Phase competition and negative piezoelectricity in interlayer-sliding ferroelectric ZrI$_2$

Abstract

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 WTe2 bilayer even though it is metallic. Very recently, another van der Waals system, i.e., the ZrI2 bilayer, was predicted to exhibit the interlayer-sliding ferroelectricity with both in-plane and out-of-plane polarizations [Phys. Rev. B 103, 165420 (2021)]. Here the ZrI2 bulk is studied, which owns two competitive phases (α vs β), both of which are derived from the common parent s-phase. The β-ZrI2 owns a considerable out-of-plane polarization (0.39 µC/cm 2 ), while its inplane 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-ZrI2 is dominantly contributed by the enhanced dipole of ZrI2 layers as a unique characteristic of interlayer-sliding ferroelectricity, which is different from many other layered ferroelectrics with negative longitudinal piezoelectricity like CuInP2S6.