Symmetry-Guided Functional Pathways of Intercalation-Free Rhombohedral (R3) Hafnia Derived from the Fluorite Phase...

Rhombohedral hafnia-based ferroelectrics promise low-coercive, scalable nonvolatile memories, yet their realization has traditionally relied on complex cation intercalation or external stress. Here, we demonstrate a possible intrinsic route to the rhombohedral (R3) phase in Hf_1-xZr_xO₂ through symmetry breaking of the parent fluorite lattice. First-principles calculations under R3 symmetry-constrained equation-of-state conditions show that the 12-atom fluorite-derived configuration, at equiatomic composition (x = 0.5), stabilizes an intrinsically polar R3 ground state with spontaneous polarization P_s = 44.9 μC cm^-2, dielectric permittivity ε_r = 51.3, an ultralow switching barrier of 27.8 meV f.u^.-1, and a coercive field of 0.46 MV cm^-1. Distinct from orthorhombic Pca2₁, the R3 structure shows nonmonotonic dielectric behavior, revealing a symmetry-renormalized polarization mechanism beyond conventional Vegard-type ferroelectricity. Moreover, the R3 phase stabilizes at reduced thickness, with low built-in potential at proper electrodes preserving its low coercive field. Experiments using fast Fourier transform and geometric-phase analysis validate these predictions, and R3-phase-dominant Hf_1-xZr_xO₂ capacitors exhibit comparably low coercive fields (0.65 MV cm^-1) and enhanced dielectric permittivity ε_r = 39.1.