Expanding the trilayer Ruddlesden-Popper nickelate family: Synthesis and characterization of Sm₄Ni₃O₁₀-δ single crystals

摘要

本文报道了利用高压高温（3.25 GPa、1400 ℃）助熔剂法首次成功合成钐基三层Ruddlesden-Popper镍酸盐Sm₄Ni₃O_10-δ单晶。单晶X射线衍射和粉末衍射精修表明其结构为正交Pbca空间群，沿c轴Ni−O−Ni键角约152.4°，明显偏离180°。磁化与输运测量在常压下一致观测到约180 K的密度波转变；施加高达80 GPa压力可部分抑制绝缘行为和密度波序，但未观察到超导。密度泛函理论计算显示3d_z²和3dx²−y²轨道与其他t₂g轨道分离，是费米面的主要贡献。该结果表明Ni−O−Ni键角的严重偏离可能阻碍超导出现，新合成的Sm₄Ni₃O_10-δ为研究RP镍酸盐基础物理提供了独特平台。

材料

• Sm₄Ni₃O₁₀-δ

方法

• single-crystal X-ray diffraction (SXRD)
• powder X-ray diffraction (PXRD) with Rietveld refinement
• scanning electron microscopy (SEM)
• energy-dispersive X-ray spectroscopy (EDS)
• dc magnetization
• temperature-dependent resistance measurements
• high-pressure electrical resistance measurements
• density functional theory (DFT) calculations

关键词

• density wave transition
• ni o ni bond angle
• octahedral distortion
• tolerance factor
• fermi surface
• eg t2g orbital separation
• superconductivity absence
• ruddlesden popper nickelate

亮点

• This is the first successful synthesis of the samarium-based trilayer Ruddlesden-Popper nickelate Sm₄Ni₃O₁₀-δ.
• Sm₄Ni₃O₁₀-δ represents a trilayer RP nickelate with the smallest rare-earth ion synthesized to date.
• The HPHT flux growth method overcomes the limitations of ambient-pressure synthesis for small rare-earth RP phases.
• The distortion mediated by the tolerance factor is proposed as a key design parameter for tailoring electronic states and superconductivity.
• Sm₄Ni₃O₁₀-δ is highlighted as a unique platform for probing the fundamental physics of RP nickelates.

结论

• Sm₄Ni₃O₁₀-δ single crystals were successfully synthesized by a high-pressure, high-temperature flux method.
• The crystal adopts an orthorhombic Pbca structure with a c-axis Ni-O-Ni bond angle of about 152.4°, which deviates significantly from 180°.
• Magnetization and transport measurements consistently reveal a density-wave transition at about 180 K at ambient pressure.
• Under pressures up to 80 GPa, the insulating behavior and density-wave order are partially suppressed, but superconductivity is not observed.
• Density functional theory calculations show that Ni eg orbitals (3dz2 and 3dx2-y2) are separated from the t2g orbitals and dominate the Fermi surface.
• The strong NiO₆ octahedral distortion is suggested to hinder the emergence of superconductivity.

主要论断

• First successful high-pressure high-temperature synthesis of Sm₄Ni₃O₁₀-δ single crystals.
  • 证据: Abstract: 'Here we report the first successful high-pressure and high-temperature (HPHT) synthesis of samarium-based compound Sm₄Ni₃O₁₀-δ.',Full text: 'We report the successful HPHT (3.25 GPa, 1400°C) synthesis of Sm₄Ni₃O₁₀-δ single crystals',Experimental Section describes KCl/KClO₄ flux growth under 3.25 GPa.
• Sm₄Ni₃O₁₀-δ adopts an orthorhombic Pbca structure with c-axis Ni–O–Ni bond angle of about 152.4°, much smaller than 180°.
  • 证据: SXRD refinement gives orthorhombic Pbca (No. 61) space group,Full text: 'The Ni−O−Ni bond angle along the c axis is about 152.4(10)°',PXRD Rietveld refinement supports the Pbca model over P₂₁/a
• Magnetization and transport measurements consistently reveal a density-wave transition at about 180 K at ambient pressure.
  • 证据: Resistance curve shows an anomalous kink at approximately 180 K,Polycrystalline pellet resistivity shows a kink at ≈180 K,Magnetic susceptibility and 1/χ show a transition at ≈180K
• Applying pressure up to 80 GPa partially suppresses insulating behavior and density-wave order but does not produce superconductivity.
  • 证据: Resistance anomaly weakens and becomes almost invisible above 18.2 GPa,Resistance decreases with pressure, but no superconducting transition is observed up to 80 GPa,Abstract: 'despite partial suppression of insulating behavior and the DW order, but superconductivity is not observed'
• Density functional theory calculations indicate that 3d_z² and 3dx²-y² orbitals are separated from t2g orbitals and dominate the Fermi surface.
  • 证据: Band structure and DOS show Ni eg orbitals broadly distributed across the Fermi level,Full text: 'the 3d_z² and 3dx²−y² orbitals are separated from the other t2g orbitals and are the main contributors to the Fermi surface'
• The severe Ni–O–Ni bond angle deviation may hinder superconductivity in Sm₄Ni₃O₁₀-δ.
  • 证据: Sm₄Ni₃O₁₀-δ has c-axis Ni–O–Ni bond angle about 152.4°, versus 165.6° in La₄Ni₃O₁₀-δ,No superconductivity observed up to 80 GPa despite pressure suppressing density-wave order,Authors compare with La and Pr trilayer nickelates, where smaller bond angle corresponds to higher required pressure for superconductivity

