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

摘要

本研究利用压力-温度单晶X射线衍射与从头算密度泛函理论计算，揭示助熔剂生长的三层镍酸盐La₄Ni₃O₁₀中四方相（I₄/mmm）到单斜相（P₂₁/c）的转变不存在中间斜方Bmab相，而是一个直接的结构相变，并伴随两倍超结构的形成，体现为公度超晶格反射的出现。该转变温度可在14 GPa压力下从约1030 K连续抑制至20 K，说明压力有效稳定了四方相。此外，在助熔剂生长晶体中首次通过X射线衍射探测到与非公度密度波有序相关的微弱卫星反射，补充了此前仅见于浮区法晶体的结果，拉曼光谱在130 K以下观察到额外声子模式也进一步印证了该有序态。从头算计算与实验观察吻合。这项工作澄清了La₄Ni₃O₁₀结构对称性的长期争议，并支持超导电性在高压恢复四方对称性后出现，为后续电子结构及超导机理研究提供了关键的晶体学基础。

材料

• La₄Ni₃O₁₀

方法

• 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

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• The structural phase transition in La₄Ni₃O₁₀ from tetragonal (I₄/mmm) to monoclinic (P₂₁/c) is direct, without an intermediate orthorhombic Bmab phase.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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).

研究流程

• sample_synthesis — High-quality single crystals of La₄Ni₃O₁₀ were successfully synthesized via the flux method.
  • 材料: La₂O₃; Ni; K₂CO₃ flux
  • 方法: flux growth method
  • 观察: 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.
  • 材料: ground powder of La₄Ni₃O₁₀
  • 方法: synchrotron powder X-ray diffraction at BM₀₁ (ESRF)
  • 观察: 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.
  • 材料: single crystal of La₄Ni₃O₁₀; diamond anvil cell; helium pressure transmitting medium
  • 方法: single-crystal X-ray diffraction at ID₁₅b (ESRF)
  • 观察: 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.
  • 材料: single crystal of La₄Ni₃O₁₀ (flux-grown)
  • 方法: single-crystal X-ray diffraction at P₂₄ (DESY) and ID₂₈ (ESRF)
  • 观察: 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.
  • 材料: single crystal of La₄Ni₃O₁₀
  • 方法: polarized Raman spectroscopy; 532 nm laser excitation
  • 观察: 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.
  • 方法: ab initio density functional theory (DFT); ABINIT code; PBE-GGA functional; pseudopotentials from PseudoDojo
  • 观察: 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.
  • 方法: crystallographic refinement; symmetry analysis; phase diagram construction
  • 观察: 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