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This study systematically investigates the strain effect in trilayer nickelate La₄Ni₃O₁₀ thin films through atomically precise synthesis, electrical transport measurements, picometer-resolution electron microscopy, and synchrotron X-ray diffraction. While compressive epitaxial strain effectively suppresses the parent density-wave order and enhances crystal symmetry (e.g., eliminating out-of-plane octahedral rotations), no superconductivity is observed even under the maximum compressive strain of -2.8%. Critical structural characterization reveals that compressive strain fails to completely eliminate the characteristic in-plane octahedral rotations in the thin films, which exhibit interlayer inequivalence between the inner and outer layers of each trilayer unit and persist robustly. Synchrotron X-ray diffraction shows that the amplitude of in-plane rotations decreases monotonically with compressive strain but does not vanish entirely. In contrast, in the bilayer system La₃Ni₂O₇, compressive strain fully suppresses all octahedral rotations, thereby inducing superconductivity. These results uncover a key difference between trilayer and bilayer systems, indicating that ambient-pressure superconductivity in trilayer nickelates cannot be achieved solely through epitaxial strain engineering, and alternative tuning methods need to be explored.

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This study employs high-brightness synchrotron X-ray diffraction for precise structural analysis of high-quality single crystals of the bilayer nickelate La₃Ni₂O₇. Using a large dynamic range detector, we successfully resolved previously overlooked weak diffraction signals, whose intensities are nearly four orders of magnitude weaker than the main Bragg reflections. These observations indicate the presence of glide mirror symmetry breaking in the crystal, leading to a polar structure (space group Ima2) instead of the previously assumed centrosymmetric model (Fmmm). Further structural refinement reveals two inequivalent nickel sites with significantly different Ni–O bond lengths. Combined with bond valence sum calculations, this suggests a checkerboard-like charge ordering of nickel sites, which, together with oxygen octahedral tilting, endows the crystal with polarity. The charge-ordered phase is structurally analogous to the polar state observed in bilayer manganites. This study establishes the polar charge-ordered state of La₃Ni₂O₇ at ambient pressure, indicating its competition with pressure-induced superconductivity, and provides critical structural insights for understanding phase competition mechanisms and the origin of pressure-induced superconductivity in bilayer nickelates.

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