  Binary transition metal chalcogenides have attracted increasing attention for
their unique structural and electronic properties. High pressure is powerful
tool for tuning their crystal structure and electronic structure away from
their pristine states.In this work, we systematically studied the in situ
structural and optical behavior of silver sulfide (Ag2S) under pressure by
X-ray diffraction (XRD) and Infrared (IR) spectroscopy measurements in a
diamond anvil cell. Upon compression, Ag2S undergoes structural symmetrization
from monoclinic to orthorhombic, represented by the decrease of angle from 99
to 90 degrees through a series of structural transitions coupled with lattice
contractions. IR transmission and reflectivity measurements showed that
pressure effectively tunes semiconducting Ag2S into a metal at ~ 22 GPa. Drude
model analysis of the IR reflectivity indicates that the optical conductivity
evolves significantly, reaching the highest conductivity at ~ 40 GPa. Our
results highlight pressure's dramatic role in tuning the structural and
electronic state of silver chacogenides.
