Compression of Silver Sulfide: X-ray Diffraction Measurements and Total-Energy Calculations

Angle-dispersive X-ray diffraction measurements have
been performed
in acanthite, Ag2S, up to 18 GPa in order to investigate
its high-pressure structural behavior. They have been complemented
by ab initio electronic structure calculations. From our experimental
data, we have determined that two different high-pressure phase transitions
take place at 5 and 10.5 GPa. The first pressure-induced transition
is from the initial anti-PbCl2-like monoclinic structure
(space group P21/n) to
an orthorhombic Ag2Se-type structure (space group P212121). The compressibility
of the lattice parameters and the equation of state of both phases
have been determined. A second phase transition to a P21/n phase has been found, which is a
slight modification of the low-pressure structure (Co2Si-related
structure). The initial monoclinic phase was fully recovered after
decompression. Density functional and, in particular, GGA+U calculations
present an overall good agreement with the experimental results in
terms of the high-pressure sequence, cell parameters, and their evolution
with pressure.