Insulator-to-metal transition in Co-doped La₃Ni₂O₇−δ with high oxygen pressure annealing

摘要

The effects of Co doping in a series of La₃Ni₂−xCo_xO₇−δ samples before and after high oxygen pressure annealing have been investigated. The structural refinemen

材料

• La₃Ni_2-xCo_xO₇-δ

方法

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关键词

• insulator to metal transition
• oxygen content

亮点

本批次暂无数据。

结论

本批次暂无数据。

主要论断

• High oxygen pressure annealing induces an insulator-to-metal transition in Co-doped La₃Ni₂O₇−δ, leading to metallic conductivity above 20 K.
  • 证据: The annealed La₃Ni₂−xCo_xO₇−δ samples exhibit an insulator-to-metal transition, and the metallic-like conductivity could be achieved above ≈20 K in Co-doped samples
• The increase in oxygen content during annealing raises the average Ni valence state, which is responsible for the enhanced electrical conductivity.
  • 证据: The annealing treatment results in an increase in the oxygen content and the average valence state of Ni ions of the La₃Ni₂−xCo_xO₇−δ samples,The approach of the average valence state of Ni to 2.5 is believed to be favorable for the occurrence of superconductivity

研究流程

• sample preparation — Co-doped La₃Ni₂O₇−δ samples with high doping level (x up to 0.3) were successfully synthesized via sol-gel.
  • 材料: Ni(NO₃)₂·6H₂O; La₂O₃; Co(NO₃)₂·6H₂O; citric acid; nitric acid
  • 方法: sol-gel method; stepwise heating (160°C, 400°C, 800°C); pelleting; sintering at 1100°C in air for 24 h
  • 观察: blackish-gray powders after organic removal; single-phase orthorhombic (Amam) for all doping
• high oxygen pressure annealing — High oxygen pressure annealing effectively increases oxygen content and reduces structural distortion.
  • 材料: as-prepared La₃Ni₂−xCo_xO₇−δ pellets
  • 方法: high-temperature high-pressure tube furnace (OTF-1200X-HP-30)
  • 观察: mass increase ≈1.13% on average indicating oxygen uptake; lattice parameters decrease (a and c) and Ni-O-Ni bond angles increase
• structural and electronic characterization — Insulator-to-metal transition is achieved in Co-doped samples after high oxygen pressure annealing, with metallic conduction above ≈20 K.
  • 材料: powder samples before and after annealing
  • 方法: X-ray diffraction (XRD) with Rietveld refinement (GSAS); X-ray photoelectron spectroscopy (XPS); four-probe resistivity; SQUID magnetometry
  • 观察: unit cell volume increases with Co doping; Ni valence increases from ≈2.43 to ≈2.48 after annealing; magnetic susceptibility largely unchanged by annealing; resistivity shows insulator-to-metal transition after annealing: metallic behavior above 20K