High temperature transitions in Ruddlesden-Popper nickelates Lan+1NinO₃n+1

摘要

本研究系统性地分析了Ruddlesden-Popper镍酸盐Lan+1Ni_nO₃n+1（n=1,2,3,∞）系列单晶及粉末样品，结合粉末与单晶X射线衍射、热容、差示扫描量热等多种实验手段，在2–1000 K宽温域内揭示了此前被忽视的高温相变。对n=2和n=3化合物，在约560 K处观测到明显的晶格参数异常，例如双层2222相中面外晶格常数突增、面内收缩，表明八面体倾斜的突然释放；单层-三层1313多形体则表现为各向同性的体积骤缩；三层相n=3在该温度附近单斜角β出现清晰拐折，热容与DSC数据也证实该转变的热力学特征。该转变与已知的高温四方相变及低温密度波相变截然不同，且n=∞钙钛矿LaNiO₃未呈现类似行为。研究确认该高温相变是镍酸盐RP系列的普遍特性，强调在探索超导电性时必须重视此高温结构不稳定性对低温物性的潜在影响。

材料

• La₂NiO₄
• La₃Ni₂O₇ (bilayer 2222)
• La₃Ni₂O₇ (monolayer-trilayer 1313)
• La₄Ni₃O₁₀
• LaNiO₃

方法

• Powder X-ray diffraction (PXRD)
• Single-crystal X-ray diffraction (scXRD)
• STEM
• Heat capacity measurements
• Differential scanning calorimetry (DSC)
• Magnetic susceptibility measurements (SQUID)
• Electrical transport measurements

关键词

• high temperature structural transition
• octahedral tilting
• charge ordering
• density wave transition
• oxygen intercalation
• polymorphism
• superstructure

亮点

• First observation of a bulk single crystal of a higher-order Ruddlesden–Popper phase (La₃Ni₂O₇–1313) with oxygen intercalation, yielding La₃Ni₂O₇.15 and an Imma superstructure.
• Discovery of a polar room-temperature structure in La₃Ni₂O₇–2222 with Ni-O bond-length differences indicative of charge order.
• Identification of a previously underappreciated high-temperature structural transition near 560 K in layered RP nickelates, distinct from the tetragonal and density-wave transitions.
• High-temperature structural anomalies dominate the electrical transport behavior, far outweighing the effects of low-temperature density-wave transitions.

结论

• A previously underappreciated high-temperature structural transition near 560 K exists in mixed-valence layered Ruddlesden–Popper nickelates (n=2,3), distinct from the tetragonal-to-orthorhombic transition.
• This transition is characterized by lattice-parameter anomalies and a sharp, fully reversible entropy release.
• Its absence in the three-dimensional perovskite LaNiO₃ suggests it is an intrinsic feature of the mixed-valence layered RP topology.
• High-temperature structural anomalies have a dominant impact on electrical transport, exceeding the signatures of low-temperature density-wave transitions.
• In La₃Ni₂O₇, a sharp resistance increase and a polar room-temperature structure indicate a charge ordering scenario.
• The bilayer 2222 and monolayer–trilayer 1313 polymorphs of La₃Ni₂O₇ exhibit different oxygen stoichiometries; the 1313 phase incorporates interstitial oxygen (up to La₃Ni₂O₇.15) stabilizing an Imma superstructure.
• Oxygen content is a key factor influencing the structural and electronic properties of Ruddlesden–Popper nickelates.

