Single-crystal structural phase diagram of stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure

Summary

Using helium as a pressure-transmitting medium, this study constructs the intrinsic pressure–temperature structural phase diagram of stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure via synchrotron single-crystal X-ray diffraction. At ambient pressure, the material adopts a polar orthorhombic Am₂m structure, accompanied by charge ordering between inequivalent Ni sites and tilting of NiO₆ octahedra. With increasing pressure, the orthorhombic lattice distortion gradually decreases, while the intensity of superstructure reflections indicative of charge ordering diminishes linearly. At approximately 10 GPa, a direct transition occurs from the charge-ordered Am₂m phase to a tetragonal I₄/mmm phase, with no intervening Amam phase; orthorhombic twinning splitting disappears, and unit cell parameters approach tetragonal symmetry. This tetragonal phase is characterized by the complete elimination of octahedral tilting, resulting in linear interlayer Ni–O–Ni bonds, and it persists in the pressure–temperature region where superconductivity emerges. This structural transition coincides precisely with the onset of bulk superconductivity at ~68 K as detected in resistance measurements, indicating that the superconducting state resides within the I₄/mmm tetragonal framework. These findings resolve controversies over the structural identity of the superconducting phase, establish the intrinsic structural evolution pathway of La₃Ni₂O₇, and provide a critical structural basis for understanding the microscopic origin of high-temperature superconductivity in nickelates.

Materials

• La₃Ni₂O₇

Methods

• synchrotron single-crystal X-ray diffraction
• high-pressure diamond anvil cell with helium medium
• ruby fluorescence pressure calibration

Keywords

• charge order
• octahedral tilting
• polar structure
• structural phase transition
• i4/mmm phase
• am2m phase
• hydrostatic pressure

Highlights

• Hydrostatic helium pressure conditions and single-crystal X-ray diffraction overcome extrinsic effects to establish the intrinsic structural phase diagram.
• Direct Am₂m → I₄/mmm transition observed, resolving earlier controversies over intermediate Amam or Fmmm phases.
• Superstructure reflections violating a‑glide symmetry (indicative of charge order) decrease linearly with pressure and vanish precisely at the orthorhombic‑to‑tetragonal transition.
• No evidence for the Fmmm phase throughout the entire investigated pressure range, highlighting the importance of hydrostaticity and single‑crystal samples.

Conclusions

• We find that the polar Am₂m phase persists at low pressure and transforms directly into a tetragonal I₄/mmm phase near 10 GPa, coincident with the onset of bulk superconductivity.
• The previously proposed orthorhombic Amam and Fmmm phases are not observed throughout the investigated pressure–temperature range, up to 19.5 GPa and 9–300 K.
• The pressure-induced transition to the tilt-free I₄/mmm structure establishes a high-symmetry crystallographic framework with linear interlayer Ni–O–Ni bonds for the superconducting state.
• Our results indicate that this structural transition alone is unlikely to be sufficient for superconductivity, as the tetragonal phase persists over a broader pressure–temperature range than the superconducting state itself.

Main claims

• La₃Ni₂O₇ undergoes a direct structural transition from charge-ordered Am₂m to tetragonal I₄/mmm near 10 GPa under hydrostatic pressure.
  • Evidence: At₁₃.7 GPa and 9 K, orthorhombic domain splitting disappears and all peaks merge, consistent with a tetragonal lattice (Fig. 3c).,The 3 0 0_o superstructure reflection, which violates the Amam a-glide extinction, decreases linearly in intensity with pressure and vanishes precisely at the pressure where the orthorhombic distortion disappears (Fig. 4e).,Diffraction patterns at 7.0 GPa still show reflections that violate F-centering (e.g., 2̅ 5 13̅̅ o), ruling out the Fmmm phase (Fig. 3b).
• The onset of bulk superconductivity coincides with the stability region of the I₄/mmm phase.
  • Evidence: Electrical transport measurements on crystals from the same batch show Tc_onset ≈ 68 K above 10 GPa (Ref. [40]).,The constructed P–T phase diagram (Fig. 1d) places the superconducting region entirely within the I₄/mmm stability field.
• No intermediate Amam phase exists; charge order and octahedral tilting disappear simultaneously at the transition.
  • Evidence: The 3 0 0_o reflection (allowed in Am₂m, forbidden in Amam) is clearly observed at 0.2 GPa and persists until disappearing along with the orthorhombic splitting at ≈10 GPa, indicating that the Am₂m phase transforms directly to I₄/mmm without passing through Amam.

