HfO$_2$中反极性$Pbcn$相的意外密度泛函依赖性
Unexpected Density Functional Dependence of the Antipolar $Pbcn$ Phase in HfO$_2$
AI总结：
本研究通过DFT计算和DPMD模拟发现HfO2的Pbcn相稳定性高度依赖交换关联泛函，但在固定力学边界条件下各泛函预测的相稳定性和翻转势垒一致，揭示了Pbcn相对铁电氧化铪相变机制的重要影响。
摘要：
HfO$_2$的反极性$Pbcn$相被认为在铁电氧化铪的相变和极化翻转机制中发挥重要作用。在本研究中，我们对密度泛函理论（DFT）计算和深度势分子动力学（DPMD）模拟进行了全面的基准测试，以研究氧化铪的热力学稳定性和相变行为，特别关注$Pbcn$和铁电$Pca2_1$相之间的关系。我们的结果揭示了不同交换关联泛函在预测$Pbcn$相相对于$Pca2_1$相的稳定性方面存在显著差异。值得注意的是，PBE和杂化HSE06泛函表现出一致的趋势，这与PBEsol和SCAN泛函的预测不同。对于给定的密度泛函，基于DFT的准谐自由能计算预测的温度驱动相变与使用在同一密度泛函上训练的深度势进行的有限温度MD模拟相一致。具体而言，PBE泛函预测随着温度升高会发生从$Pca2_1$到$Pbcn$的转变，而PBEsol预测从$Pca2_1$到$P4_2/nmc$的转变。一个特别引人注目且令人安心的发现是，在由$Pca2_1$基态结构定义的固定力学边界条件下，所有泛函都预测了一致的相对相稳定性、可比的翻转势垒以及畴壁能量。这些发现强调了$Pbcn$相在影响铁电氧化铪相变和翻转机制方面的独特特征。
英文摘要：
The antipolar $Pbcn$ phase of HfO$_2$ has been suggested to play an important role in the phase transition and polarization switching mechanisms in ferroelectric hafnia. In this study, we perform a comprehensive benchmark of density functional theory (DFT) calculations and deep potential molecular dynamics (DPMD) simulations to investigate the thermodynamic stability and phase transition behavior of hafnia, with a particular focus on the relationship between the $Pbcn$ and ferroelectric $Pca2_1$ phases. Our results reveal significant discrepancies in the predicted stability of the $Pbcn$ phase relative to the $Pca2_1$ phase across different exchange-correlation functionals. Notably, the PBE and hybrid HSE06 functionals exhibit consistent trends, which diverge from the predictions of the PBEsol and SCAN functionals. For a given density functional, temperature-driven phase transitions predicted by DFT-based quasi-harmonic free energy calculations aligns with finite-temperature MD simulations using a deep potential trained on the same density functional. Specifically, the PBE functional predicts a transition from $Pca2_1$ to $Pbcn$ with increasing temperature, while PBEsol predicts a transition from $Pca2_1$ to $P4_2/nmc$. A particularly striking and reassuring finding is that under fixed mechanical boundary conditions defined by the ground-state structure of $Pca2_1$, all functionals predict consistent relative phase stabilities and comparable switching barriers as well as domain wall energies. These findings underscore the unique characteristics of the $Pbcn$ phase in influencing phase transitions and switching mechanisms in ferroelectric hafnia.
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