CsEr2Cu3Se5 crystallizes in the orthorhombic Cmcm space group. Cs(1) is bonded in a 10-coordinate geometry to two equivalent Cu(1), two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Er(1) is bonded to one Se(1), two equivalent Se(2), and three equivalent Se(3) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(1)Se6 octahedra, corners with four equivalent Cu(2)Se4 tetrahedra, edges with four equivalent Er(1)Se6 octahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with three equivalent Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Cs(1), two equivalent Se(1), and two equivalent Se(2) atoms to form distorted CuCs2Se4 tetrahedra that share corners with four equivalent Se(3)CsEr3Cu2 octahedra, corners with four equivalent Cu(2)Se4 tetrahedra, edges with four equivalent Er(1)Se6 octahedra, and edges with two equivalent Cu(1)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles are 67°. In the second Cu site, Cu(2) is bonded to two equivalent Se(2) and two equivalent Se(3) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(1)Se6 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with three equivalent Er(1)Se6 octahedra, and  an edgeedge with one Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 15-60°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), one Cu(1), and two equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded to one Cs(1), three equivalent Er(1), and two equivalent Cu(2) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(3)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with four equivalent Se(3)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 54°.
CsEr2Cu3Se5 crystallizes in the orthorhombic Cmcm space group. Cs(1) is bonded in a 10-coordinate geometry to two equivalent Cu(1), two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Both Cs(1)-Cu(1) bond lengths are 3.79 Å. Both Cs(1)-Se(1) bond lengths are 3.60 Å. Both Cs(1)-Se(3) bond lengths are 3.49 Å. All Cs(1)-Se(2) bond lengths are 3.63 Å. Er(1) is bonded to one Se(1), two equivalent Se(2), and three equivalent Se(3) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(1)Se6 octahedra, corners with four equivalent Cu(2)Se4 tetrahedra, edges with four equivalent Er(1)Se6 octahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with three equivalent Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. The Er(1)-Se(1) bond length is 2.82 Å. Both Er(1)-Se(2) bond lengths are 2.83 Å. There is one shorter (2.81 Å) and two longer (2.92 Å) Er(1)-Se(3) bond lengths. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Cs(1), two equivalent Se(1), and two equivalent Se(2) atoms to form distorted CuCs2Se4 tetrahedra that share corners with four equivalent Se(3)CsEr3Cu2 octahedra, corners with four equivalent Cu(2)Se4 tetrahedra, edges with four equivalent Er(1)Se6 octahedra, and edges with two equivalent Cu(1)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles are 67°. Both Cu(1)-Se(1) bond lengths are 2.39 Å. Both Cu(1)-Se(2) bond lengths are 2.75 Å. In the second Cu site, Cu(2) is bonded to two equivalent Se(2) and two equivalent Se(3) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(1)Se6 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, corners with two equivalent Cu(2)Se4 tetrahedra, edges with three equivalent Er(1)Se6 octahedra, and  an edgeedge with one Cu(2)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 15-60°. Both Cu(2)-Se(2) bond lengths are 2.53 Å. Both Cu(2)-Se(3) bond lengths are 2.50 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), one Cu(1), and two equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded to one Cs(1), three equivalent Er(1), and two equivalent Cu(2) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(3)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with four equivalent Se(3)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 54°.