Origin of Negative Longitudinal Piezoelectricity Based on 2D Bipolar OOP Piezoelectric Metal Halides

Two-dimensional (2D) piezoelectric materials exhibiting negative longitudinal piezoelectricity have garnered much attention due to their unique electromechanical properties, while the candidate material has rarely been reported. In this study, we present a novel paradigm for 2D negative longitudinal piezoelectric materials (<i>e</i> ₃₃ < 0) through the manipulation of bipolar out-of-plane (OOP) piezoelectric materials (<i>e</i> ₃₁ × <i>e</i> ₃₂ < 0), characterized by the competitive relationship between <i>e</i> ₃₁ and <i>e</i> ₃₂. Through symmetry analysis, we establish the fundamental requirements for achieving bipolar piezoelectricity and elucidate the underlying mechanism of negative longitudinal piezoelectricity in engineered bipolar OOP materials. Using first-principles calculations, we systematically investigate a family of 2D metal halides MX₂ (M = Mg/Ca/Sr/Ba/Zn/Cd/Hg; X = F/Cl/Br/I) materials and identify 24 Janus MXY materials as promising candidates exhibiting both bipolar OOP piezoelectricity and negative longitudinal piezoelectricity. The theoretical predictions are further validated through the application of electric fields and strain along the <i>z</i> direction. This work not only provides an ideal platform for further comprehension of the electromechanical coupling effect but also paves the way for the development of novel direction-dependent electromechanical devices.