Pressure-Driven Structural Transitions without a Displacive Charge-Density Wave in La₂SmNi₂O₇

Summary

This study employs synchrotron X-ray diffraction to systematically investigate the structural evolution of the bilayer nickelate La₂SmNi₂O₇ under low temperature and high pressure. At ambient conditions, single-crystal diffraction reveals a new monoclinic superstructure (space group P₂₁/a) with a c-axis doubling primarily driven by antiferrodistortive oxygen displacements; no satellite reflections associated with charge-density-wave order are detected, indicating that any displacive charge ordering, if present, has an amplitude below a few thousandths of an ångström. Under applied pressure, a sequence of structural transitions is observed at room temperature using both powder and single-crystal diffraction: a monoclinic-to-orthorhombic transition at approximately 15 GPa, followed by a further transition to tetragonal symmetry near 21 GPa, with the intermediate orthorhombic phase persisting stably over a finite pressure interval. In the pressure–temperature regime where superconductivity emerges, high-quality single-crystal data enable structural refinement and provide precise lattice parameters and bond angles, establishing a structural basis for understanding the onset of superconductivity. The results demonstrate that the pressure-driven structural transformations in La₂SmNi₂O₇ occur without the participation of a displacive charge-density wave, and the successive symmetry changes impose crucial constraints on theoretical models aimed at exploring the interplay of charge, lattice, and magnetism.

Materials

• La₂SmNi₂O₇

Methods

• synchrotron X-ray diffraction
• single-crystal X-ray diffraction
• powder X-ray diffraction
• Le Bail fitting
• diamond anvil cell
• ruby fluorescence pressure measurement

Keywords

• charge density wave
• superconductivity
• structural phase transition
• monoclinic superstructure
• antiferrodistortive distortion
• ni o ni angle
• interlayer coupling
• orbital redistribution

Highlights

• Single-crystal synchrotron diffraction reveals a new monoclinic superstructure (space group P₂₁/a) with antiferrodistortive oxygen displacements not previously reported.
• The detection sensitivity sets an upper bound on any possible displacive CDW modulation of less than a few thousandths of an ångström, ruling out a significant structural component.
• Accurate structural parameters are provided within the superconducting dome, enabling direct input for theoretical models of superconductivity.
• The successive symmetry changes from monoclinic to orthorhombic to tetragonal are mapped with single-crystal diffraction under low temperature and high pressure for the first time.

Conclusions

• The material crystallizes as a monoclinic superstructure with c-axis doubling driven by antiferrodistortive oxygen displacements.
• No signatures of satellite reflections associated with charge density wave ordering are detected, placing an upper limit on any displacive CDW amplitude of less than a few thousandths of an ångström.
• A sequence of pressure-induced structural transitions from monoclinic to orthorhombic at approximately 15 GPa and then to tetragonal near 21 GPa is observed.
• The onset of superconductivity coincides with the disappearance of the monoclinic phase, while the maximum T_c lies within the tetragonal phase, highlighting the importance of linear Ni-O-Ni bonds.
• The absence of a sizeable displacive CDW suggests weak electron-phonon coupling or a purely electronic CDW, and indicates that electron-electron interactions are weaker in the n=2 nickelate family compared to n=3.

