Towards structural softness and enhanced electromechanical responses in HfO2 ferroelectrics

TL;DR

This study predicts that applying epitaxial strain to HfO2 ferroelectrics can induce structural instabilities and soft phonon modes, leading to enhanced electromechanical responses, especially in superlattice configurations.

Contribution

It demonstrates, through first principles calculations, that strain can induce and control structural softness and electromechanical enhancements in HfO2 ferroelectrics, a novel insight.

Findings

Epitaxial tensile strain causes mechanical instability in HfO2 ferroelectrics.

Soft phonon modes lead to increased dielectric and piezoelectric responses.

Superlattices lower the strain threshold needed for these effects.

Abstract

Structural softness - often characterized by unstable phonon modes and large electromechanical responses - is a hallmark of ferroelectric perovskites like BaTiO3 or Pb(Ti,Zr)O3. Whether HfO2 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, which transforms discontinuously into an antipolar polymorph. Then, upon release of the tensile strain, the antipolar polymorph transforms back to the ferroelectric state by 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 HfO2, we show that the required deformations are considerably lowered in superlattices with other simple oxides, which may facilitate realizing these effects experimentally.