CsEr2Cu3Se5 crystallizes in the triclinic P1 space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 10-coordinate geometry to two equivalent Cu(1), one Se(8), one Se(9), two equivalent Se(1), two equivalent Se(4), and two equivalent Se(5) atoms. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to two equivalent Cu(2), one Se(10), one Se(7), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(6) atoms. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), one Se(7), two equivalent Se(4), and two equivalent Se(8) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(4)Se6 octahedra, corners with four equivalent Cu(6)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Cu(6)Se4 tetrahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with two equivalent Cu(5)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. In the second Er site, Er(2) is bonded to one Se(2), one Se(8), two equivalent Se(3), and two equivalent Se(7) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with four equivalent Cu(5)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Cu(5)Se4 tetrahedra, edges with two equivalent Cu(2)Cs2Se4 tetrahedra, and edges with two equivalent Cu(6)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 29°. In the third Er site, Er(3) is bonded to one Se(2), one Se(9), two equivalent Se(10), and two equivalent Se(6) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(2)Se6 octahedra, corners with four equivalent Cu(4)Se4 tetrahedra, edges with two equivalent Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra,  an edgeedge with one Cu(4)Se4 tetrahedra, edges with two equivalent Cu(2)Cs2Se4 tetrahedra, and edges with two equivalent Cu(3)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 29°. In the fourth Er site, Er(4) is bonded to one Se(1), one Se(10), two equivalent Se(5), and two equivalent Se(9) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(1)Se6 octahedra, corners with four equivalent Cu(3)Se4 tetrahedra, edges with two equivalent Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra,  an edgeedge with one Cu(3)Se4 tetrahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with two equivalent Cu(4)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. There are six inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Cs(1), one Se(4), one Se(5), and two equivalent Se(1) atoms to form distorted CuCs2Se4 tetrahedra that share corners with two equivalent Se(8)CsEr3Cu2 octahedra, corners with two equivalent Se(9)CsEr3Cu2 octahedra,  a cornercorner with one Cu(3)Se4 tetrahedra,  a cornercorner with one Cu(4)Se4 tetrahedra,  a cornercorner with one Cu(5)Se4 tetrahedra,  a cornercorner with one Cu(6)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and edges with two equivalent Cu(1)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-68°. In the second Cu site, Cu(2) is bonded to two equivalent Cs(2), one Se(3), one Se(6), and two equivalent Se(2) atoms to form distorted CuCs2Se4 tetrahedra that share corners with two equivalent Se(10)CsEr3Cu2 octahedra, corners with two equivalent Se(7)CsEr3Cu2 octahedra,  a cornercorner with one Cu(3)Se4 tetrahedra,  a cornercorner with one Cu(4)Se4 tetrahedra,  a cornercorner with one Cu(5)Se4 tetrahedra,  a cornercorner with one Cu(6)Se4 tetrahedra, edges with two equivalent Er(2)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and edges with two equivalent Cu(2)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-69°. In the third Cu site, Cu(3) is bonded to one Se(5), one Se(6), and two equivalent Se(9) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Er(4)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(3)Se4 tetrahedra,  an edgeedge with one Er(4)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and  an edgeedge with one Cu(4)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 11-61°. In the fourth Cu site, Cu(4) is bonded to one Se(5), one Se(6), and two equivalent Se(10) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Er(3)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(4)Se4 tetrahedra,  an edgeedge with one Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and  an edgeedge with one Cu(3)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 13-61°. In the fifth Cu site, Cu(5) is bonded to one Se(3), one Se(4), and two equivalent