Anharmonic thermodynamics redefines metastability and parent phases in ferroelectric HfO2

TL;DR

This study uses advanced thermodynamic modeling to show that anharmonic effects significantly influence the stability of ferroelectric phases in HfO2, challenging previous harmonic-based predictions.

Contribution

It introduces a machine learning force field combined with self-consistent phonon theory to accurately account for anharmonicity in HfO2 thermodynamics.

Findings

Ferroelectric orthorhombic phase oIII is metastable below 0.1kBT across a range of conditions.

Previous harmonic models predicted metastability only above 1500 K, which is contradicted by this study.

Evidence of temperature- and pressure-dependent ferroelectric parent phases was found.

Abstract

Hafnia (HfO2) is a silicon-compatible dielectric material, yet stabilizing its desired but metastable ferroelectric phase remains challenging. Phase stability predictions by density functional theory (DFT) have provided crucial guidance, but most simulations neglected or only treated finite temperature effects with (quasi-)harmonic approximation due to high computational cost of DFT. Here, we develop a machine learning force field and perform thermodynamic calculations for HfO2 using self-consistent phonon theory to address growing evidence of anharmonicity. Our results reveal that the ferroelectric orthorhombic phase oIII exhibits metastability below 0.1kBT under most conditions within the simulated regime of temperature and pressure (600 K <= T <= 1500 K and 0 <= p <= 7.5 GPa), contradicting previous harmonic predictions of metastability above 1500 K at ambient pressure. We further report evidence for temperature- and pressure-dependent ferroelectric parent phase despite efforts to identify a universal one. This study highlights the importance of anharmonicity and provides an effective approach for its treatment in the design of HfO2-based ferroelectrics.