Ce5CuSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent Ce sites. In the first Ce site, Ce(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. In the second Ce site, Ce(2) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. Cu(1) is bonded in a square co-planar geometry to four equivalent Se(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Ce(2) and four equivalent Ce(1) atoms to form distorted SeCe5 square pyramids that share corners with eight equivalent Se(2)Ce5Cu octahedra, corners with five equivalent Se(1)Ce5 square pyramids, edges with three equivalent Se(2)Ce5Cu octahedra, edges with five equivalent Se(1)Ce5 square pyramids, and faces with two equivalent Se(2)Ce5Cu octahedra. The corner-sharing octahedral tilt angles range from 19-88°. In the second Se site, Se(2) is bonded to one Ce(2), four equivalent Ce(1), and one Cu(1) atom to form distorted SeCe5Cu octahedra that share corners with seven equivalent Se(2)Ce5Cu octahedra, corners with eight equivalent Se(1)Ce5 square pyramids, edges with four equivalent Se(2)Ce5Cu octahedra, edges with three equivalent Se(1)Ce5 square pyramids,  a faceface with one Se(2)Ce5Cu octahedra, and faces with two equivalent Se(1)Ce5 square pyramids. The corner-sharing octahedral tilt angles range from 16-48°.
Ce5CuSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent Ce sites. In the first Ce site, Ce(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are a spread of Ce(1)-Se(1) bond distances ranging from 2.97-3.12 Å. There are a spread of Ce(1)-Se(2) bond distances ranging from 2.93-3.35 Å. In the second Ce site, Ce(2) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All Ce(2)-Se(1) bond lengths are 2.95 Å. All Ce(2)-Se(2) bond lengths are 3.19 Å. Cu(1) is bonded in a square co-planar geometry to four equivalent Se(2) atoms. All Cu(1)-Se(2) bond lengths are 2.77 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Ce(2) and four equivalent Ce(1) atoms to form distorted SeCe5 square pyramids that share corners with eight equivalent Se(2)Ce5Cu octahedra, corners with five equivalent Se(1)Ce5 square pyramids, edges with three equivalent Se(2)Ce5Cu octahedra, edges with five equivalent Se(1)Ce5 square pyramids, and faces with two equivalent Se(2)Ce5Cu octahedra. The corner-sharing octahedral tilt angles range from 19-88°. In the second Se site, Se(2) is bonded to one Ce(2), four equivalent Ce(1), and one Cu(1) atom to form distorted SeCe5Cu octahedra that share corners with seven equivalent Se(2)Ce5Cu octahedra, corners with eight equivalent Se(1)Ce5 square pyramids, edges with four equivalent Se(2)Ce5Cu octahedra, edges with three equivalent Se(1)Ce5 square pyramids,  a faceface with one Se(2)Ce5Cu octahedra, and faces with two equivalent Se(1)Ce5 square pyramids. The corner-sharing octahedral tilt angles range from 16-48°.