Daily Overview: Today’s highlight focuses on revealing a ubiquitous high-temperature structural phase transition in the Ruddlesden-Popper nickelate La$_{n+1}$Ni$_n$O$_{3n+1}$ series. Through multi-technique characterization across a broad temperature range, previously overlooked lattice-parameter anomalies were discovered in the n=2 and n=3 compounds at around 560 K: the bilayer phase exhibits a sudden increase in the out-of-plane lattice constant coupled with an in-plane contraction, indicating an abrupt release of octahedral tilting; in contrast, the monolayer–trilayer polymorphs display isotropic volume shrinkage, and the monoclinic angle β of the trilayer phase shows a clear kink. Heat capacity and DSC measurements confirm the thermodynamic nature of this transition. This high-temperature transformation is distinct from the known tetragonal transition and the low-temperature density-wave transitions, and it is absent in the n=∞ perovskite LaNiO₃. The study emphasizes that such high-temperature structural instability must be seriously considered for its potential impact on the low-temperature physical properties when exploring superconductivity in nickelates. arXiv submission processing window: 2026-06-26 00:00 to 2026-06-26 00:00 UTC.
1. High temperature transitions in Ruddlesden-Popper nickelates La$_{n+1}$Ni$_{n}$O$_{3n+1}$
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5.7479 - Authors: P. Reiss, A. Shevchenko, P. S. Lizama, J. Nuss, R. Dinnebier, P. A. van Aken, M. Hepting, M. Isobe, Y. E. Suyolcu, H. Takagi, B. Keimer, P. Puphal
- Affiliations: Max Planck Institute for Solid State Research
- Link: https://arxiv.org/abs/2606.27040
- Paper page: High temperature transitions in Ruddlesden-Popper nickelates Lan+1NinO₃n+1
Summary: This study systematically investigates single-crystal and powder samples of Ruddlesden-Popper nickelates La_{n+1}Ni_nO_{3n+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.