Workflow

• sample_preparation — High-quality stoichiometric single crystals under truly hydrostatic pressure conditions prepared for diffraction studies.
  • Materials: Stoichiometric La₃Ni₂O₇ single crystals (Crystal 1: 50×50×20 μm, Crystal 2: 30×20×10 μm)
  • Methods: High-pressure synthesis technique (Ref. [40]); Loading into diamond anvil cell (DAC) with helium gas as pressure transmitting medium; Helium precompressed to 200 MPa before sealing
  • Observations: Crystals from same batch exhibit bulk superconductivity with Tc_onset ≈ 68 K above 10 GPa in transport measurements (Ref. [40]); Ambient-pressure structure confirmed as polar Am₂m with charge order and no oxygen deficiency (Ref. [40])
• measurement — High-pressure, low-temperature XRD datasets collected with sensitivity to both in-plane and h0l reflections, capturing structural evolution.
  • Materials: Synchrotron X-rays (30 keV, beam diameter 60 μm); 2D imaging plate detector
  • Methods: High-pressure single-crystal X-ray diffraction at BL₁₀XU beamline, SPring-8; Data collection at various pressures (0.2–19.5 GPa) and temperatures (9 K, 300 K); Crystal 1 oriented with X-rays parallel to c-axis to probe in-plane lattice changes; Crystal 2 oriented to access h0l plane for superstructure reflection detection; Pressure tuning via helium gas membrane system; temperature by cryostat
  • Observations: At₀.2 GPa and 9 K, orthorhombic domain splitting observed; all reflections satisfy A-centering; At₇.0 GPa and 9 K, peak splitting reduced; reflections violating F-centering extinction are present, confirming A-centered lattice; At₁₃.7 GPa and 9 K, orthorhombic splitting disappears, peaks merge into I-centered tetragonal pattern; At₀.2 GPa and 300 K (Crystal 2), 3 0 0_o reflection (forbidden in Amam) clearly detected; Upon compression, intensity of 3 0 0_o decreases linearly with pressure and vanishes at ≈10 GPa
• analysis — Crystallographic analysis shows a direct transition from polar, charge-ordered Am₂m to tilt-free I₄/mmm at ≈10 GPa, with no intermediate Amam or Fmmm phase.
  • Materials: Diffraction patterns from Crystal 1 and Crystal 2
  • Methods: Indexing diffraction spots and checking centering extinction conditions (A, I, F); Refinement of lattice parameters a_o, b_o, c_o and ratio a_o/b_o; Integration of 3 0 0_o superstructure reflection intensity; Comparison with candidate space groups: Amam, Am₂m, Fmmm, I₄/mmm
  • Observations: At low P: A-centered lattice, a_o/b_o < 1, finite 3 0 0_o intensity → Am₂m phase; Above ≈10 GPa: A-centering lost, tetragonal lattice with I-centering, 3 0 0_o absent → I₄/mmm phase; No pressure region shows F-centering; reflections violating Amam absent only at high P; Lattice parameters and 3 0 0_o intensity evolve continuously but transition is direct, not gradual symmetry change
• interpretation — The superconducting state of La₃Ni₂O₇ resides within the tetragonal I₄/mmm structure, and while the structural transition is necessary, additional electronic or magnetic factors are required to explain the superconductivity.
  • Materials: Structural phase data from diffraction, transport data from Ref. [40]
  • Methods: Correlation of structural transition pressure with reported superconducting T_c onset; Construction of pressure–temperature phase diagram (Fig. 1d); Comparison with previously proposed structural models
  • Observations: Superconducting transition (Tc_onset ≈68 K) appears only above ≈10 GPa, matching the pressure where I₄/mmm phase stabilizes; The I₄/mmm structure exists over a wider P–T range than the superconducting dome; No evidence for other orthorhombic structures (e.g., Fmmm, Amam) under hydrostatic conditions