Structural softness–often characterized by unstable phonon modes and large electromechanical responses–is a hallmark of ferroelectric perovskites, like ${\mathrm{BaTiO}}_3$ or $\mathrm{Pb}\left(\mathrm{Ti},\mathrm{Zr}\right){\mathrm{O}}_3$. Whether ${\mathrm{HfO}}_2$ 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 $\left(Pca{2}_1\right)$, 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 ${\mathrm{HfO}}_2$, we show that the required deformations are considerably lowered in superlattices with other simple oxides, which may facilitate realizing these effects experimentally.

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• Dielectric properties
• Ferroelasticity
• Ferroelectricity
• First-principles calculations
• Piezoelectricity
• Dielectrics
• Ferroelectrics
• Piezoelectrics
• Density functional theory