Evolution of structure and density wave order in La₃Ni₂O₇−δ single crystals at ambient pressure

摘要

The recent discovery of high-temperature superconductivity in bilayer nickelate La₃Ni₂O₇−δ single crystals under high pressure has sparked significant interest in the correlated electronic physics and unconventional pairing mechanisms within Ruddlesden-Popper phase nickelates, where the oxygen content plays a crucial role in both structure and superconductivity. Here we report the evolution of the structure and density wave order in La₃Ni₂O₇−δ single crystals at ambient pressure under various oxygen annealing conditions. Structural analysis reveals that with the increase of annealing oxygen pressure, La₃Ni₂O₇−δ undergoes a structural phase transition from an orthorhombic to a tetragonal phase. Magnetic torque measurements indicate that the density wave order, present in the orthorhombic phase, vanishes upon transition to the tetragonal structure, accompanied by an abrupt change in carrier concentration. A phase diagram illustrating the dependence of both the crystal structure and density wave order on the annealing oxygen pressure is mapped. Our findings suggest a promising pathway for studying the interplay between structure, density wave order, and superconductivity in bulk nickel-based materials.

材料

• La₃Ni₂O₇-δ

方法

• structural analysis
• magnetic torque measurements
• oxygen annealing

关键词

• density wave order
• structural phase transition
• carrier concentration change
• phase diagram

亮点

• Suggests pathway for studying interplay between structure, density wave order, and superconductivity in bulk nickelates.

结论

• Density wave order present in orthorhombic phase vanishes upon transition to tetragonal structure with increased annealing oxygen pressure, accompanied by abrupt carrier concentration change.

主要论断

• Increasing oxygen annealing pressure drives a structural phase transition from orthorhombic (Amam) to tetragonal (I₄/mmm) in La₃Ni₂O₇−δ single crystals.
  • 证据: Table 1: lattice parameters change abruptly at 50 bar, with a and b becoming equal and c decreasing,Figure S₁: distinct diffraction patterns on (hk0) plane
• Density wave order, present in the orthorhombic phase, vanishes upon transition to tetragonal structure.
  • 证据: Figure 2(a,d): magnetic torque shows clear kink near 150 K for orthorhombic sample (15 bar),Figure 2(b,c,e,f): no kink for tetragonal samples (50 bar and 150 bar)
• The structural transition is accompanied by an abrupt change in carrier concentration.
  • 证据: Figure 3(c): carrier concentration versus oxygen pressure shows a jump at ≈50 bar,Hall resistivity slope changes at the transition
• All annealed samples exhibit metallic behavior in resistivity, but no density wave anomaly is observed in resistivity curves.
  • 证据: Figure 3(a): resistivity curves decrease with temperature,absence of anomaly at ≈150 K in resistivity

研究流程

• Crystal growth and oxygen annealing — Increasing oxygen annealing pressure induces a structural phase transition from orthorhombic to tetragonal in La₃Ni₂O₇−δ single crystals.
  • 材料: La(NO₃)₃·6H₂O; Ni(NO₃)₂·6H₂O; citric acid
  • 方法: sol-gel method; high-pressure (1.5 GPa) and high-temperature (1350°C) synthesis; oxygen annealing at 500°C for 10 h under various oxygen pressures (5-150 bar)
  • 观察: as-grown crystals are orthorhombic (Amam); after annealing at ≥50 bar, structure becomes tetragonal (I₄/mmm)
• Structural characterization — The structural transition is accompanied by a reduction in unit cell volume and straightening of Ni-O-Ni bond angles.
  • 方法: single crystal X-ray diffraction (SC-XRD); energy dispersive X-ray spectroscopy (EDS)
  • 观察: lattice parameters a, b, c and volume change abruptly at ≈50 bar; in-plane Ni-O-Ni bond angle increases from ≈173° to 180°, out-of-plane from ≈170° to 180°
• Physical property measurements — Density wave order exists only in the orthorhombic phase and vanishes upon structural transition to tetragonal; carrier concentration changes abruptly at the transition.
  • 方法: magnetic torque magnetometry; longitudinal resistivity; Hall effect measurements
  • 观察: torque curves show kink near 150 K for orthorhombic samples (density wave order); no kink for tetragonal samples; resistivity is metallic for all annealed samples; carrier concentration jumps at ≈50 bar
• Phase diagram construction — A phase diagram of structure and density wave order as a function of annealing oxygen pressure is established, showing the critical role of oxygen content.
  • 方法: combining structural and transport data
  • 观察: density wave transition temperature (≈150 K) is nearly independent of oxygen pressure in orthorhombic region