Bulk superconductivity in quintuple-layer square-planar nickelate La₆Ni₅O₁₂

Summary

This study achieved bulk superconductivity at ambient pressure by synthesizing the five-layer Ruddlesden-Popper phase La₆Ni₅O₁₆ under high temperature and high pressure conditions, followed by a topological reduction reaction to produce the five-layer square planar (SP) nickelate La₆Ni₅O₁₂. The material exhibits a superconducting transition at approximately 18 K, a property confirmed by zero resistance, diamagnetic shielding, and specific heat measurements, with the latter revealing a superconducting volume fraction of about 25% that strongly supports the presence of bulk superconductivity. Magnetization and torque magnetometry measurements revealed distinct magnetic hysteresis loops, verifying the existence of vortex pinning effects and Type-II superconducting characteristics. This result overcomes the previous limitation that nickelate superconductivity was confined to thin films or high-pressure environments, thereby providing a critical platform for the in-depth investigation of the mechanisms underlying nickel-based high-temperature superconductivity in ambient-pressure bulk samples.

Materials

• La₆Ni₅O₁₂
• La₆Ni₅O₁₆

Methods

• High-temperature, high-pressure synthesis
• Topotactic reduction
• Single-crystal X-ray diffraction
• Powder X-ray diffraction
• HAADF-STEM
• iDPC-STEM
• Electrical resistivity measurement
• Magnetic susceptibility measurement
• Torque magnetometry
• Specific heat measurement
• Thermogravimetric analysis

Keywords

• bulk superconductivity
• quintuple layer nickelate
• square planar structure
• vortex pinning
• type ii superconductivity
• superconducting volume fraction
• oxygen stoichiometry
• strong coupling superconductivity

Highlights

• This work establishes the first realization of bulk superconductivity in nickelate superconductors at ambient pressure, overcoming previous limitations to thin films or high-pressure environments.
• The availability of ambient-pressure superconducting single crystals provides a platform for neutron scattering, NMR, STM, and ARPES without interfacial or strain-related ambiguities.
• The superconducting gap analysis suggests strong-coupling behavior, with a ratio of 2Δ/k_BT_c of 4.8, exceeding weak-coupling BCS values.

Conclusions

• Bulk superconductivity at ambient pressure with an onset transition temperature of ≈18 K is realized in quintuple-layer square-planar nickelate La₆Ni₅O₁₂.
• The superconducting state is verified by zero resistance, diamagnetic shielding, and specific heat measurements, with a superconducting volume fraction of ≈25%.
• Magnetization and torque magnetometry reveal pronounced hysteresis loops, demonstrating the occurrence of vortex pinning and Type-II superconducting characteristics.
• The material exhibits a nominal Ni valence of +1.2, comparable to optimally doped infinite-layer nickelate superconductors.
• Clean superconductivity is stabilized within a narrow topotactic reduction temperature window, with insulating behavior observed at higher reduction temperatures.

Main claims

• Bulk superconductivity at ambient pressure is realized in quintuple-layer square-planar nickelate La₆Ni₅O₁₂ with T_c ≈ 18 K.
  • Evidence: Zero resistance and diamagnetic shielding observed at ≈18 K.,Specific heat anomaly at ≈18 K with superconducting volume fraction of ≈25%.,Magnetic torque and susceptibility measurements confirm transition at ≈18 K.
• The superconducting state is stabilized within a narrow topotactic reduction temperature window.
  • Evidence: Resistivity measurements show superconductivity at 310 °C and 320 °C annealing, but insulating behavior at 330 °C.,Oxygen content analysis correlates with transport behavior changes.
• La₆Ni₅O₁₂ exhibits Type-II superconducting characteristics with significant vortex pinning.
  • Evidence: Pronounced hysteresis loops in magnetization and torque magnetometry.,Upper critical field μ0Hc2 estimated at ≈41.5 T.,Critical current density Jc ≈ 2000 A cm^-2.

Workflow

• Synthesis of Parent Phase — Quintuple-layer Ruddlesden-Popper nickelate La₆Ni₅O₁₆ was successfully synthesized in bulk form.
  • Materials: Lanthanum nitrate; Nickel nitrate; Citric acid; Alumina crucible
  • Methods: Sol-gel precursor preparation; High-temperature, high-pressure six-anvil synthesis
  • Observations: Formation of monoclinic Ruddlesden-Popper La₆Ni₅O₁₆ single crystals
• Topotactic Reduction — Topotactic reduction converts La₆Ni₅O₁₆ to square-planar La₆Ni₅O₁₂, with superconductivity emerging in a narrow temperature window (310-320 °C).
  • Materials: La₆Ni₅O₁₆ crystals; LiAlH₄ (oxygen getter); Quartz tube
  • Methods: Topochemical reduction via thermal decomposition of LiAlH₄
  • Observations: Conversion to tetragonal T'-type La₆Ni₅O₁₂; Removal of apical oxygen atoms
• Structural Characterization — Structural transformation from Ruddlesden-Popper to square-planar phase is confirmed by diffraction and atomic-resolution microscopy.
  • Materials: La₆Ni₅O₁₆ and La₆Ni₅O₁₂ samples
  • Methods: Single-crystal X-ray diffraction; Powder X-ray diffraction; HAADF-STEM and iDPC-STEM imaging; Thermogravimetric analysis (TGA)
  • Observations: Confirmation of monoclinic RP structure for La₆Ni₅O₁₆; Confirmation of tetragonal SP structure for La₆Ni₅O₁₂; Oxygen content determination showing reduction from ≈15.9 to ≈12.7
• Transport and Magnetic Measurements — La₆Ni₅O₁₂ exhibits superconducting transition at ≈18 K with Type-II superconducting characteristics.
  • Materials: La₆Ni₅O₁₂ single crystals
  • Methods: Four-probe AC resistivity; DC magnetic susceptibility; Magnetization hysteresis; Magnetic torque magnetometry
  • Observations: Zero resistance and diamagnetic shielding at ≈18 K; Upper critical field μ0Hc2 ≈ 41.5 T; Hysteresis loops indicating vortex pinning; Torque anomaly at ≈18 K
• Specific Heat Analysis — Specific heat measurements confirm bulk superconductivity with a volume fraction of ≈25% and strong-coupling characteristics.
  • Materials: Polycrystalline La₆Ni₅O₁₂ pellet
  • Methods: Relaxation-time specific heat measurement; BCS α-model fitting; Entropy balance analysis
  • Observations: Specific heat anomaly at ≈18 K; Superconducting volume fraction ≈25%; Mean-field T_c = 17.5 K; Superconducting gap Δ = 3.6 meV