Isosymmetric phase transitions, ultrahigh ductility, and topological nodal lines in α−Ag2S

Abstract

We report two reversible pressure-induced isosymmetric phase transitions in $\ensuremath{\alpha}\ensuremath{-}\mathrm{A}{\mathrm{g}}_{2}\mathrm{S}$ that are accompanied by two compressive anomalies at 7.5 and 16 GPa, respectively. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in $\ensuremath{\alpha}\ensuremath{-}\mathrm{A}{\mathrm{g}}_{2}\mathrm{S}$. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. The band-crossing points near the Fermi energy form a nodal-line structure due to the preservation of the time-reversal and space-inversion symmetries under pressure. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals.