Spontaneous polarization is essential for ferroelectric functionality in non-centrosymmetric crystals. High-integration-density ferroelectric devices require the stabilization of ferroelectric polarization in small volumes. Here, atomic-resolution transmission electron microscopy imaging reveals that twinning-induced symmetry breaking in colloidal nanocrystals of centrosymmetric HfO2 leads to the formation of multiple polarization orders, which are associated with sub-nanometer ferroelectric and antiferroelectric phases. The minimum size limit of the ferroelectric phase is found to be ~4 nm3. Density functional theory calculations indicate that transformations between the ferroelectric and antiferroelectric phases can be modulated by lattice strain and are energetically possible in either direction. The results of this work provide a route towards applications of HfO2 nanocrystals in information storage at densities that are more than an order of magnitude higher than the scaling limit defined by the nanocrystal size.
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