Boosting Inversion Symmetry Breaking in Epitaxial Tetragonal ZrO 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) ZrO. Epitaxial HfZrO 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-ZrO phase deviates from the expected phase transition from nonpolar HfO through ferroelectric HfZrO to antiferroelectric Zr-rich HfZrO. Second harmonic generation measurements reveal unexpected inversion symmetry breaking in T-phase ZrO. 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 HfO-ZrO-based materials and provide critical insight into the polar nature of T-phase ZrO.