Low-Temperature Predicted Structures of Ag(2)S (Silver Sulfide)

Silver sulfide phases, such as body-centered cubic argentite and monoclinic acanthite, are widely known. Traditionally, acanthite is regarded as the only low-temperature phase of silver sulfide. However, the possible existence of other low-temperature phases of silver sulfide cannot be ruled out. Until now, there have been only a few suggestions about low-temperature Ag(2)S phases that differ from monoclinic acanthite. The lack of a uniform approach has hampered the prediction of such phases. In this work, the use of such an effective tool as an evolutionary algorithm for the first time made it possible to perform a broad search for the model Ag(2)S phases of silver sulfide, which are low-temperature with respect to cubic argentite. The possibility of forming Ag(2)S phases with cubic, tetragonal, orthorhombic, trigonal, monoclinic, and triclinic symmetry is considered. The calculation of the cohesion energy and the formation enthalpy show, for the first time, that the formation of low-symmetry Ag(2)S phases is energetically most favorable. The elastic stiffness constants c(ij) of all predicted Ag(2)S phases are computed, and their mechanical stability is determined. The densities of the electronic states of the predicted Ag(2)S phases are calculated. The prediction of low-temperature Ag(2)S structures indicates the possibility of synthesizing new silver sulfide phases with improved properties.