Heavily Sr-Doped La₂SrNi₂O₇-δ as a Tetragonal Ruddlesden-Popper Phase at Ambient Pressure

摘要

该研究通过高压高温助熔剂法首次成功合成了重Sr掺杂的La₂SrNi₂O_7-δ单晶。X射线衍射和扫描透射电子显微镜确认，该材料在常压下具有四方I₄/mmm结构，沿c轴Ni−O−Ni键角为180°，表明重掺杂有效抑制了NiO₆八面体畸变。电阻测量显示金属性行为并伴有低温上翘，未观察到密度波特征；但施加压力或改变氧含量均未能诱发超导。密度泛函理论计算表明，Sr掺杂引入的空穴主要进入Ni-3d_z²轨道，在费米面上形成很大的γ口袋，并显著降低该轨道占据，使Ni-3d_z²能带严重偏离半填充态。作者据此认为，这种对半填充态的偏离削弱了层间反铁磁相互作用和配对，是该体系未能出现超导的关键原因。该工作为理解双层镍酸盐中实现超导所需条件提供了重要线索。

材料

• La₂SrNi₂O₇-δ

方法

• single-crystal X-ray diffraction (SXRD)
• powder X-ray diffraction (PXRD)
• Rietveld refinement
• scanning electron microscopy (SEM)
• energy-dispersive X-ray spectroscopy (EDS)
• high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)
• selected area electron diffraction (SAED)
• integrated differential phase contrast (iDPC) imaging
• dc magnetization
• temperature-dependent resistance measurements
• high-pressure electrical resistance measurements
• density functional theory (DFT) calculations
• Wannier downfolding

关键词

• ruddlesden popper nickelate
• hole doping
• tetragonal phase
• density wave suppression
• ni 3dz2 half filling
• interlayer antiferromagnetic coupling
• γ fermi pocket
• superconductivity absence

亮点

• This is the first successful synthesis of heavily Sr-doped La₂SrNi₂O₇-δ single crystals under high-pressure, high-temperature flux conditions.
• Heavy Sr doping suppresses NiO₆ octahedral distortion and stabilizes a tetragonal phase with a straight Ni-O-Ni bond along the c-axis at ambient pressure.
• The precise position of the γ pocket, rather than its mere presence, is proposed to be crucial for superconductivity in bilayer nickelates.
• Complementary strategies of co-doping with Ce and smaller rare-earth ions are proposed as potential routes to stabilize superconductivity at ambient pressure.

结论

• La₂SrNi₂O₇-δ single crystals were successfully synthesized under HPHT conditions and adopt a tetragonal I₄/mmm structure with a 180° Ni-O-Ni bond angle along the c-axis at ambient pressure.
• The material exhibits metallic behavior with a low-temperature upturn and no signatures of density-wave order.
• Neither external pressure nor oxygen variation induces superconductivity in this heavily Sr-doped bilayer nickelate.
• Density functional theory calculations show that holes introduced by Sr doping are predominantly doped into the Ni-3dz2 orbital, yielding a significantly enlarged γ Fermi pocket and reducing the Ni-3dz2 occupation.
• The absence of superconductivity is attributed to a serious deviation from half-filling of the Ni-3dz2 band, which weakens interlayer antiferromagnetic interaction and pairing.