Main claims

• At ambient conditions, La₂SmNi₂O₇ adopts a new monoclinic superstructure (space group P₂₁/a) with c‑axis doubling driven by antiferrodistortive oxygen displacements, and no displacive charge‑density wave is present.
  • Evidence: Single‑crystal synchrotron XRD reveals weak superlattice reflections at (h,k,l±½) below 300 K.,Structure refined in P₂₁/a with systematic extinctions; O₄ and O₅ atoms occupy general positions causing the doubling.,Search for CDW satellites at 15 K in (a*,b*) plane shows no extra reflections down to 5×10^-5 of main Bragg intensity → atomic displacement < few thousandths Å.
• Under compression, La₂SmNi₂O₇ undergoes a sequence of pressure‑induced structural transitions: monoclinic → orthorhombic at ≈15 GPa and orthorhombic → tetragonal at ≈21 GPa (completed ≈24 GPa), without participation of a displacive CDW.
  • Evidence: Powder XRD shows abrupt change in cos(β) at 15 GPa; single‑crystal intensity ratio of (hkl)/(h̅k̅l) pairs drops from 50% to 0% at 15 GPa, indicating onset of orthorhombic symmetry.,Above 21 GPa, (135)/(315) reflections merge progressively, becoming identical at 24 GPa, confirming tetragonal symmetry.,No CDW satellites observed at any pressure or temperature.
• The onset of the superconducting dome coincides with the disappearance of the monoclinic phase, and the maximum T_c occurs in the tetragonal phase with a linear Ni–O–Ni angle, indicating that monoclinic symmetry is incompatible with SC and that a straight Ni–O–Ni bond is favorable for optimizing T_c.
  • Evidence: Superconducting onset (≈14.8 GPa, from Li et al. 2025, same batch) matches the pressure where monoclinic phase vanishes in our structural data.,Maximum T_c (≈92 K onset, 21.6 GPa) lies within the tetragonal phase, where Ni–O–Ni angle along c reaches 180° (from our refinements).

Workflow

• sample_synthesis — High-qualityLa₂SmNi₂O₇ single crystals and powder were successfully synthesized.
  • Materials: La₂O₃ (99.99%); Sm₂O₃ (99.99%); NiO (99.99%); anhydrous K₂C flux; Al₂O₃ crucible
  • Methods: ambient-pressure flux growth; preheating oxides at 800 °C for 6 h; mixing with flux at 1:15 mass ratio; heating to 1050 °C for 72 h, slow cooling at 1 °C/h
  • Observations: single crystals obtained; composition La:Ni ≈ 0.96, La:Sm ≈ 2.1 (SEM/EDX); powder ground from single-crystal batch
• x-ray_diffraction_measurements — Comprehensive structural data across the (P,T) phase diagram were collected with high sensitivity.
  • Materials: single crystals (≈30×30×25 µm³); ground powder; diamond anvil cell (DAC); He gas pressure medium; ruby fluorescence pressure gauge; stainless steel/rhenium gasket; NaCl internal calibrant
  • Methods: synchrotron XRD at CRISTAL beamline (SOLEIL); single-crystal diffraction (4‑circle diffractometer, CCD detector); powder diffraction in DAC; low‑temperature He cryostat
  • Observations: ≈1500 Bragg reflections per (P,T) point; weak superstructure reflections (intensity ≈10^-2 of main peaks) at low T; no satellite reflections attributable to CDW; powder patterns used for lattice parameter extraction
• structural_refinement_and_phase_analysis — A sequence of pressure‑driven structural transitions occurs without a displacive charge‑density wave; the new ambient‑pressure monoclinic structure is distinct from previously reported models.
  • Materials: single-crystal and powder diffraction data; JANA₂₀₂₀ software
  • Methods: Le Bail fit (powder data); least‑squares refinement (single‑crystal data); space group determination via extinction rules and reflection intensity ratios; bulk modulus derivation from equation of state
  • Observations: new monoclinic superstructure (P₂₁/a, a≈3.9, b≈3.8, c≈20.2 Å, β≈105°) with c‑axis doubling from alternating O displacements; no CDW; upper bound on displacive modulation < few thousandths Å; bulk modulus B₀ = 147.3 GPa (isotropic compression); monoclinic→orthorhombic transition at ≈15 GPa (confirmed by 50%→0% ratio of unequal reflection pairs); orthorhombic→tetragonal onset at ≈21 GPa, fully completed at ≈24 GPa; Ni–Ni apical distance nearly constant under pressure; Ni–O–Ni angle increases from 162° to 180° in tetragonal phase
• structure–superconductivity_correlation — Monoclinic symmetry is incompatible with superconductivity; a linear Ni–O–Ni angle, achieved in the tetragonal phase, optimizes T_c.
  • Materials: structural P‑T data from this work; transport data from Li et al. (2025) Nature 649, 871 (same synthesis batch)
  • Methods: comparison of structural phase boundaries with measured superconducting dome
  • Observations: superconducting onset (≈14.8 GPa) coincides with disappearance of monoclinic phase; maximum T_c (≈92 K onset at 21.6 GPa) lies within tetragonal phase where Ni–O–Ni angle becomes linear (180°)