层间滑动铁电体ZrI $_2$的相竞争和负压电性
Phase competition and negative piezoelectricity in interlayer-sliding ferroelectric ZrI$_2$
摘要：
层间滑动铁电性是近年来提出的一种非传统的在范德华多层膜中追求电极性的方法，在WTe $_2$双层膜中得到了实验证实，尽管它是金属的。最近，另一个范德瓦尔斯系统，即，$_2$双层被预测表现出具有面内和面外极化的层间滑动铁电性[Phys. Rev. \textbf{103}，165420（2021）]。本文研究了$_2$块体，它具有两个竞争相（$α$ \textit{vs} $β$），这两个相都是从共同的母体$s$ -相衍生而来的。$β$具有相当大的面外偏振（$0.39$ $μ$ C/cm $^2$），而其面内分量得到完全补偿.它们的近似能量提供了通过适度的静水压力和单轴应变来调谐基态相位的机会。此外，$β$ $_2$中的负纵向压电性主要由作为层间滑动铁电性的独特特征的ZrI $_2$层的增强偶极子贡献，这不同于许多具有负纵向压电性的其它层状铁电体如CuInP $_2$ S $_6$。
英文摘要：
The so-called interlayer-sliding ferroelectricity was recently proposed as an unconventional route to pursuit electric polarity in van der Waals multi-layers, which was already experimentally confirmed in WTe$_2$ bilayer even though it is metallic. Very recently, another van der Waals system, i.e., the ZrI$_2$ bilayer, was predicted to exhibit the interlayer-sliding ferroelectricity with both in-plane and out-of-plane polarizations [Phys. Rev. B \textbf{103}, 165420 (2021)]. Here the ZrI$_2$ bulk is studied, which owns two competitive phases ($α$ \textit{vs} $β$), both of which are derived from the common parent $s$-phase. The $β$-ZrI$_2$ owns a considerable out-of-plane polarization ($0.39$ $μ$C/cm$^2$), while its in-plane component is fully compensated. Their proximate energies provide the opportunity to tune the ground state phase by moderate hydrostatic pressure and uniaxial strain. Furthermore, the negative longitudinal piezoelectricity in $β$-ZrI$_2$ is dominantly contributed by the enhanced dipole of ZrI$_2$ layers as a unique characteristic of interlayer-sliding ferroelectricity, which is different from many other layered ferroelectrics with negative longitudinal piezoelectricity like CuInP$_2$S$_6$.
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