Tracing the horizon of tetragonal-to-monoclinic distortion in pressurized trilayer nickelate La₄Ni₃O₁₀

Summary

This study employs pressure-temperature single-crystal X-ray diffraction and ab initio density functional theory calculations to reveal that the transition from the tetragonal phase (I₄/mmm) to the monoclinic phase (P₂₁/c) in flux-grown trilayer nickelate La₄Ni₃O₁₀ does not involve an intermediate orthorhombic Bmab phase; instead, it is a direct structural phase transition accompanied by the formation of a two-fold superstructure, manifested by the appearance of commensurate superlattice reflections. The transition temperature can be continuously suppressed from approximately 1030 K to 20 K under a pressure of 14 GPa, indicating that pressure effectively stabilizes the tetragonal phase. Furthermore, weak satellite reflections associated with incommensurate density wave ordering are detected for the first time in X-ray diffraction from flux-grown crystals, complementing previous results observed only in float-zone crystals, and Raman spectroscopy reveals additional phonon modes below 130 K, further corroborating this ordered state. The ab initio calculations are in good agreement with the experimental observations. This work clarifies the long-standing dispute over the structural symmetry of La₄Ni₃O₁₀, supports the emergence of superconductivity after the restoration of tetragonal symmetry under high pressure, and provides a critical crystallographic foundation for subsequent studies of electronic structure and superconducting mechanisms.

Materials

• La₄Ni₃O₁₀

Methods

• Pressure-temperature single-crystal X-ray diffraction
• Temperature-dependent powder X-ray diffraction
• Raman spectroscopy
• Ab initio density functional theory
• Scanning electron microscopy
• Electron probe micro-analysis

Keywords

• density wave ordering
• incommensurate modulation
• superstructure
• tetragonal to monoclinic phase transition
• phonon modes
• twinning

Highlights

• First observation of incommensurate satellite reflections in flux-grown La₄Ni₃O₁₀ single crystals, establishing that the density wave ordering is intrinsic and not dependent on synthesis method.
• Direct tetragonal-to-monoclinic transition without an intermediate orthorhombic phase, resolving structural ambiguities.
• Pressure suppresses the structural transition temperature from 1030 K to 20 K at 14 GPa.
• The monoclinic phase is a twinned two-fold superstructure with minute distortion, maintaining pseudo-tetragonal symmetry.
• Raman spectroscopy detects new phonon modes below 130 K, linked to the onset of incommensurate density wave state.

Conclusions

• The monoclinic phase consistently manifests as a twinned two-fold superstructure below 1030 K.
• The monoclinic distortion is exceedingly small, maintaining an overall pseudo-tetragonal crystallographic framework.
• Pressurizing from ambient to 14 GPa gradually suppresses the structural transition temperature from 1030 K down to 20 K.
• The tetragonal-to-monoclinic phase transition is direct, without an intermediate orthorhombic phase, as confirmed by both XRD and DFT calculations.
• Incommensurate satellite reflections associated with density wave ordering are observed for the first time in flux-grown crystals, demonstrating that the lattice modulation is intrinsic.
• Raman spectroscopy reveals additional phonon modes below 130 K, corroborating the DW-driven lattice distortions.
• A consistent crystallographic description resolves long-standing ambiguities among tetragonal, orthorhombic, and monoclinic structures.

