Cr7Se8 crystallizes in the monoclinic C2/m space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent Se(2) and four equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 49-51°. In the second Cr site, Cr(2) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-50°. In the third Cr site, Cr(3) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 48-51°. In the fourth Cr site, Cr(4) is bonded to one Se(1), one Se(2), and four equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-51°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Cr(4), two equivalent Cr(2), and two equivalent Cr(3) atoms to form distorted SeCr5 square pyramids that share corners with three equivalent Se(2)Cr6 pentagonal pyramids, corners with two equivalent Se(1)Cr5 square pyramids, corners with four equivalent Se(3)Cr5 trigonal bipyramids, edges with three equivalent Se(2)Cr6 pentagonal pyramids,  an edgeedge with one Se(1)Cr5 square pyramid, and edges with four equivalent Se(3)Cr5 trigonal bipyramids. In the second Se site, Se(2) is bonded to one Cr(1), one Cr(4), two equivalent Cr(2), and two equivalent Cr(3) atoms to form distorted SeCr6 pentagonal pyramids that share corners with three equivalent Se(2)Cr6 pentagonal pyramids, corners with three equivalent Se(1)Cr5 square pyramids,  an edgeedge with one Se(2)Cr6 pentagonal pyramid, edges with three equivalent Se(1)Cr5 square pyramids, and edges with eight equivalent Se(3)Cr5 trigonal bipyramids. In the third Se site, Se(3) is bonded to one Cr(1), one Cr(2), one Cr(3), and two equivalent Cr(4) atoms to form distorted SeCr5 trigonal bipyramids that share corners with two equivalent Se(1)Cr5 square pyramids, corners with seven equivalent Se(3)Cr5 trigonal bipyramids, edges with four equivalent Se(2)Cr6 pentagonal pyramids, edges with two equivalent Se(1)Cr5 square pyramids, and edges with two equivalent Se(3)Cr5 trigonal bipyramids.
Cr7Se8 crystallizes in the monoclinic C2/m space group. There are four inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to two equivalent Se(2) and four equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 49-51°. Both Cr(1)-Se(2) bond lengths are 2.65 Å. All Cr(1)-Se(3) bond lengths are 2.66 Å. In the second Cr site, Cr(2) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-50°. Both Cr(2)-Se(1) bond lengths are 2.60 Å. Both Cr(2)-Se(2) bond lengths are 2.69 Å. Both Cr(2)-Se(3) bond lengths are 2.64 Å. In the third Cr site, Cr(3) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 48-51°. Both Cr(3)-Se(1) bond lengths are 2.57 Å. Both Cr(3)-Se(2) bond lengths are 2.71 Å. Both Cr(3)-Se(3) bond lengths are 2.61 Å. In the fourth Cr site, Cr(4) is bonded to one Se(1), one Se(2), and four equivalent Se(3) atoms to form a mixture of edge, corner, and face-sharing CrSe6 octahedra. The corner-sharing octahedral tilt angles range from 47-51°. The Cr(4)-Se(1) bond length is 2.57 Å. The Cr(4)-Se(2) bond length is 2.75 Å. There are two shorter (2.59 Å) and two longer (2.62 Å) Cr(4)-Se(3) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Cr(4), two equivalent Cr(2), and two equivalent Cr(3) atoms to form distorted SeCr5 square pyramids that share corners with three equivalent Se(2)Cr6 pentagonal pyramids, corners with two equivalent Se(1)Cr5 square pyramids, corners with four equivalent Se(3)Cr5 trigonal bipyramids, edges with three equivalent Se(2)Cr6 pentagonal pyramids,  an edgeedge with one Se(1)Cr5 square pyramid, and edges with four equivalent Se(3)Cr5 trigonal bipyramids. In the second Se site, Se(2) is bonded to one Cr(1), one Cr(4), two equivalent Cr(2), and two equivalent Cr(3) atoms to form distorted SeCr6 pentagonal pyramids that share corners with three equivalent Se(2)Cr6 pentagonal pyramids, corners with three equivalent Se(1)Cr5 square pyramids,  an edgeedge with one Se(2)Cr6 pentagonal pyramid, edges with three equivalent Se(1)Cr5 square pyramids, and edges with eight equivalent Se(3)Cr5 trigonal bipyramids. In the third Se site, Se(3) is bonded to one Cr(1), one Cr(2), one Cr(3), and two equivalent Cr(4) atoms to form distorted SeCr5 trigonal bipyramids that share corners with two equivalent Se(1)Cr5 square pyramids, corners with seven equivalent Se(3)Cr5 trigonal bipyramids, edges with four equivalent Se(2)Cr6 pentagonal pyramids, edges with two equivalent Se(1)Cr5 square pyramids, and edges with two equivalent Se(3)Cr5 trigonal bipyramids.