Ambient-pressure superconductivity in bulk single crystals of La₅Ni₄O₁₀

摘要

本研究采用高压高温法生长四层Ruddlesden–Popper镍酸盐La₅Ni₄O₁₃块体单晶，并复现五层La₆Ni₅O₁₆的合成；随后以CaH₂对La₅Ni₄O₁₃单晶进行拓扑还原，得到方平面相La₅Ni₄O₁₀。该还原相在常压下表现为块体超导体，起始转变温度T_c^onset约18 K，零场冷屏蔽分数超过60%，比热跃变证实发生块体配对转变；同时所有晶体中超导与密度波或金属—绝缘转变共存。上临界场各向异性γ≈2.4且在T→T_c时仍保持有限，表明超导具有三维特征且四层块之间存在显著耦合。作者据此提出，方平面镍酸盐R_n+1Ni_nO_2n+2可仅通过层数调控镍价态进入超导区，无需化学替代、外加压力或外延约束。

材料

• La₅Ni₄O₁₃
• La₆Ni₅O₁₆
• La₅Ni₄O₁₀

方法

• 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

关键词

• 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

亮点

• 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.

结论

• 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.

主要论断

• Bulk single crystals of the quadruple-layer RP nickelate La₅Ni₄O₁₃ and quintuple-layer La₆Ni₅O₁₆ can be grown by HPHT synthesis.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.
  • 证据: superconductivity realized in bulk reduced La₅Ni₄O₁₀ at ambient pressure,systematic trend across reduced RP series with layer number

研究流程

• precursor_synthesis — Homogeneous cation mixing via sol–gel is essential for single-phase high-order RP nickelates.
  • 材料: La₂O₃; Ni(NO₃)₂·6H₂O; HNO₃; deionized water; citric acid
  • 方法: sol–gel route; auto-combustion; grinding; oxygen annealing at 800 °C
  • 观察: 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.
  • 材料: sol–gel precursor; Pt capsule; LaCrO₃ heater; 18/11 octahedral pressure assembly
  • 方法: Walker-type multi-anvil HPHT synthesis; melting and slow cooling; quenching
  • 观察: 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.
  • 材料: as-grown single crystals; crushed crystals
  • 方法: single-crystal XRD reciprocal-space mapping; Rietveld refinement of PXRD
  • 观察: 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.
  • 材料: La₅Ni₄O₁₃ single crystals; CaH₂
  • 方法: sealed-ampoule topotactic reduction; direct-contact and indirect gas-phase reduction
  • 观察: 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%.
  • 材料: reduced La₅Ni₄O₁₀ crystals; quartz holders
  • 方法: DC SQUID magnetometry; ZFC/FC shielding; field-dependent hysteresis at 2 K
  • 观察: 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.
  • 材料: reduced single crystals; silver epoxy contacts
  • 方法: four-probe van der Pauw resistivity; field-dependent resistance
  • 观察: 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.
  • 材料: several single crystals on sapphire platform; Apiezon N grease
  • 方法: PPMS thermal-relaxation heat capacity; Debye-plus-two-Einstein phonon model; two-τ relaxation analysis
  • 观察: 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.
  • 方法: comparison with thin-film and high-pressure nickelates; analysis of anisotropy and dimensionality
  • 观察: layer number alone tunes Ni valence into the superconducting regime; no chemical substitution, pressure, or epitaxial constraint needed