstract: 'Under pressure, the tetragonal P₄/mmm structure becomes stable… Octahedral tilting is not a prerequisite for superconductivity'

研究流程

• Structural stability analysis — Imma is the most stable ambient-pressure structure
  • 材料: La₃Ni₂O₇-1313 polymorph
  • 方法: DFT structural optimization; Phonon dispersion (DFPT); Group theory (ISODISTORT)
  • 观察: 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
  • 材料: Optimized structures
  • 方法: LDA+U calculations for various magnetic configurations
  • 观察: 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
  • 材料: Structures at various pressures
  • 方法: Enthalpy calculations; Phonon dispersion
  • 观察: Tetragonal P₄/mmm becomes stable under pressure