Se(7) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(2)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(5)Se4 tetrahedra,  an edgeedge with one Er(2)Se6 octahedra, edges with two equivalent Er(1)Se6 octahedra, and  an edgeedge with one Cu(6)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 12-58°. In the sixth Cu site, Cu(6) is bonded to one Se(3), one Se(4), and two equivalent Se(8) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(1)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(6)Se4 tetrahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and  an edgeedge with one Cu(5)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 16-60°. There are ten inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), one Er(1), one Er(4), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Cs(2), one Er(2), one Er(3), and two equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded in a 7-coordinate geometry to two equivalent Cs(2), two equivalent Er(2), one Cu(2), one Cu(5), and one Cu(6) atom. In the fourth Se site, Se(4) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), one Cu(1), one Cu(5), and one Cu(6) atom. In the fifth Se site, Se(5) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(4), one Cu(1), one Cu(3), and one Cu(4) atom. In the sixth Se site, Se(6) is bonded in a 7-coordinate geometry to two equivalent Cs(2), two equivalent Er(3), one Cu(2), one Cu(3), and one Cu(4) atom. In the seventh Se site, Se(7) is bonded to one Cs(2), one Er(1), two equivalent Er(2), and two equivalent Cu(5) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(10)CsEr3Cu2 octahedra, corners with two equivalent Cu(2)Cs2Se4 tetrahedra, edges with two equivalent Se(7)CsEr3Cu2 octahedra, and edges with two equivalent Se(8)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 53°. In the eighth Se site, Se(8) is bonded to one Cs(1), one Er(2), two equivalent Er(1), and two equivalent Cu(6) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(9)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, edges with two equivalent Se(7)CsEr3Cu2 octahedra, and edges with two equivalent Se(8)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 56°. In the ninth Se site, Se(9) is bonded to one Cs(1), one Er(3), two equivalent Er(4), and two equivalent Cu(3) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(8)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, edges with two equivalent Se(10)CsEr3Cu2 octahedra, and edges with two equivalent Se(9)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 56°. In the tenth Se site, Se(10) is bonded to one Cs(2), one Er(4), two equivalent Er(3), and two equivalent Cu(4) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(7)CsEr3Cu2 octahedra, corners with two equivalent Cu(2)Cs2Se4 tetrahedra, edges with two equivalent Se(10)CsEr3Cu2 octahedra, and edges with two equivalent Se(9)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 53°.
CsEr2Cu3Se5 crystallizes in the triclinic P1 space group. There are two inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 10-coordinate geometry to two equivalent Cu(1), one Se(8), one Se(9), two equivalent Se(1), two equivalent Se(4), and two equivalent Se(5) atoms. There is one shorter (3.73 Å) and one longer (3.74 Å) Cs(1)-Cu(1) bond length. The Cs(1)-Se(8) bond length is 3.53 Å. The Cs(1)-Se(9) bond length is 3.52 Å. Both Cs(1)-Se(1) bond lengths are 3.61 Å. There is one shorter (3.60 Å) and one longer (3.64 Å) Cs(1)-Se(4) bond length. There is one shorter (3.58 Å) and one longer (3.59 Å) Cs(1)-Se(5) bond length. In the second Cs site, Cs(2) is bonded in a 10-coordinate geometry to two equivalent Cu(2), one Se(10), one Se(7), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(6) atoms. There is one shorter (3.79 Å) and one longer (3.84 Å) Cs(2)-Cu(2) bond length. The Cs(2)-Se(10) bond length is 3.49 Å. The Cs(2)-Se(7) bond length is 3.47 Å. There is one shorter (3.54 Å) and one longer (3.56 Å) Cs(2)-Se(2) bond length. There is one shorter (3.67 Å) and one longer (3.70 Å) Cs(2)-Se(3) bond length. Both Cs(2)-Se(6) bond lengths are 3.63 Å. