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The recent discovery of superconductivity below 80 K in bulk Ruddlesden–Popper (RP) La3Ni2O7 (LNO327) under high pressure (>14 GPa) has reignited intense interest in nickel-based superconductors1. This breakthrough marks a new chapter in the “Nickel Age”, demonstrating superconductivity in a compound with a nominal Ni2.5+ valence state and a non–square-planar coordination geometry, clearly distinct from both cuprates and infinite-layer nickelates (which are closer to 3d9). As such, LNO327 may represent the prototype of an entirely new family of high-Tc superconductors, where the straightening out of the Ni-O-Ni bond angle accompanied by the loss of the octahedral tilting is thought playing the main role1,2. However, the requirement of relatively high hydrostatic pressure severely limits fundamental investigation of its superconducting state, as most spectroscopic probes are incompatible with such conditions. By exploiting the unique tuning parameters offered by thin-film growth, such as epitaxial strain, dimensional confinement, and interface engineering, significant progress has already been achieved. In late 2025, LNO327 thin films displayed a superconducting transition with a critical temperature below 42 K following ozone annealing3. However, the superconducting phase remains highly elusive, leaving considerable room for further optimization and discoveries.
In this work, we present a detailed structural study of compressively strained LNO327 thin films by using scanning transmission electron microscopy combined with electron energy loss spectroscopy. The films were grown onto (001)-oriented SrLaAlO4 substrates by pulsed laser deposition assisted by reflection high energy electron diffraction. By comparing differently annealed films, resulting in distinct superconducting properties, we show a structural reference framework for LNO327 thin films and highlight the range of structural configurations accessible in this system (LNO-2222 vs LNO-1313), offering new insight into the structure that possibly stabilize superconductivity at ambient pressure and guiding future efforts to engineer more stable superconducting nickelates.
References :
1. Sun, H. et al. Signatures of superconductivity near 80 K in a nickelate under high pressure. Nature 621, 493–498 (2023).
2. Wang, L. et al. Structure Responsible for the Superconducting State in La3Ni2O7 at High-Pressure and Low-Temperature Conditions. J. Am. Chem. Soc. 146, 7506–7514 (2024).
3. Ko, E. K. et al. Signatures of ambient pressure superconductivity in thin film La3Ni2O7. Nature 638, 935–940 (2025).