Phase competition and negative piezoelectricity in interlayer-sliding ferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Zr</mml:mi><mml:msub><mml:mi mathvariant="normal">I</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

The so-called interlayer-sliding ferroelectricity was recently proposed as an unconventional route to pursuit electric polarity in van der Waals multi-layers, which already experimentally confirmed WTe$_2$ bilayer even though it is metallic. Very recently, another system, i.e., the ZrI$_2$ bilayer, predicted exhibit with both in-plane and out-of-plane polarizations [Phys. Rev. B \textbf{103}, 165420 (2021)]. Here bulk studied, owns two competitive phases ($\alpha$ \textit{vs} $\beta$), of are derived from common parent $s$-phase. $\beta$-ZrI$_2$ a considerable polarization ($0.39$ $\mu$C/cm$^2$), while its component fully compensated. Their proximate energies provide opportunity tune ground state phase by moderate hydrostatic pressure uniaxial strain. Furthermore, negative longitudinal piezoelectricity dominantly contributed enhanced dipole layers unique characteristic ferroelectricity, different many other layered ferroelectrics like CuInP$_2$S$_6$.