Stress-induced ferroelectricity in hafnia core-shell nanoparticles

In contrast to hafnia ( HfO 2 ) thin films, where the appearance of switchable ferroelectric polarization can be induced by strain or defect engineering, reliable methods for controlling ferroelectricity in HfO 2 nanoparticles are absent. Direct experimental observations of ferroelectric hysteresis and ferroelectric domains in these nanoparticles have also not been reported. To the best of our knowledge, stress-induced ferroelectric states in the HfO 2 nanoparticles have not been explored. In this work, we study the influence of chemical stress on phase diagrams, dielectric properties, and polar properties of spherical HfO 2 core-shell nanoparticles using a Landau-Ginzburg-Devonshire free energy functional that includes trilinear and biquadratic couplings involving polar, antipolar, and nonpolar order parameters. The ferroelectric phase exhibits reentrant behavior as a function of nanoparticle size, such that the spontaneous polarization exists only within a limited range of core radii R c , namely R cr min < R c < R cr max . The minimal critical radius R cr min is primarily determined by the size dependence of the depolarization field and correlation effects, whereas the maximal critical radius R cr max is governed mainly by the size dependence of chemical stresses induced by the elastic defects in the shell. Thus, this work identifies a stress-driven mechanism for reentrant ferroelectricity stabilization in nanoscale hafnia systems, arising from the competition between depolarization field-induced suppression of ferroelectricity and its stabilization by shell-induced chemical stress. We revealed that relatively large compressive chemical strains are necessary to induce the ferroelectric phase in the HfO 2 nanoparticles. Successful chemical strain engineering opens the way for significant enhancement of nanoscale HfO 2 polar properties for applications in advanced memory cells and logic devices.