Mukherjee B., Fedorova N.S., Íñiguez-González J.
Physical Review B, vol. 112, n° 6, art. no. L060102, 2025
Structural softness–often characterized by unstable phonon modes and large electromechanical responses–is a hallmark of ferroelectric perovskites, like BaTiO<sub>3</sub> or Pb(Ti, Zr)O<sub>3</sub>. Whether HfO<sub>2</sub> ferroelectrics present any such structural softness is still a matter of debate. Here, using first principles calculations, we predict that it is possible to induce structural instabilities in hafnia. More specifically, our calculations show that in-plane epitaxial tensile strain causes a mechanical instability of the ferroelectric phase (Pca2<sub>1</sub>), which transforms discontinuously into an antipolar polymorph (Pbcn). Then, upon reduction of the tensile strain, the antipolar polymorph transforms back to the ferroelectric state through a soft phonon instability. We show that the softening is accompanied by enhancements in the dielectric and piezoelectric responses. While these transitions occur at high epitaxial strains for pure ferroelectric HfO<sub>2</sub>, we show that the required deformations are considerably lowered in superlattices with other simple oxides, which may facilitate realizing these effects experimentally.
