Argentite–Acanthite Transition in Silver Sulfide as a Two-Sublattice Ordering

Abstract

We propose a model of phase transformation cubic argentite–monoclinic acanthite in silver sulfide Ag2S (AgS0.5) as ordering in two argentite sublattices. We have determined the channel of the disorder–order transition including four nonequivalent superstructure vectors of stars {k9} and {k4}. For monoclinic acanthite α-Ag2S, we have calculated the distribution function for silver atoms occupying b positions in argentite, as well as the distribution function for sulfur atoms. Ordering in both sublattices is complicated by static atomic displacements. The displacement of S atoms distort the body-centered cubic (bcc) nonmetallic argentite sublattice, forming a monoclinic lattice in which silver atoms are at large distances from one another and occupy their crystallographic positions with unit probability. We have determined the range of admissible values of long-range order parameters η9 and η4 for the model monoclinic ordered α-Ag2S phase.