Co9Se8 crystallizes in the cubic Fm-3m space group. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to six equivalent Se(2) atoms to form corner-sharing CoSe6 octahedra. In the second Co site, Co(2) is bonded to one Se(1) and three equivalent Se(2) atoms to form distorted CoSe4 tetrahedra that share corners with three equivalent Co(1)Se6 octahedra, corners with six equivalent Co(2)Se4 tetrahedra, and edges with three equivalent Co(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 51°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a tetrahedral geometry to four equivalent Co(2) atoms. In the second Se site, Se(2) is bonded in a 5-coordinate geometry to one Co(1) and four equivalent Co(2) atoms.
Co9Se8 crystallizes in the cubic Fm-3m space group. There are two inequivalent Co sites. In the first Co site, Co(1) is bonded to six equivalent Se(2) atoms to form corner-sharing CoSe6 octahedra. All Co(1)-Se(2) bond lengths are 2.42 Å. In the second Co site, Co(2) is bonded to one Se(1) and three equivalent Se(2) atoms to form distorted CoSe4 tetrahedra that share corners with three equivalent Co(1)Se6 octahedra, corners with six equivalent Co(2)Se4 tetrahedra, and edges with three equivalent Co(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 51°. The Co(2)-Se(1) bond length is 2.24 Å. All Co(2)-Se(2) bond lengths are 2.29 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a tetrahedral geometry to four equivalent Co(2) atoms. In the second Se site, Se(2) is bonded in a 5-coordinate geometry to one Co(1) and four equivalent Co(2) atoms.