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), one Se(7), two equivalent Se(4), and two equivalent Se(8) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(4)Se6 octahedra, corners with four equivalent Cu(6)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Cu(6)Se4 tetrahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with two equivalent Cu(5)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. The Er(1)-Se(1) bond length is 2.83 Å. The Er(1)-Se(7) bond length is 2.77 Å. There is one shorter (2.86 Å) and one longer (2.87 Å) Er(1)-Se(4) bond length. There is one shorter (2.92 Å) and one longer (2.95 Å) Er(1)-Se(8) bond length. In the second Er site, Er(2) is bonded to one Se(2), one Se(8), two equivalent Se(3), and two equivalent Se(7) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with four equivalent Cu(5)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Cu(5)Se4 tetrahedra, edges with two equivalent Cu(2)Cs2Se4 tetrahedra, and edges with two equivalent Cu(6)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 29°. The Er(2)-Se(2) bond length is 2.78 Å. The Er(2)-Se(8) bond length is 2.83 Å. There is one shorter (2.80 Å) and one longer (2.82 Å) Er(2)-Se(3) bond length. There is one shorter (2.89 Å) and one longer (2.98 Å) Er(2)-Se(7) bond length. In the third Er site, Er(3) is bonded to one Se(2), one Se(9), two equivalent Se(10), and two equivalent Se(6) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(2)Se6 octahedra, corners with four equivalent Cu(4)Se4 tetrahedra, edges with two equivalent Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra,  an edgeedge with one Cu(4)Se4 tetrahedra, edges with two equivalent Cu(2)Cs2Se4 tetrahedra, and edges with two equivalent Cu(3)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 29°. The Er(3)-Se(2) bond length is 2.80 Å. The Er(3)-Se(9) bond length is 2.83 Å. There is one shorter (2.88 Å) and one longer (2.97 Å) Er(3)-Se(10) bond length. There is one shorter (2.82 Å) and one longer (2.92 Å) Er(3)-Se(6) bond length. In the fourth Er site, Er(4) is bonded to one Se(1), one Se(10), two equivalent Se(5), and two equivalent Se(9) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(1)Se6 octahedra, corners with four equivalent Cu(3)Se4 tetrahedra, edges with two equivalent Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra,  an edgeedge with one Cu(3)Se4 tetrahedra, edges with two equivalent Cu(1)Cs2Se4 tetrahedra, and edges with two equivalent Cu(4)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 28°. The Er(4)-Se(1) bond length is 2.82 Å. The Er(4)-Se(10) bond length is 2.82 Å. There is one shorter (2.82 Å) and one longer (2.84 Å) Er(4)-Se(5) bond length. There is one shorter (2.84 Å) and one longer (2.91 Å) Er(4)-Se(9) bond length. There are six inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to two equivalent Cs(1), one Se(4), one Se(5), and two equivalent Se(1) atoms to form distorted CuCs2Se4 tetrahedra that share corners with two equivalent Se(8)CsEr3Cu2 octahedra, corners with two equivalent Se(9)CsEr3Cu2 octahedra,  a cornercorner with one Cu(3)Se4 tetrahedra,  a cornercorner with one Cu(4)Se4 tetrahedra,  a cornercorner with one Cu(5)Se4 tetrahedra,  a cornercorner with one Cu(6)Se4 tetrahedra, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and edges with two equivalent Cu(1)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-68°. The Cu(1)-Se(4) bond length is 2.82 Å. The Cu(1)-Se(5) bond length is 2.65 Å. There is one shorter (2.40 Å) and one longer (2.44 Å) Cu(1)-Se(1) bond length. In the second Cu site, Cu(2) is bonded to two equivalent Cs(2), one Se(3), one Se(6), and two equivalent Se(2) atoms to form distorted CuCs2Se4 tetrahedra that share corners with two equivalent Se(10)CsEr3Cu2 octahedra, corners with two equivalent Se(7)CsEr3Cu2 octahedra,  a cornercorner with one Cu(3)Se4 tetrahedra,  a cornercorner with one Cu(4)Se4 tetrahedra,  a cornercorner with one Cu(5)Se4 tetrahedra,  a cornercorner with one Cu(6)Se4 tetrahedra, edges with two equivalent Er(2)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and edges with two equivalent Cu(2)Cs2Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 67-69°. The Cu(2)-Se(3) bond length is 2.82 Å. The Cu(2)-Se(6) bond length is 2.71 Å. There is one shorter (2.35 Å) and one longer (2.46 Å) Cu(2)-Se(2) bond length. In the third Cu site, Cu(3) is bonded to one Se(5), one Se(6), and two equivalent Se(9) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Er(4)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(3)Se4 tetrahedra,  an edgeedge with one Er(4)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and  an edgeedge with one