主要论断

• A previously underappreciated high-temperature phase transition exists in Lan+1Ni_nO₃n+1 (n=2,3) near 560 K.
  • 证据: Abstract: 'identify a previously underappreciated high-temperature phase transition… distinct from the one going to a tetragonal phase',Summary: 'pronounced lattice-parameter anomalies are observed around 560 K… heat-capacity and DSC data further confirm the thermodynamic character'
• The transition is entirely distinct from both the high-temperature tetragonal-orthorhombic transition and the low-temperature density-wave transition.
  • 证据: Summary: 'This transition is entirely distinct from the known high-temperature tetragonal transition and the low-temperature density-wave transition',Figure 3: lattice-parameter anomalies at 560 K occur while a and b remain split (orthorhombic) for 2222, far from the tetragonal transition at >680 K; density-wave anomalies are at much lower temperatures
• The transition is universal for layered RP nickelates (n=2,3) and absent in the 3D perovskite end member LaNiO₃.
  • 证据: Summary: 'the n=∞ perovskite LaNiO₃ shows no analogous behavior',DSC and magnetic susceptibility: no anomaly in LaNiO₃ up to highest measured temperatures,Figures 5(d) and 6(d): featureless DSC and susceptibility for n=∞
• The structural instability manifested in this high-temperature transition may influence low-temperature physical properties and the search for superconductivity.
  • 证据: Summary: 'the potential influence of this high-temperature structural instability on low-temperature physical properties must be carefully considered',Body: 'This sharp increase in resistance… combined with confirmation of a polar room temperature structure suggest a charge ordering scenario',Conclusion: 'Future pressure-dependent studies… may shed further light on its possible relevance to superconductivity'

研究流程

• sample_preparation — High-quality single crystals and phase-pure polycrystalline samples of Lan+1Ni_nO₃n+1 (n=1,2,3,∞) were successfully grown and prepared for comprehensive characterization.
  • 材料: La₂O₃; NiO; Ar; O₂
  • 方法: optical float-zone growth; high-pressure optical float-zone growth; solid-state sintering; ball-milling
  • 观察: single crystals of n=1, 2 (2222 and 1313 polymorphs), 3, ∞ obtained; phase-pure polycrystalline rods for n=2 used as reference
• measurement — A consistent structural and thermodynamic anomaly around 560 K is observed in the layered nickelates La₃Ni₂O₇ and La₄Ni₃O₁₀ by multiple techniques, with no counterpart in the 3D perovskite LaNiO₃.
  • 材料: single crystals; crushed crystals; powders
  • 方法: powder X-ray diffraction (PXRD); single-crystal XRD; scanning transmission electron microscopy (STEM-HAADF); heat capacity; differential scanning calorimetry (DSC); magnetic susceptibility (SQUID); electrical transport
  • 观察: PXRD contour plots for n=2,3 show intensity changes and peak shifts near 560K; lattice-parameter anomalies: out-of-plane expansion and in-plane contraction in 2222; isotropic volume collapse in 1313; kink in monoclinic angle β for n=3; DSC: sharp reversible entropy peaks at ≈560 K for n=2,3; no feature for n=∞; magnetic susceptibility: pronounced high-temperature increase near 590 K for n=2,3; electrical transport: sharp resistance increase by factor 3–6 at transition for n=2; weak anomaly for n=3
• analysis — The 560 K anomaly is confirmed as a distinct structural phase transition characterized by a release of octahedral tilting in the bilayer and isotropic volume collapse in the monolayer-trilayer polymorph, unrelated to previously known transitions.
  • 材料: PXRD patterns; DSC traces; susceptibility curves; transport data
  • 方法: Rietveld refinement; lattice-parameter temperature dependence extraction; entropy change evaluation; comparison of different n members
  • 观察: the transition is first-order with sudden lattice-parameter jumps in n=2; the anomaly does not correspond to the tetragonal-orthorhombic transition (which occurs at different temperatures) nor to the low-temperature density-wave transition (seen below 150 K); entropy release at 560 K is small but clearly resolved; magnetic and transport signatures coincide with the structural change
• interpretation — The high-temperature phase transition near 560 K is a universal feature of mixed-valence layered Ruddlesden-Popper nickelates (n=2,3), distinct from the tetragonal and density-wave transitions, and its structural instability must be taken into account when searching for or interpreting superconductivity in these systems.
  • 观察: the transition appears in both polymorphs of La₃Ni₂O₇ and in La₄Ni₃O₁₀, but not in LaNiO₃; oxygen content and polymorph type influence the transition details; the transition may underlie the large sample-dependent discrepancies in reported properties; its impact on low-temperature electronic states could be relevant for superconductivity