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

Summary

This study reports the first successful synthesis of heavily Sr-doped La₂SrNi₂O_7-δ single crystals by a high-pressure, high-temperature flux method. X-ray diffraction and scanning transmission electron microscopy confirm that the material adopts a tetragonal I₄/mmm structure at ambient pressure, with a Ni−O−Ni bond angle of 180° along the c-axis, indicating that heavy doping effectively suppresses the distortion of NiO₆ octahedra. Resistivity measurements reveal metallic behavior with a low-temperature upturn, and no density-wave features are observed; however, neither applying pressure nor varying the oxygen content induces superconductivity. Density functional theory calculations show that holes introduced by Sr doping predominantly enter the Ni-3d_z² orbital, creating a large γ pocket on the Fermi surface and markedly reducing the occupation of this orbital, so that the Ni-3d_z² band deviates strongly from half-filling. The authors accordingly argue that this departure from half-filling weakens interlayer antiferromagnetic interactions and pairing, which is the key reason superconductivity does not emerge in this system. This work provides important clues for understanding the conditions required to realize superconductivity in bilayer nickelates.

Materials

• La₂SrNi₂O₇-δ

Methods

• 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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• The first successful synthesis of heavily Sr-doped La₂SrNi₂O₇-δ single crystals in a tetragonal I₄/mmm phase at ambient pressure.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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

Workflow

• sample_preparation — Heavily Sr-doped La₂SrNi₂O₇-δ single crystals were synthesized for the first time under HPHT flux conditions.
  • Materials: La₂O₃; SrCO₃; Ni(OH)₂; nitric acid; citric acid; ethylene glycol; LaSrNiO₄; LaNiO₃; SrCl₂; KCl; SrO₂
  • Methods: sol-gel precursor synthesis; high-pressure high-temperature flux growth
  • Observations: 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.
  • Materials: La₂SrNi₂O₇-δ single crystals; crushed powders
  • Methods: 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
  • Observations: 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.
  • Materials: polycrystalline pellets from crushed single crystals; as-grown single crystals
  • Methods: temperature-dependent resistance measurements; dc magnetization measurements
  • Observations: 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.
  • Materials: as-grown single crystals; polycrystalline pellets; La metal; ozone
  • Methods: oxygen reduction with La metal; ozone annealing; high-pressure electrical resistance measurements in diamond anvil cell
  • Observations: 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.
  • Materials: La₂SrNi₂O₇ structural model
  • Methods: density functional theory with PBE+U; Wannier downfolding; Fermi surface calculations
  • Observations: 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