Boosting Inversion Symmetry Breaking in Epitaxial Tetragonal ZrO₂ Via Atomic Layer Deposition

The stabilization of intermediate polar phases in fluorite-structured oxides is critical for advancing ferroelectric and antiferroelectric applications. Here, we report the stabilization of epitaxial polar tetragonal (T) ZrO₂. Epitaxial Hf_xZr_1-xO₂ thin films (x = 1, 0.75, 0.5, 0.25, and 0) are synthesized on (001) yttria-stabilized zirconia substrates via atomic layer deposition. The emergence of the unprecedented polar T-ZrO₂ phase deviates from the expected phase transition from nonpolar HfO₂ through ferroelectric Hf_0.5Zr_0.5O₂ to antiferroelectric Zr-rich Hf_xZr_1-xO₂. Second harmonic generation measurements reveal unexpected inversion symmetry breaking in T-phase ZrO₂. High-resolution 4D-scanning transmission electron microscopy further confirms the presence of electric dipoles originating from off-centered oxygen displacements. These findings establish a pathway for the low-temperature epitaxial synthesis of HfO₂-ZrO₂-based materials and provide critical insight into the polar nature of T-phase ZrO₂.