Ambient-pressure superconductivity in bulk single crystals of La₅Ni₄O₁₀

Summary

In this study, bulk single crystals of the four-layer Ruddlesden–Popper nickelate La₅Ni₄O₁₃ were grown by a high-pressure high-temperature method, and the synthesis of the five-layer La₆Ni₅O₁₆ was reproduced; La₅Ni₄O₁₃ single crystals were then topotactically reduced with CaH₂ to yield the square-planar phase La₅Ni₄O₁₀. This reduced phase behaves as a bulk superconductor at ambient pressure, with an onset transition temperature T_c^onset of approximately 18 K and a zero-field-cooled shielding fraction exceeding 60%, while a specific-heat jump confirms a bulk pairing transition; superconductivity coexists with density-wave or metal–insulator transitions in all crystals. The upper critical field anisotropy γ≈2.4 remains finite as T→T_c, indicating three-dimensional superconducting characteristics and significant coupling between the four-layer blocks. On this basis, the authors propose that square-planar nickelates R_n+1Ni_nO_2n+2 can enter the superconducting regime solely by tuning the Ni valence through the layer number, without chemical substitution, applied pressure, or epitaxial constraint.

Materials

• La₅Ni₄O₁₃
• La₆Ni₅O₁₆
• La₅Ni₄O₁₀

Methods

• high-pressure high-temperature (HPHT) growth
• Walker-type multi-anvil module
• topotactic reduction with CaH₂
• single-crystal X-ray diffraction (XRD)
• powder XRD with Rietveld refinement
• energy-dispersive X-ray spectroscopy (EDX)
• thermogravimetric analysis (TGA)
• magnetization measurements (SQUID)
• transport measurements (PPMS)
• specific heat measurements

Keywords

• ambient pressure superconductivity
• ruddlesden popper nickelates
• square planar phase
• density wave transition
• metal insulator transition
• upper critical field anisotropy
• three dimensional superconductivity
• bulk pairing transition
• self doping
• interlayer coupling

Highlights

• Bulk single crystals of the quadruple-layer RP nickelate La₅Ni₄O₁₃ were grown by HPHT, and the quintuple-layer La₆Ni₅O₁₆ synthesis was reproduced, two members previously accessible only as thin films.
• Topotactic reduction preserved single crystallinity, in contrast to the reduction of perovskite LaNiO₃ where extensive domain formation limits retained crystallinity.
• The two-component structure of the specific-heat anomaly is attributed to regions of differing local oxygen content rather than an intrinsic multi-gap pairing state.
• The out-of-plane coherence length exceeds the separation of neighboring four-layer blocks, indicating substantial interlayer coupling across the La–O spacer.
• The availability of bulk single crystals opens the door to probes such as inelastic neutron scattering, muon spin rotation, and ARPES.

Conclusions

• Topotactic reduction of La₅Ni₄O₁₃ single crystals with CaH₂ yields the square-planar phase La₅Ni₄O₁₀, which is a bulk ambient-pressure superconductor with T_c^onset ≈ 18 K and a shielding fraction above 60%.
• A specific-heat jump confirms a bulk pairing transition, with superconducting volume fractions of about 60% from shielding and 56% from the specific-heat jump.
• All crystals show coexistence of superconductivity with a density-wave or metal-insulator transition.
• The upper critical field shows a small anisotropy γ ≈ 2.4 that remains finite as T → T_c, indicating three-dimensional superconductivity with substantial coupling between the four-layer blocks.
• The square-planar nickelates R_n+1Ni_nO_2n+2 form a bulk platform in which the layer number alone tunes the Ni valence into the superconducting regime without chemical substitution, pressure, or epitaxial constraint.

Main claims

• Bulk single crystals of the quadruple-layer RP nickelate La₅Ni₄O₁₃ and quintuple-layer La₆Ni₅O₁₆ can be grown by HPHT synthesis.
  • Evidence: single-crystal XRD reciprocal-space maps show sharp reflections without intergrowth,Rietveld refinement of PXRD reproduces a single phase,c-axis increases from 18.0299(10) Å to 21.806(2) Å from n=4 to n=5
• Topotactic reduction of La₅Ni₄O₁₃ with CaH₂ yields the square-planar phase La₅Ni₄O₁₀ while preserving single crystallinity.
  • Evidence: single-crystal XRD maps and BSE imaging after reduction,EDX stoichiometry La5.0(3)Ni3.9(6)O9.5(17),TGA mass loss of 3.11 wt% consistent with near-complete reduction,linear c-axis contraction trend across the reduced series
• La₅Ni₄O₁₀ is a bulk ambient-pressure superconductor with T_c^onset ≈ 18 K.
  • Evidence: zero-field-cooled shielding fraction above 60% at 10 Oe,resistive transition with 98% resistance drop,specific-heat jump at T_c consistent with bulk pairing
• Superconductivity in La₅Ni₄O₁₀ coexists with a density-wave or metal-insulator-like transition.
  • Evidence: resistive anomaly near 90 K with hysteresis and no field dependence,anomaly at 150 K in as-grown La₅Ni₄O₁₃ transport and susceptibility
• La₅Ni₄O₁₀ is a three-dimensional anisotropic superconductor with substantial coupling between four-layer blocks.
  • Evidence: upper critical field anisotropy γ ≈ 2.4 remains finite as T→T_c,α ≈ −0.95 from power-law fit rather than 2D behavior,out-of-plane coherence length exceeds inter-block spacing
• The layer number alone tunes Ni valence into the superconducting regime without chemical substitution, pressure, or epitaxial constraint.
  • Evidence: superconductivity realized in bulk reduced La₅Ni₄O₁₀ at ambient pressure,systematic trend across reduced RP series with layer number

