Ferroelectricity, triple-well potential, and negative piezoelectricity in the antiperovskite γ-Ag 3 SI

Various perovskites, such as BaTiO3, exhibit ferroelectric distortion driven by long-range Coulomb interactions and d-p orbital hybridization. Antiperovskites, with a structure resembling perovskites but with inverted cations and anions, display significantly distinct physical properties in terms of ferroelectricity and piezoelectricity despite their structural similarities. However, the literature on these materials is scarce. Among them, Ag3SI, a three-dimensional antiperovskite superionic conductor, demonstrates exceptional ionic conduction in nonpolar phases (alpha and 3 ) but shows reduced conductivity in the polar phase (gamma). Our study uncovers the unconventional mechanism of ferroelectricity in Ag3SI: an anomalous s-d orbital self-mixing in the polar gamma phase, originating from Ag+ short-range interactions. Additionally, this antiperovskite exhibits two rare phenomena in three-dimensional solid state: a triple-well potential and negative longitudinal piezoelectric effect. Our investigation of Ag3SI, establishes a theoretical framework to grasp ferroelectricity and negative piezoelectricity in antiperovskites, enriching our understanding of these captivating materials.