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This study presents a detailed structural analysis of epitaxial La₃Ni₂O₇₋δ thin films using scanning transmission electron microscopy combined with electron energy loss spectroscopy. The films were prepared on SrLaAlO₄ substrates via pulsed laser deposition and exhibited significantly distinct superconducting properties after different ozone annealing treatments. It was found that the stabilization of the superconducting phase is closely related to oxygen stoichiometry uniformity, epitaxial strain, and specific stacking structural motifs such as bilayers and polytypes. By correlating the rich morphology of stacking polytypes with transport behavior, a theoretical framework for understanding metastable superconducting phases in bilayer nickelate thin films was established. The results reveal the critical roles of oxygen content, lattice strain, and structural ordering in achieving ambient-pressure superconductivity, providing a clear pathway for designing new nickel-based superconducting materials.

Through high-throughput first-principles simulations, this study systematically compares the thermodynamic stability of delafossite (D1), ordered rock salt variant (D2), and infinite-layer (IL) oxides at ABO₂ stoichiometry, constructing phase diagrams encompassing 2,346 elemental combinations. The results demonstrate that for nickelates, palladates, and platinate, the delafossite structure exhibits stability comparable to or even superior to the infinite-layer phase, with competition between these two phases and the perovskite phase. Electronic structure analysis reveals that delafossite compounds feature an inverted cation order, with the Fermi surface dominated by d_{z^2} orbital contributions, distinctly different from the d_{x^2-y^2} characteristics of the infinite-layer phase. Among all candidate systems, the La-Ni combination is the thermodynamically optimal choice for stabilizing the infinite-layer structure. Furthermore, hole doping via Ca, Sr, and Ba systematically enhances the relative stability of the infinite-layer phase across the three transition metal families. These findings elucidate the fundamental challenges in synthesizing substrate-free bulk infinite-layer oxides and provide guidance for the experimental exploration of novel superconducting compounds.

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