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 <i>P</i>2₁/<i>n</i>) to an orthorhombic Ag₂Se-type structure (space group <i>P</i>2₁2₁2₁). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a <i>P</i>2₁/<i>n</i> 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