Argentite-Acanthite Transformation in Silver Sulfide as a Disorder-Order Transition

An alternative model has been proposed for the phase transition from cubic argentite ß-Ag₂S to monoclinic acanthite α-Ag₂S in silver sulfide as a disorder–order transition. It has been shown that, as the temperature decreases below the transition temperature T_trans, S atoms equiprobably occupying the sites of the body centered cubic (bcc) nonmetal sublattice of argentite are concentrated at four sites of the monoclinic nonmetal sublattice, whereas the other sites remain vacant. A disorder-order transition channel including three superstructure vectors of k₉ and k₄ stars has been determined. The distribution function of sulfur atoms in monoclinic acanthite α-Ag₂S has been calculated. It has been shown that displacements of sulfur atoms distort the bcc nonmetal sublattice of argentite, forming a monoclinic lattice, where silver atoms are spaced by quite large distances and occupy their crystallographic positions with a probability of 1. The region of allowed values of the long-range order parameters η₉ and η₄ for the model monoclinic ordered phase α-Ag₂S has been determined.