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

Summary

Here we report the first successful synthesis of single crystals of the samarium-based trilayer Ruddlesden–Popper nickelate Sm₄Ni₃O_10-δ using a high-pressure, high-temperature (3.25 GPa, 1400 °C) flux method. Single-crystal X-ray diffraction and powder diffraction refinement show that the structure adopts the orthorhombic Pbca space group, with an Ni–O–Ni bond angle along the c axis of about 152.4°, significantly deviating from 180°. Magnetization and transport measurements at ambient pressure consistently reveal a density-wave transition at about 180 K; applying pressure up to 80 GPa partially suppresses the insulating behavior and density-wave order, but no superconductivity is observed. Density functional theory calculations indicate that the 3d_z² and 3dx²−y² orbitals are separated from the other t₂g orbitals and are the main contributors to the Fermi surface. These results suggest that the severe deviation of the Ni–O–Ni bond angle may hinder the emergence of superconductivity, and the newly synthesized Sm₄Ni₃O_10-δ provides a unique platform for studying the fundamental physics of Ruddlesden–Popper nickelates.

Materials

• Sm₄Ni₃O₁₀-δ

Methods

• 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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• First successful high-pressure high-temperature synthesis of Sm₄Ni₃O₁₀-δ single crystals.
  • Evidence: 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°.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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

Workflow

• sample_preparation — First successful HPHT synthesis of samarium-based trilayer Ruddlesden-Popper nickelate Sm₄Ni₃O₁₀-δ single crystals.
  • Materials: Sm₂O₃; Ni(OH)₂; citric acid; ethylene glycol; KCl flux; KClO₄
  • Methods: modified sol-gel precursor synthesis; high-pressure high-temperature flux growth; KClO₄ oxygen-source optimization
  • Observations: 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°.
  • Materials: Sm₄Ni₃O₁₀-δ single crystals; powder ground from single crystals
  • Methods: single-crystal X-ray diffraction; powder X-ray diffraction with Rietveld refinement; scanning electron microscopy and energy-dispersive X-ray spectroscopy
  • Observations: 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.
  • Materials: stacked Sm₄Ni₃O₁₀-δ single crystals; Sm₄Ni₃O₁₀-δ polycrystalline pellet annealed at 500 °C under 8 MPa O₂
  • Methods: temperature-dependent resistance measurement; dc magnetization and susceptibility measurement; magnetic hysteresis loop measurement
  • Observations: 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.
  • Materials: Sm₄Ni₃O₁₀-δ single crystals; KBr pressure-transmitting medium
  • Methods: diamond-anvil-cell measurement; four-probe van der Pauw resistance measurement; ruby fluorescence pressure calibration
  • Observations: 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.
  • Materials: crystal structure from single-crystal XRD
  • Methods: density functional theory; GGA-PBE; DFT+U calculations
  • Observations: 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