Pressure-induced insulator to metal transitions in potential 3D topological insulators Ag$_{2}$Se and Ag$_{2}$Te

Silver chalcogenides Ag$_{2}$Se and Ag$_{2}$Te are non-magnetic compounds exhibiting interesting physics like linear magnetoresistance and dimensionality tunable band gap. To explore their behaviors under high pressure, we performed synchrotron X-ray diffraction and infrared experiments combined with evolutionary algorithm structure predictions and \textit{ab-initio} band structure calculations. For Ag$_{2}$Se, the unusual increase of phase I's band gap and topologically nontrivial features of its band structure support $\beta -$Ag$_{2}$Se as a potential 3D topological insulator. The bulk insulating phase I first transforms to a bulk metallic phase II with 2.4 percent volume drop, marked by the appearance of the distinctive Se(Ag1)-Ag2-Se(Ag1) triple layers stacking pattern. At higher pressure, a transition from the phase II to a completely metallic phase III was observed. And for Ag$_{2}$Te, our study shows that the bulk insulating phase I first transforms into a semi-metallic phase II with the perseverance of its topologically non-trivial nature, and then to a bulk metallic phase III. Our study highlights pressure's role in tuning the electronic structures of Ag$_{2}$Se and Ag$_{2}$Te.