Direct observation of charge density and electronic polarization in fluorite ferroelectrics by 4D-STEM

<p>The fluorite ferroelectrics is extremely promising for memory applications due to the silicon compatibility and the robust ferroelectricity with decreasing size. However, the direct observation of local electronic polarization remains elusive, thereby hindering the comprehension of the atomic-scale origin of ferroelectricity. Here, we directly map the real-space charge density of the ZrO<sub>2</sub> nanocrystal in its polar, nonpolar, as well as interphase regions with sub-Ångström resolution by four-dimensional scanning transmission electron microscopy (4D-STEM). Based on the variation of the electric dipole moments, we analyze the electronic contribution to the total spontaneous polarization, which reaches a maximum of 17.8%. In comparison to the continuous polarization in conventional ferroelectric units, the local polarization profile looks like a maple leaf edge at the tetragonal-orthorhombic phase interface, which suggests a gradual increase in the electronic polarization and the covalent nature of the Zr-O bond. We validate these findings with 4D-STEM simulations and calculations based on density functional theory. These findings provide atomic insights into the bonding nature and phase transition feature in fluorite oxides, and unravel the likely origin of ferroelectricity in ferroelectrics.</p>