Workflow

• precursor_synthesis — Homogeneous cation mixing via sol–gel is essential for single-phase high-order RP nickelates.
  • Materials: La₂O₃; Ni(NO₃)₂·6H₂O; HNO₃; deionized water; citric acid
  • Methods: sol–gel route; auto-combustion; grinding; oxygen annealing at 800 °C
  • Observations: ash-like powder obtained; residual organics removed
• high_pressure_crystal_growth — HPHT synthesis yields phase-pure bulk single crystals of quadruple- and quintuple-layer RP nickelates, with precursor stoichiometry rather than pressure selecting the member.
  • Materials: sol–gel precursor; Pt capsule; LaCrO₃ heater; 18/11 octahedral pressure assembly
  • Methods: Walker-type multi-anvil HPHT synthesis; melting and slow cooling; quenching
  • Observations: bulk single crystals of La₅Ni₄O₁₃ obtained; La₆Ni₅O₁₆ obtained with La:Ni 7:6 precursor; pressure 5 GPa and La:Ni 5:4 still gave La₅Ni₄O₁₃
• structural_characterization — The HPHT product is a macroscopically homogeneous quadruple-layer phase, not an intergrowth.
  • Materials: as-grown single crystals; crushed crystals
  • Methods: single-crystal XRD reciprocal-space mapping; Rietveld refinement of PXRD
  • Observations: sharp reflections without intergrowth; monoclinic space group with two inequivalent Ni sites; c axis increases from 18.0299(10) Å (n=4) to 21.806(2) Å (n=5)
• topotactic_reduction — Topotactic reduction of La₅Ni₄O₁₃ yields the square-planar phase La₅Ni₄O₁₀ essentially to completion while preserving single crystallinity.
  • Materials: La₅Ni₄O₁₃ single crystals; CaH₂
  • Methods: sealed-ampoule topotactic reduction; direct-contact and indirect gas-phase reduction
  • Observations: single crystallinity preserved (XRD, BSE); linear c-axis contraction across the series; EDX stoichiometry La5.0(3)Ni3.9(6)O9.5(17); TGA mass loss 3.11 wt% corresponding to near-complete reduction
• magnetization_measurements — Reduced La₅Ni₄O₁₀ is a bulk ambient-pressure superconductor with T_c ≈ 18 K and shielding fraction above 60%.
  • Materials: reduced La₅Ni₄O₁₀ crystals; quartz holders
  • Methods: DC SQUID magnetometry; ZFC/FC shielding; field-dependent hysteresis at 2 K
  • Observations: T_c^onset ≈ 18 K independent of reduction conditions; shielding fraction above 60%; lower critical field Hc1 ≈ 200 Oe; ferromagnetic Ni impurity background subtracted
• transport_measurements — La₅Ni₄O₁₀ is an anisotropic but three-dimensional superconductor with substantial coupling between four-layer blocks, coexisting with a density-wave or metal-insulator-like transition.
  • Materials: reduced single crystals; silver epoxy contacts
  • Methods: four-probe van der Pauw resistivity; field-dependent resistance
  • Observations: superconducting transition with 98% resistance drop; resistive anomaly near 90 K with hysteresis and no field dependence; upper critical field anisotropy γ ≈ 2.4 remaining finite as T→T_c; coherence lengths exceeding inter-block spacing
• specific_heat_measurements — Specific heat provides thermodynamic evidence of bulk superconductivity in about half of the sample, with inhomogeneous oxygen reduction explaining the two-component anomaly.
  • Materials: several single crystals on sapphire platform; Apiezon N grease
  • Methods: PPMS thermal-relaxation heat capacity; Debye-plus-two-Einstein phonon model; two-τ relaxation analysis
  • Observations: clear specific-heat jump at T_c; ΔC/γT_c close to weak-coupling BCS value for the superconducting fraction; superconducting volume fraction ≈56%; broad transition with shoulder near 12 K
• interpretation — Square-planar nickelates R_n+1Ni_nO_2n+2 form a bulk platform where layer number self-dopes Ni valence into superconductivity.
  • Methods: comparison with thin-film and high-pressure nickelates; analysis of anisotropy and dimensionality
  • Observations: layer number alone tunes Ni valence into the superconducting regime; no chemical substitution, pressure, or epitaxial constraint needed