Low temperature phase transformation in superionic conductors: a molecular dynamics study of silver sulfide

Upon lowering the temperature below 450 K the superionic solid Ag2 S undergoes a structural phase transformation from a bcc sulfur lattice with the space group Im3m to a monoclinic lattice with the space group P21 /c. We have successfully modeled this transformation using the variable size–shape form of molecular dynamics and a pair potential constructed earlier to model structural and transport properties in the superionic phase. Very good agreement is found between the partial pair distribution functions for the transformed crystal and the distribution functions calculated using the atomic positions obtained in x-ray studies. We also find good agreement between the three-body angular distributions calculated for the transformed system and distributions calculated for the structure based on the x-ray positions.