Sm5CuSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. In the second Sm site, Sm(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(1) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Sm(2), four equivalent Sm(1), and one Cu(1) atom to form distorted SeSm5Cu octahedra that share corners with seven equivalent Se(1)Sm5Cu octahedra, corners with eight equivalent Se(2)Sm5 square pyramids, edges with four equivalent Se(1)Sm5Cu octahedra, edges with three equivalent Se(2)Sm5 square pyramids,  a faceface with one Se(1)Sm5Cu octahedra, and faces with two equivalent Se(2)Sm5 square pyramids. The corner-sharing octahedral tilt angles range from 17-47°. In the second Se site, Se(2) is bonded to one Sm(2) and four equivalent Sm(1) atoms to form distorted SeSm5 square pyramids that share corners with eight equivalent Se(1)Sm5Cu octahedra, corners with five equivalent Se(2)Sm5 square pyramids, edges with three equivalent Se(1)Sm5Cu octahedra, edges with five equivalent Se(2)Sm5 square pyramids, and faces with two equivalent Se(1)Sm5Cu octahedra. The corner-sharing octahedral tilt angles range from 20-89°.
Sm5CuSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent Sm sites. In the first Sm site, Sm(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are a spread of Sm(1)-Se(1) bond distances ranging from 2.89-3.33 Å. There are a spread of Sm(1)-Se(2) bond distances ranging from 2.93-3.17 Å. In the second Sm site, Sm(2) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All Sm(2)-Se(1) bond lengths are 3.20 Å. All Sm(2)-Se(2) bond lengths are 2.91 Å. Cu(1) is bonded in a square co-planar geometry to four equivalent Se(1) atoms. All Cu(1)-Se(1) bond lengths are 2.76 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Sm(2), four equivalent Sm(1), and one Cu(1) atom to form distorted SeSm5Cu octahedra that share corners with seven equivalent Se(1)Sm5Cu octahedra, corners with eight equivalent Se(2)Sm5 square pyramids, edges with four equivalent Se(1)Sm5Cu octahedra, edges with three equivalent Se(2)Sm5 square pyramids,  a faceface with one Se(1)Sm5Cu octahedra, and faces with two equivalent Se(2)Sm5 square pyramids. The corner-sharing octahedral tilt angles range from 17-47°. In the second Se site, Se(2) is bonded to one Sm(2) and four equivalent Sm(1) atoms to form distorted SeSm5 square pyramids that share corners with eight equivalent Se(1)Sm5Cu octahedra, corners with five equivalent Se(2)Sm5 square pyramids, edges with three equivalent Se(1)Sm5Cu octahedra, edges with five equivalent Se(2)Sm5 square pyramids, and faces with two equivalent Se(1)Sm5Cu octahedra. The corner-sharing octahedral tilt angles range from 20-89°.