Finite-size effects on the ferroelectricity in rhombohedral HfO$_2$

TL;DR

This study investigates how finite-size effects influence the ferroelectric properties of rhombohedral HfO$_2$, revealing that surface charges and strain significantly reduce polarization, with implications for the material's ferroelectric stability.

Contribution

It provides a detailed analysis of finite-size effects on HfO$_2$'s ferroelectricity, including atomistic and symmetry perspectives, and suggests no critical thickness is needed for ferroelectricity.

Findings

Surface charges reduce polarization in thin HfO$_2$ slabs.

No critical thickness for ferroelectricity in HfO$_2$.

Finite-size effects are independent of slab thickness within studied range.

Abstract

In this work we analyze the finite-size effects on the structural properties and on the polarization of the rhombohedral phase of HfO$_2$ subjected to a biaxial compressive strain. We show how the presence of surface charges affects the polarization, leading to a strong reduction with respect to its bulk value. This reduction can be ascribed to two mechanisms: the coupling between compressive strain and the phase-transition order parameter; the changes in the ferroelectric distortion. We give two alternative explanations of this phenomenon: from an atomistic point of view, analyzing the evolution of the bond lengths, and from a symmetry-analysis point of view, considering the changes in the amplitude of the symmetry-allowed distortions, when a slab configuration is considered. These results are independent on the slab-thickness in the considered range, suggesting the absence of a critical thickness for ferroelectricity in HfO$_2$, in agreement with the proposed improper nature of hafnia ferroelectricity.