研究流程

• sample_preparation — First successful HPHT synthesis of samarium-based trilayer Ruddlesden-Popper nickelate Sm₄Ni₃O₁₀-δ single crystals.
  • 材料: Sm₂O₃; Ni(OH)₂; citric acid; ethylene glycol; KCl flux; KClO₄
  • 方法: modified sol-gel precursor synthesis; high-pressure high-temperature flux growth; KClO₄ oxygen-source optimization
  • 观察: Sm₄Ni₃O₁₀-δ single crystals obtained; rectangular crystals about 100 × 100 × 40 µm³; EDS Sm:Ni ratio about 4:2.82; minor Sm₂NiO₄ impurity about 9.827 wt% from Rietveld refinement
• structural_characterization — Sm₄Ni₃O₁₀-δ adopts an orthorhombic Pbca structure with severely reduced c-axis Ni–O–Ni bond angle of about 152.4°.
  • 材料: Sm₄Ni₃O₁₀-δ single crystals; powder ground from single crystals
  • 方法: single-crystal X-ray diffraction; powder X-ray diffraction with Rietveld refinement; scanning electron microscopy and energy-dispersive X-ray spectroscopy
  • 观察: orthorhombic Pbca space group; lattice parameters a=5.3844(4) Å, b=5.4062(5) Å, c=27.287(2) Å; c-axis Ni–O–Ni bond angle about 152.4(10)°; higher symmetry than monoclinic P₂₁/a of La, Pr, Nd trilayer nickelates
• ambient_pressure_transport_and_magnetic_measurements — Ambient-pressure magnetization and transport consistently show a density-wave transition at approximately 180 K with an insulating ground state.
  • 材料: stacked Sm₄Ni₃O₁₀-δ single crystals; Sm₄Ni₃O₁₀-δ polycrystalline pellet annealed at 500 °C under 8 MPa O₂
  • 方法: temperature-dependent resistance measurement; dc magnetization and susceptibility measurement; magnetic hysteresis loop measurement
  • 观察: density-wave transition at about 180 K in resistance and susceptibility; weak insulating behavior down to 2K; paramagnetic or antiferromagnetic-like M(H) without hysteresis and unsaturated to 70 kOe
• high_pressure_transport_measurements — Pressure partially suppresses insulating behavior and density-wave order but does not induce superconductivity up to 80 GPa.
  • 材料: Sm₄Ni₃O₁₀-δ single crystals; KBr pressure-transmitting medium
  • 方法: diamond-anvil-cell measurement; four-probe van der Pauw resistance measurement; ruby fluorescence pressure calibration
  • 观察: density-wave anomaly weakens and becomes nearly invisible above about 18.2 GPa; insulating behavior partially suppressed by pressure; no superconductivity observed up to 80 GPa
• electronic_structure_analysis_and_interpretation — DFT calculations indicate that eg orbitals dominate the Fermi surface, and the large Ni–O–Ni bond-angle deviation may hinder superconductivity.
  • 材料: crystal structure from single-crystal XRD
  • 方法: density functional theory; GGA-PBE; DFT+U calculations
  • 观察: Ni 3d_z² and 3dx²-y² orbitals separated from t2g orbitals and dominate the Fermi surface; bonding, nonbonding, and antibonding bands described near the Fermi level; Fermi surface has an electron pocket at Γ and an additional small pocket along Γ-Y