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Rb2S is Fluorite structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Rb1+ is bonded to four equivalent S2- atoms to form a mixture of edge and corner-sharing RbS4 tetrahedra. All Rb–S bond lengths are 3.39 Å. S2- is bonded in a body-centered cubic geometry to eight equivalent Rb1+ atoms.
Rb2S crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. there are two inequivalent Rb1+ sites. In the first Rb1+ site, Rb1+ is bonded to six equivalent S2- atoms to form distorted RbS6 octahedra that share corners with twelve equivalent RbS6 octahedra, corners with twelve equivalent RbS5 trigonal bipyramids, edges with six equivalent RbS6 octahedra, faces with two equivalent RbS6 octahedra, and faces with six equivalent RbS5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 51°. All Rb–S bond lengths are 3.75 Å. In the second Rb1+ site, Rb1+ is bonded to five equivalent S2- atoms to form distorted RbS5 trigonal bipyramids that share corners with twelve equivalent RbS6 octahedra, corners with eight equivalent RbS5 trigonal bipyramids, edges with six equivalent RbS5 trigonal bipyramids, and faces with six equivalent RbS6 octahedra. The corner-sharing octahedra tilt angles range from 31–59°. There are three shorter (3.21 Å) and two longer (3.88 Å) Rb–S bond lengths. S2- is bonded in a 3-coordinate geometry to eleven Rb1+ atoms.
Rb2S is Cotunnite structured and crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. there are two inequivalent Rb1+ sites. In the first Rb1+ site, Rb1+ is bonded in a 5-coordinate geometry to five equivalent S2- atoms. There are a spread of Rb–S bond distances ranging from 3.42–3.90 Å. In the second Rb1+ site, Rb1+ is bonded to four equivalent S2- atoms to form a mixture of corner and edge-sharing RbS4 tetrahedra. There are three shorter (3.29 Å) and one longer (3.36 Å) Rb–S bond lengths. S2- is bonded in a 9-coordinate geometry to nine Rb1+ atoms.