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)O₂ (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.