Enhancement of metallicity by Na doping in La₃Ni₂O₇+δ

Summary

Polycrystalline samples of La_3-xNa_xNi₂O₇₊δ with various sodium doping concentrations were synthesized via a solid-state method, and their structural, thermal, magnetic, and electrical transport properties were systematically investigated using X-ray diffraction, thermogravimetric analysis, and measurements of magnetic susceptibility and electrical resistivity. X-ray diffraction analysis revealed that when the sodium doping level x ≥ 0.075, the samples undergo a structural transition from the ‘327’ Amam phase to the ‘4310’ Bmab phase, accompanied by gradual lattice expansion. Resistivity measurements indicated that sodium doping significantly enhances metallicity while slightly suppressing the density wave transition temperature; applying external pressure further suppresses the density wave transition, yet the low-temperature insulating behavior remains insensitive to pressure. These findings demonstrate that hole doping introduced by substituting sodium for lanthanum effectively modulates competing electronic phases in layered nickelates, providing crucial experimental evidence for understanding the roles of elemental substitution and carrier doping in stabilizing high-pressure superconducting phases.

Materials

• La_3-xNa_xNi₂O₇+δ polycrystalline

Methods

• 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)

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• Na doping induces a structural transition from '327' Amam phase to '4310' Bmab phase when x ≥ 0.075, accompanied by lattice expansion.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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'

Workflow

• sample_synthesis — Na doping induces structural transition.
  • Materials: La_3-xNa_xNi₂O₇₊δ (x = 0 to 0.5)
  • Methods: sol-gel method with metal nitrates; calcination at 800°C and sintering at 1100°C
  • Observations: 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.
  • Materials: as-synthesized polycrystalline samples
  • Methods: XRD; TGA; magnetic susceptibility (SQUID); electrical resistivity (four-probe)
  • Observations: Lattice expansion with Na doping; DW transition suppressed; Metallicity enhanced
• high_pressure_measurements — Pressure suppresses DW but leaves insulating state robust.
  • Materials: x = 0.05 and x = 0.1 samples
  • Methods: piston cylinder pressure cell; four-probe resistance
  • Observations: DW transition further suppressed; Low-T insulating behavior insensitive to pressure