Cu(4)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 11-61°. The Cu(3)-Se(5) bond length is 2.53 Å. The Cu(3)-Se(6) bond length is 2.54 Å. There is one shorter (2.54 Å) and one longer (2.56 Å) Cu(3)-Se(9) bond length. In the fourth Cu site, Cu(4) is bonded to one Se(5), one Se(6), and two equivalent Se(10) atoms to form distorted CuSe4 tetrahedra that share corners with four equivalent Er(3)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(4)Se4 tetrahedra,  an edgeedge with one Er(3)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and  an edgeedge with one Cu(3)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 13-61°. The Cu(4)-Se(5) bond length is 2.60 Å. The Cu(4)-Se(6) bond length is 2.47 Å. There is one shorter (2.47 Å) and one longer (2.51 Å) Cu(4)-Se(10) bond length. In the fifth Cu site, Cu(5) is bonded to one Se(3), one Se(4), and two equivalent Se(7) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(2)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(5)Se4 tetrahedra,  an edgeedge with one Er(2)Se6 octahedra, edges with two equivalent Er(1)Se6 octahedra, and  an edgeedge with one Cu(6)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 12-58°. The Cu(5)-Se(3) bond length is 2.47 Å. The Cu(5)-Se(4) bond length is 2.54 Å. There is one shorter (2.46 Å) and one longer (2.55 Å) Cu(5)-Se(7) bond length. In the sixth Cu site, Cu(6) is bonded to one Se(3), one Se(4), and two equivalent Se(8) atoms to form CuSe4 tetrahedra that share corners with four equivalent Er(1)Se6 octahedra,  a cornercorner with one Cu(1)Cs2Se4 tetrahedra,  a cornercorner with one Cu(2)Cs2Se4 tetrahedra, corners with two equivalent Cu(6)Se4 tetrahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and  an edgeedge with one Cu(5)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 16-60°. The Cu(6)-Se(3) bond length is 2.47 Å. The Cu(6)-Se(4) bond length is 2.53 Å. There is one shorter (2.42 Å) and one longer (2.50 Å) Cu(6)-Se(8) bond length. There are ten inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Cs(1), one Er(1), one Er(4), and two equivalent Cu(1) atoms. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Cs(2), one Er(2), one Er(3), and two equivalent Cu(2) atoms. In the third Se site, Se(3) is bonded in a 7-coordinate geometry to two equivalent Cs(2), two equivalent Er(2), one Cu(2), one Cu(5), and one Cu(6) atom. In the fourth Se site, Se(4) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(1), one Cu(1), one Cu(5), and one Cu(6) atom. In the fifth Se site, Se(5) is bonded in a 7-coordinate geometry to two equivalent Cs(1), two equivalent Er(4), one Cu(1), one Cu(3), and one Cu(4) atom. In the sixth Se site, Se(6) is bonded in a 7-coordinate geometry to two equivalent Cs(2), two equivalent Er(3), one Cu(2), one Cu(3), and one Cu(4) atom. In the seventh Se site, Se(7) is bonded to one Cs(2), one Er(1), two equivalent Er(2), and two equivalent Cu(5) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(10)CsEr3Cu2 octahedra, corners with two equivalent Cu(2)Cs2Se4 tetrahedra, edges with two equivalent Se(7)CsEr3Cu2 octahedra, and edges with two equivalent Se(8)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 53°. In the eighth Se site, Se(8) is bonded to one Cs(1), one Er(2), two equivalent Er(1), and two equivalent Cu(6) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(9)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, edges with two equivalent Se(7)CsEr3Cu2 octahedra, and edges with two equivalent Se(8)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 56°. In the ninth Se site, Se(9) is bonded to one Cs(1), one Er(3), two equivalent Er(4), and two equivalent Cu(3) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(8)CsEr3Cu2 octahedra, corners with two equivalent Cu(1)Cs2Se4 tetrahedra, edges with two equivalent Se(10)CsEr3Cu2 octahedra, and edges with two equivalent Se(9)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 56°. In the tenth Se site, Se(10) is bonded to one Cs(2), one Er(4), two equivalent Er(3), and two equivalent Cu(4) atoms to form distorted SeCsEr3Cu2 octahedra that share  a cornercorner with one Se(7)CsEr3Cu2 octahedra, corners with two equivalent Cu(2)Cs2Se4 tetrahedra, edges with two equivalent Se(10)CsEr3Cu2 octahedra, and edges with two equivalent Se(9)CsEr3Cu2 octahedra. The corner-sharing octahedral tilt angles are 53°.