Unlocking the phase evolution of the hidden non-polar to ferroelectric transition in HfO₂-based bulk crystals

The discovery of ferroelectricity in hafnium dioxide (HfO₂) thin films over the past decade has revolutionized the landscape of ferroelectrics, providing a promising candidate for next-generation ferroelectrics beyond the constraints of Moore's law. However, the underlying formation mechanism of their metastable and volatile ferroelectric phase is under debate. Herein, we successfully grow HfO₂-based (Lu:Hf_1-xZr_xO₂) bulk crystals and gain a comprehensive understanding of the non-polar to ferroelectric phase evolution. We achieve a controllable polymorphic engineering by elucidating the synergistic modulation of co-doped Lu³⁺ and Zr⁴⁺ ions. Our investigation unveils the intricate local structural transitions involved in the formation of the ferroelectric orthorhombic Pbc2₁ phase from the metastable tetragonal phase. We also establish a controllable tetragonal-to-orthorhombic transformation route, effectively improving the ferroelectric phase component within bulk crystals. Our findings will advance the comprehension of ferroelectric mechanisms in fluorite-structured materials, paving the way for significant strides in developing HfO₂-based nonvolatile electronic and photonic devices.