主要论断

• The first successful synthesis of heavily Sr-doped La₂SrNi₂O₇-δ single crystals in a tetragonal I₄/mmm phase at ambient pressure.
  • 证据: SXRD and PXRD refinements show tetragonal I₄/mmm with a=b=3.8264 Å, c=19.9908 Å,STEM/SAED confirm fourfold symmetry and body-centered reflection condition,Ni-O-Ni bond angle is 180° along c-axis
• The tetragonal structure with suppressed density-wave order is not sufficient for superconductivity in this system.
  • 证据: Resistance measurements show metallic behavior and no density-wave features,High-pressure resistance up to 51.9 GPa and oxygen/ozone treatments show no superconductivity,Samples remain non-superconducting despite straight Ni-O-Ni bonds
• Holes introduced by Sr doping predominantly occupy the Ni-3d_z2 orbital, producing an enlarged γ Fermi surface pocket and reducing 3d_z2 occupation.
  • 证据: DFT calculations show holes are mainly doped into Ni-3d_z2,Fermi surface contains α, β electron pockets and a large γ hole pocket at ambient pressure,Ni-3d_z2 occupation is 0.92 compared with 3dx2-y2 occupation 1.08
• The absence of superconductivity is attributed to strong deviation from half-filling of the Ni-3d_z2 band, which weakens interlayer antiferromagnetic coupling and pairing.
  • 证据: Experimental observations show no superconductivity under pressure or oxygen tuning,DFT shows reduced Ni-3d_z2 occupation (0.92) and increased interlayer hopping under pressure,Authors argue reduced occupation weakens interlayer superexchange despite comparable hopping

研究流程

• sample_preparation — Heavily Sr-doped La₂SrNi₂O₇-δ single crystals were synthesized for the first time under HPHT flux conditions.
  • 材料: La₂O₃; SrCO₃; Ni(OH)₂; nitric acid; citric acid; ethylene glycol; LaSrNiO₄; LaNiO₃; SrCl₂; KCl; SrO₂
  • 方法: sol-gel precursor synthesis; high-pressure high-temperature flux growth
  • 观察: single crystals of La₂SrNi₂O₇-δ obtained; EDS La:Sr:Ni ratio 1.86:1.14:2.02
• structural_characterization — The material adopts a tetragonal I₄/mmm structure at ambient pressure with straight Ni-O-Ni bonds along the c-axis.
  • 材料: La₂SrNi₂O₇-δ single crystals; crushed powders
  • 方法: single-crystal X-ray diffraction; powder X-ray diffraction with Rietveld refinement; scanning electron microscopy; energy-dispersive X-ray spectroscopy; HAADF-STEM; selected area electron diffraction; integrated differential phase contrast imaging
  • 观察: tetragonal I₄/mmm structure; a=b=3.8264 Å, c=19.9908 Å; Ni-O-Ni bond angle 180° along c-axis; no octahedral tilting; body-centered reflection condition h+k+l=2n; Sr preferentially occupies La sites
• transport_and_magnetic_measurements — The material shows metallic transport with a low-temperature upturn and no density-wave order.
  • 材料: polycrystalline pellets from crushed single crystals; as-grown single crystals
  • 方法: temperature-dependent resistance measurements; dc magnetization measurements
  • 观察: metallic behavior with low-temperature upturn at 88 K (S₁) and 68 K (S₂); Curie-Weiss paramagnetism; effective moment ≈1.05 μB per formula; no density-wave features in resistance or magnetization
• superconductivity_search — Neither pressure nor oxygen variation induces superconductivity in this tetragonal heavily Sr-doped bilayer nickelate.
  • 材料: as-grown single crystals; polycrystalline pellets; La metal; ozone
  • 方法: oxygen reduction with La metal; ozone annealing; high-pressure electrical resistance measurements in diamond anvil cell
  • 观察: reduced sample S₃ is strongly insulating; ozone-annealed samples S₄/S₅ show decreased resistivity and S₅ metallic above 210K; low-temperature resistance upturn persists to highest pressure; no superconductivity observed under any pressure or oxygen treatment
• electronic_structure_calculation — Sr doping drives the Ni-3d_z2 band away from half-filling, enlarging the γ pocket and weakening interlayer antiferromagnetic superexchange.
  • 材料: La₂SrNi₂O₇ structural model
  • 方法: density functional theory with PBE+U; Wannier downfolding; Fermi surface calculations
  • 观察: holes mainly doped into Ni-3d_z2 orbital; large γ hole pocket appears at ambient pressure; Ni-3d_z2 occupation 0.92 vs Ni-3dx2-y2 occupation 1.08; interlayer 3d_z2 hopping increases from -0.551 eV to -0.648 eV at 25 GPa