Atomic perspective on plasticity mechanism of ionic-covalent silver sulfide

Abstract

Due to the diversity of atomic bonding, good plasticity and machinability are often considered hallmark characteristics of metals. Novel plastic inorganic semiconductors like α-Ag₂S have challenged this conventional thinking, but relevant first-principles calculations still lack an intuitive and comprehensive understanding of the underlying plasticity mechanisms. From the perspective of machine learning molecular dynamics that can describe the microstructure evolution aptly, this work reveals the plasticity mechanism of the ionic-covalent system α-Ag₂S. It is found that the S sublattice stabilized by movable Ag acts as a framework during deformation, while the mechanical response of α-Ag₂S is sublattice-dependent. Shear bands or kink bands originating from random and local micro-kinks signify the plastic features, and the subsequent amorphization enables sustained deformation under high strains. Different from features in metals, the oppositely signed dislocation pairs in α-Ag₂S can achieve nucleation and motion through coordinated lattice expansion and contraction, while the twining-like kink triggered in a staggered manner allows the material to accommodate large shear strains. The established idealized models capture the unconventional dislocation pair and pseudo-twinning kink, narrowing the blind area in our understanding of plasticity mechanisms within similar systems. The summarized novel structural and deformation features provide clear clues for identifying other plastic ionic-covalent crystals in superionic conductors.