Cs2Se crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded to six equivalent Se(1) atoms to form distorted CsSe6 octahedra that share corners with twelve equivalent Cs(1)Se6 octahedra, corners with twelve equivalent Cs(2)Se5 trigonal bipyramids, edges with six equivalent Cs(1)Se6 octahedra, faces with two equivalent Cs(1)Se6 octahedra, and faces with six equivalent Cs(2)Se5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 51°. In the second Cs site, Cs(2) is bonded to five equivalent Se(1) atoms to form distorted CsSe5 trigonal bipyramids that share corners with twelve equivalent Cs(1)Se6 octahedra, corners with eight equivalent Cs(2)Se5 trigonal bipyramids, edges with six equivalent Cs(2)Se5 trigonal bipyramids, and faces with six equivalent Cs(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 30-60°. Se(1) is bonded in a 11-coordinate geometry to five equivalent Cs(2) and six equivalent Cs(1) atoms.
Cs2Se crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded to six equivalent Se(1) atoms to form distorted CsSe6 octahedra that share corners with twelve equivalent Cs(1)Se6 octahedra, corners with twelve equivalent Cs(2)Se5 trigonal bipyramids, edges with six equivalent Cs(1)Se6 octahedra, faces with two equivalent Cs(1)Se6 octahedra, and faces with six equivalent Cs(2)Se5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 51°. All Cs(1)-Se(1) bond lengths are 4.06 Å. In the second Cs site, Cs(2) is bonded to five equivalent Se(1) atoms to form distorted CsSe5 trigonal bipyramids that share corners with twelve equivalent Cs(1)Se6 octahedra, corners with eight equivalent Cs(2)Se5 trigonal bipyramids, edges with six equivalent Cs(2)Se5 trigonal bipyramids, and faces with six equivalent Cs(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 30-60°. There are three shorter (3.50 Å) and two longer (4.11 Å) Cs(2)-Se(1) bond lengths. Se(1) is bonded in a 11-coordinate geometry to five equivalent Cs(2) and six equivalent Cs(1) atoms.