A Critical Strain Window for Stabilizing Polar Orthorhombic Hf ₀ . ₅ Zr ₀ . ₅ O ₂ Epitaxial Thin Films with Scale‐Free Domain Walls

Ferroelectric (Hf, Zr)O 2 (HZO) films are well positioned to revolutionize the next‐generation memories and neuromorphic computing, though metastability of their polar orthorhombic phase remains a daunting challenge. Here, we demonstrate that the phase structure of epitaxial HZO films can be effectively tuned on yttria‐stabilized‐zirconia (YSZ) substrate, resulting in tetragonal, orthorhombic, and monoclinic phases with distinct strain states modulated by film thickness. A critical strain window (+1.16% < ε yy < +2.30%) is identified to stabilize the polar orthorhombic phase, outside of which tetragonal or monoclinic phase emerges. Tetragonal‐orthorhombic phase transition is observed from lattice relaxation, confirming that the phase structure of HZO is governed by strain. 90° nanoscale domains with interleaved yet atomically sharp domain walls are also revealed, rendering direct experimental evidence for scale‐free ferroelectricity. Our findings not only establish critical conditions for stable polar orthorhombic HZO, but also shed new insight into unconventional scaling of ferroelectric fluorite oxides.