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

Summary

This study systematically investigates single-crystal and powder samples of Ruddlesden-Popper nickelates Lan+1Ni_nO₃n+1 (n=1,2,3,∞) using a combination of powder and single-crystal X-ray diffraction, heat capacity, and differential scanning calorimetry measurements across a broad temperature range of 2–1000 K, revealing a previously overlooked high-temperature phase transition. For the n=2 and n=3 compounds, pronounced lattice-parameter anomalies are observed around 560 K: in the bilayer 2222 phase, the out-of-plane lattice constant exhibits a sudden increase while the in-plane parameter contracts, indicating an abrupt release of octahedral tilting, whereas the monolayer–trilayer 1313 polytype displays an isotropic volume collapse; in the trilayer n=3 phase, the monoclinic angle β shows a clear kink near this temperature, and heat-capacity and DSC data further confirm the thermodynamic character of the transition. This transition is entirely distinct from the known high-temperature tetragonal transition and the low-temperature density-wave transition, and the n=∞ perovskite LaNiO₃ shows no analogous behavior. The study establishes that this high-temperature phase transition is a universal feature of the nickelate RP series and emphasizes that, in the search for superconductivity, the potential influence of this high-temperature structural instability on low-temperature physical properties must be carefully considered.

Materials

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

Methods

• 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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• A previously underappreciated high-temperature phase transition exists in Lan+1Ni_nO₃n+1 (n=2,3) near 560 K.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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'

Workflow

• 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.
  • Materials: La₂O₃; NiO; Ar; O₂
  • Methods: optical float-zone growth; high-pressure optical float-zone growth; solid-state sintering; ball-milling
  • Observations: 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.
  • Materials: single crystals; crushed crystals; powders
  • Methods: 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
  • Observations: 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.
  • Materials: PXRD patterns; DSC traces; susceptibility curves; transport data
  • Methods: Rietveld refinement; lattice-parameter temperature dependence extraction; entropy change evaluation; comparison of different n members
  • Observations: 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.
  • Observations: 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