Decoding the Complexity of Ferroelectric Orthorhombic HfO2: A Unified Mode Expansion Approach

TL;DR

This paper introduces a unified phonon mode expansion framework to analyze the complex orthorhombic phases and ferroelectric properties of HfO2, enabling comprehensive study of phase structures, domain walls, and switching mechanisms.

Contribution

The paper presents a novel unified mode expansion approach that simplifies the analysis of orthorhombic phases and ferroelectric switching in HfO2, linking complex phases to high-symmetry cubic modes.

Findings

Clarified the origin of orthorhombic stability from mode energy functional

Enumerated and analyzed stable domain walls and their criteria

Summarized all possible switching mechanisms in HfO2

Abstract

The ferroelectricity in $\mathrm{HfO}_2$ thin films is widely attributed to the formation of a polar orthorhombic phase named OIII phase. However, the complexity of OIII phase originated from its low symmetry becomes an obstacle for studying ferroelectric properties of $\mathrm{HfO}_2$. Here, we developed a unified framework based on phonon mode expansion for studying ferroelectric $\mathrm{HfO}_2$. In this framework, phase structures, domain walls and switching paths of orthogonal crystal system can be studied from the same basis of mode analysis. The OIII phase and other orthogonal phases can be represented by the high-symmetry cubic phase with the excitation of cubic phonon modes, into which the complexity of orthogonal phases is faithfully coded. To present the capability of this mode expansion approach, we clarified the origin of orthorhombic stability from the energy functional of modes; enumerated inequivalent domain walls and calculated their stable criteria; and summarized all possible switching mechanisms. This unified framework can be used to simplify the study of domain wall structures and transition paths. Furthermore, it can provide a new perspective for ferroelectricity in $\mathrm{HfO}_2$ from phonon mode analysis.