Main claims

• The structural phase transition in La₄Ni₃O₁₀ from tetragonal (I₄/mmm) to monoclinic (P₂₁/c) is direct, without an intermediate orthorhombic Bmab phase.
  • Evidence: High-pressure single-crystal XRD shows a direct transition with 2-fold superstructure and no orthorhombic phase.,DFT relaxation shows simultaneous disappearance of all order parameters at a single critical pressure.,Crystallographic refinement favors monoclinic over orthorhombic with statistical significance.
• Density-wave ordering is an intrinsic property of La₄Ni₃O₁₀, confirmed by the observation of incommensurate satellite reflections in flux-grown crystals and additional Raman modes below 130 K.
  • Evidence: Weak incommensurate satellites observed in XRD at ID₂₈ on flux-grown crystals, previously only seen in floating-zone crystals.,Raman spectroscopy reveals new phonon modes below ≈130 K, exceeding the number allowed by the high-temperature space group.
• Hydrostatic pressure suppresses the monoclinic distortion, stabilizing the tetragonal phase down to 20 K at 14 GPa, which is the likely host for superconductivity.
  • Evidence: Pressure-temperature phase diagram from XRD shows transition temperature decreasing from 1030 K to 20 K at 14 GPa.,Previous reports link superconductivity emergence to the tetragonal phase (cited within the paper).

Workflow

• sample_synthesis — High-quality single crystals of La₄Ni₃O₁₀ were successfully synthesized via the flux method.
  • Materials: La₂O₃; Ni; K₂CO₃ flux
  • Methods: flux growth method
  • Observations: flattened cuboid black crystals; La:Ni ratio 4:3 confirmed by EPMA and EDX
• high_temperature_powder_XRD — The tetragonal-to-monoclinic transition temperature under ambient pressure is 1030 K.
  • Materials: ground powder of La₄Ni₃O₁₀
  • Methods: synchrotron powder X-ray diffraction at BM₀₁ (ESRF)
  • Observations: structural transition from tetragonal to monoclinic at Ts ≈ 1030K; minute monoclinic distortion (β ≈ 90.1–90.2°)
• high_pressure_single_crystal_XRD — The tetragonal-to-monoclinic transition is direct, without an intermediate orthorhombic phase, and can be continuously suppressed by pressure.
  • Materials: single crystal of La₄Ni₃O₁₀; diamond anvil cell; helium pressure transmitting medium
  • Methods: single-crystal X-ray diffraction at ID₁₅b (ESRF)
  • Observations: emergence of 2-fold superstructure reflections below 11.3 GPa; splitting and broadening of (h k l) reflections; deviation of β from 90°; twin domains; suppression of transition temperature to 20 K at 14 GPa
• ambient_pressure_low_temperature_SXRD — Incommensurate density-wave ordering is an intrinsic property of La₄Ni₃O₁₀, observed in flux-grown crystals for the first time.
  • Materials: single crystal of La₄Ni₃O₁₀ (flux-grown)
  • Methods: single-crystal X-ray diffraction at P₂₄ (DESY) and ID₂₈ (ESRF)
  • Observations: no incommensurate satellites at P₂₄; weak incommensurate satellite reflections observed at ID₂₈ at 80K; average intensity ≈0.01% of main Bragg reflections
• raman_spectroscopy — Additional phonon modes below 130 K indicate a symmetry lowering associated with density-wave ordering.
  • Materials: single crystal of La₄Ni₃O₁₀
  • Methods: polarized Raman spectroscopy; 532 nm laser excitation
  • Observations: 15 modes in parallel and 19 in crossed polarization at 170K; new phonon modes appear below ≈130K; total modes exceed allowed number for room-temperature space group; modes follow selection rules
• dft_calculations — Ab initio calculations confirm the direct tetragonal-to-monoclinic transition without intermediate phases.
  • Methods: ab initio density functional theory (DFT); ABINIT code; PBE-GGA functional; pseudopotentials from PseudoDojo
  • Observations: simultaneous vanishing of octahedral tilt angle, monoclinic distortion, and basal distortion at ≈9 GPa; no stable intermediate orthorhombic phases; order parameter scaling deviates from mean-field
• structural_interpretation — A unified crystallographic framework resolves ambiguities, and the restoration of tetragonal symmetry under pressure likely enables superconductivity.
  • Methods: crystallographic refinement; symmetry analysis; phase diagram construction
  • Observations: consistent description using non-standard settings P₂₁/c and I₄/mmm; monoclinic phase is a twinned 2-fold superstructure; phase diagram shows monotonic suppression of transition with pressure