Enhancement of metallicity by Na doping in La₃Ni₂O₇+δ

摘要

本研究通过固相法合成了不同钠掺杂浓度的La_3-xNa_xNi₂O₇₊δ多晶样品，利用X射线衍射、热重分析、磁化率和电阻率测量系统研究了其结构、热、磁和电输运性质。X射线衍射分析表明，当钠掺杂浓度x≥0.075时，样品会发生从‘327’ Amam相到‘4310’ Bmab相的结构转变，且晶格逐渐膨胀。电阻率测量显示，钠掺杂显著增强了金属性，同时密度波转变温度略微降低；施加外部压力可进一步抑制密度波转变，但低温绝缘行为对压力不敏感。这些结果揭示了钠替代镧引入的空穴掺杂能有效调控层状镍酸盐中竞争电子相，为理解元素替代和载流子掺杂在稳定高压超导相中的作用提供了关键实验依据。

材料

• La_3-xNa_xNi₂O₇+δ polycrystalline

方法

• X-ray diffraction (XRD)
• thermogravimetric analysis (TGA)
• magnetic susceptibility (SQUID)
• electrical resistivity (four-probe)
• high-pressure electrical resistance
• sol-gel synthesis
• energy-dispersive X-ray spectroscopy (EDS)
• transmission electron microscopy (TEM)

关键词

• na doping
• metallicity enhancement
• density wave transition
• hole doping
• structural transition
• 327 to 4310 phase
• insulating behavior
• pressure suppression
• oxygen content

亮点

• Na doping induces a structural transition from the '327' Amam phase to the '4310' Bmab phase when x≥0.075.
• The resistivity is drastically reduced, and the density wave transition temperature TDW is slightly suppressed.
• The low-T insulating behavior is very robust against pressure, in sharp contrast to the suppression of TDW.

结论

• We demonstrated that in La_3-xNa_xNi₂O₇+δ, pure '327' phase can only be stabilized within a very narrow Na composition range. For x≥0.075, the main phase transforms to the orthorhombic '4310' Bmab phase.
• Na substitution for La leads to a pronounced enhancement of metallicity and the suppression of the DW transition.
• Upon applying pressure, the DW transition can be further suppressed, whereas the low-T insulating behavior shows little sensitivity to pressure.

主要论断

• Na doping induces a structural transition from '327' Amam phase to '4310' Bmab phase when x ≥ 0.075, accompanied by lattice expansion.
  • 证据: full_text Fig. 1a-c: new peaks appear at x=0.075, peak splitting for '327' peaks,abstract: 'XRD analysis reveals that Na doping induces a structural transition'
• Na doping enhances metallicity (reduces resistivity, suppresses insulating behavior) and marginally suppresses the density wave transition temperature.
  • 证据: full_text Fig. 3a,b: resistivity drops and normalized curves become steeper above T_DW,abstract: 'resistivity measurements indicate that the density wave transition is marginally suppressed and metallicity is significantly enhanced'
• Applied pressure further suppresses the DW transition (rate ~-22 to -27 K/GPa), but the low-temperature insulating behavior remains insensitive to pressure.
  • 证据: full_text Fig. 4a,b: T_DW decreases with pressure; energy gap from activated fit remains nearly constant,abstract: 'Upon the application of pressure, DW transition can be further suppressed, whereas the low-T insulating behaviors remain insensitive to pressure'

研究流程

• sample_synthesis — Na doping induces structural transition.
  • 材料: La_3-xNa_xNi₂O₇₊δ (x = 0 to 0.5)
  • 方法: sol-gel method with metal nitrates; calcination at 800°C and sintering at 1100°C
  • 观察: Phase-pure '327' Amam phase for x < 0.075; Phase transition to '4310' Bmab phase for x ≥ 0.075
• characterization — Na doping significantly modifies electronic properties.
  • 材料: as-synthesized polycrystalline samples
  • 方法: XRD; TGA; magnetic susceptibility (SQUID); electrical resistivity (four-probe)
  • 观察: Lattice expansion with Na doping; DW transition suppressed; Metallicity enhanced
• high_pressure_measurements — Pressure suppresses DW but leaves insulating state robust.
  • 材料: x = 0.05 and x = 0.1 samples
  • 方法: piston cylinder pressure cell; four-probe resistance
  • 观察: DW transition further suppressed; Low-T insulating behavior insensitive to pressure