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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.

This study employs time-resolved optical spectroscopy to investigate the ultrafast dynamics of high-energy electronic excitations in bilayer nickelate La₃Ni₂O₇ from 10 K to room temperature at ambient pressure. Two high-energy electronic excitations originating from distinct interband transitions are identified at approximately 1.8 eV and 2.4 eV, revealing different density wave (DW) gaps of about 54 meV and 67 meV, respectively. The relaxation dynamics of these two excited states are well described by the Rothwarf-Taylor model. Additionally, four coherent Raman-active phonon modes are observed, exhibiting varying coupling strengths to the different electronic excitations. The phonon softening upon heating from about 100 K to room temperature can be explained by a semi-quantitative model incorporating thermal expansion and anharmonic phonon-phonon coupling, while the deviation of measured phonon frequencies from the model fit at low temperatures suggests an additional contribution from electron-phonon coupling. This work directly demonstrates the complex gap structure and phonon dynamics in this material, providing key insights into its density wave mechanism and many-body effects.

This study reports the discovery of high-magnetic-field-stabilized reentrant superconductivity in (Sm,Eu,Ca,Sr)NiO₂ infinite-layer nickelate thin films. Through resistive and radio-frequency inductive measurements, in addition to the low-field superconducting state, another superconducting state characterized by a sharp resistivity drop was observed in the high-field region, with a transition temperature of approximately 9.6–11.7 K in low-Tc samples, while in high-Tc samples (up to 31.7 K) the low-field and high-field superconducting phases merge, and the upper critical field far exceeds the Pauli paramagnetic limit. The phase diagram can be accurately described by a Werthamer-Helfand-Hohenberg (WHH) model modified with an internal exchange field, indicating that this phenomenon originates from the Jaccarino-Peter compensation mechanism, where the internal exchange field generated by the Eu²⁺ magnetic moments counteracts the applied magnetic field. This finding realizes for the first time field-induced reentrant superconductivity in materials with relatively high superconducting transition temperatures, offering a new pathway toward developing superconducting magnets and devices capable of operating under magnetic fields of several tens of teslas.

Single crystals of Ruddlesden–Popper nickelates La4Ni3O10 were grown by means of the floating-zone technique at oxygen pressure of 20 bar. Our results reveal the effects of the annealing process under pressure on the crystal structure. We present the requirements for crystal growth and show how a reported ferromagnetic impurity phase can be avoided. The different growth and post-annealing processes result in two distinct phases 𝑃 21∕𝑎 and Bmab in which the metal-to-metal transitions occur at 152 K and 136 K, respectively.

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