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

摘要

本研究利用氦气作传压介质，通过同步辐射单晶X射线衍射构建了化学计量比双层镍酸盐La₃Ni₂O₇在静水压下的本征压力–温度结构相图。常压下该材料呈极性正交Am₂m结构，伴随不等价Ni位间的电荷有序和NiO₆八面体倾斜。随压力升高，正交晶格畸变逐步减小，同时代表电荷有序的超结构反射强度线性下降。在约10 GPa时，体系发生从电荷有序Am₂m相到四方I₄/mmm相的直接转变，无中间Amam相出现，正交孪晶分裂消失，晶胞参数趋于四方对称。该四方相特征为八面体倾斜完全消除，形成直线型层间Ni–O–Ni键，并在超导出现的压力–温度区域稳定存在。这一结构相变恰好与电阻测量中起始温度约68 K的体超导开始出现吻合，表明超导态位于I₄/mmm四方结构框架内。上述结果澄清了超导相结构归属的争议，确立了La₃Ni₂O₇的本征结构演化路径，为理解镍酸盐高温超导的微观起源提供了关键结构基础。

材料

• La₃Ni₂O₇

方法

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

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• La₃Ni₂O₇ undergoes a direct structural transition from charge-ordered Am₂m to tetragonal I₄/mmm near 10 GPa under hydrostatic pressure.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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.

研究流程

• sample_preparation — High-quality stoichiometric single crystals under truly hydrostatic pressure conditions prepared for diffraction studies.
  • 材料: Stoichiometric La₃Ni₂O₇ single crystals (Crystal 1: 50×50×20 μm, Crystal 2: 30×20×10 μm)
  • 方法: 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
  • 观察: 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.
  • 材料: Synchrotron X-rays (30 keV, beam diameter 60 μm); 2D imaging plate detector
  • 方法: 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
  • 观察: 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.
  • 材料: Diffraction patterns from Crystal 1 and Crystal 2
  • 方法: 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
  • 观察: 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.
  • 材料: Structural phase data from diffraction, transport data from Ref. [40]
  • 方法: 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
  • 观察: 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