Yb(ErSe2)2 crystallizes in the orthorhombic Pnma space group. Yb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and three equivalent Se(3) atoms to form distorted YbSe7 pentagonal bipyramids that share corners with four equivalent Er(1)Se6 octahedra, corners with four equivalent Er(2)Se6 octahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with four equivalent Er(2)Se6 octahedra, edges with two equivalent Yb(1)Se7 pentagonal bipyramids, and faces with two equivalent Yb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 13-66°. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), two equivalent Se(2), and three equivalent Se(4) atoms to form ErSe6 octahedra that share corners with three equivalent Er(2)Se6 octahedra, corners with four equivalent Yb(1)Se7 pentagonal bipyramids, edges with two equivalent Er(2)Se6 octahedra, edges with four equivalent Er(1)Se6 octahedra, and  an edgeedge with one Yb(1)Se7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 52-59°. In the second Er site, Er(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form ErSe6 octahedra that share corners with three equivalent Er(1)Se6 octahedra, corners with four equivalent Yb(1)Se7 pentagonal bipyramids, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and edges with four equivalent Yb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 52-59°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Yb(1), one Er(1), and two equivalent Er(2) atoms to form distorted SeYb2Er3 trigonal bipyramids that share corners with two equivalent Se(2)Yb2Er3 square pyramids, corners with six equivalent Se(3)Yb3Er2 square pyramids, edges with two equivalent Se(3)Yb3Er2 square pyramids, edges with three equivalent Se(2)Yb2Er3 square pyramids, and edges with two equivalent Se(1)Yb2Er3 trigonal bipyramids. In the second Se site, Se(2) is bonded to two equivalent Yb(1), one Er(2), and two equivalent Er(1) atoms to form SeYb2Er3 square pyramids that share corners with two equivalent Se(3)Yb3Er2 square pyramids, corners with two equivalent Se(1)Yb2Er3 trigonal bipyramids, edges with two equivalent Se(2)Yb2Er3 square pyramids, edges with three equivalent Se(3)Yb3Er2 square pyramids, and edges with three equivalent Se(1)Yb2Er3 trigonal bipyramids. In the third Se site, Se(3) is bonded to three equivalent Yb(1) and two equivalent Er(2) atoms to form SeYb3Er2 square pyramids that share corners with two equivalent Se(2)Yb2Er3 square pyramids, corners with six equivalent Se(1)Yb2Er3 trigonal bipyramids, edges with three equivalent Se(2)Yb2Er3 square pyramids, edges with four equivalent Se(3)Yb3Er2 square pyramids, and edges with two equivalent Se(1)Yb2Er3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms.
Yb(ErSe2)2 crystallizes in the orthorhombic Pnma space group. Yb(1) is bonded to two equivalent Se(1), two equivalent Se(2), and three equivalent Se(3) atoms to form distorted YbSe7 pentagonal bipyramids that share corners with four equivalent Er(1)Se6 octahedra, corners with four equivalent Er(2)Se6 octahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with four equivalent Er(2)Se6 octahedra, edges with two equivalent Yb(1)Se7 pentagonal bipyramids, and faces with two equivalent Yb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 13-66°. Both Yb(1)-Se(1) bond lengths are 3.10 Å. Both Yb(1)-Se(2) bond lengths are 2.98 Å. There are two shorter (2.95 Å) and one longer (3.02 Å) Yb(1)-Se(3) bond length. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(1), two equivalent Se(2), and three equivalent Se(4) atoms to form ErSe6 octahedra that share corners with three equivalent Er(2)Se6 octahedra, corners with four equivalent Yb(1)Se7 pentagonal bipyramids, edges with two equivalent Er(2)Se6 octahedra, edges with four equivalent Er(1)Se6 octahedra, and  an edgeedge with one Yb(1)Se7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 52-59°. The Er(1)-Se(1) bond length is 2.86 Å. Both Er(1)-Se(2) bond lengths are 2.82 Å. There are two shorter (2.81 Å) and one longer (2.90 Å) Er(1)-Se(4) bond length. In the second Er site, Er(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form ErSe6 octahedra that share corners with three equivalent Er(1)Se6 octahedra, corners with four equivalent Yb(1)Se7 pentagonal bipyramids, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and edges with four equivalent Yb(1)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 52-59°. The Er(2)-Se(2) bond length is 2.82 Å. The Er(2)-Se(4) bond length is 2.88 Å. Both Er(2)-Se(1) bond lengths are 2.84 Å. Both Er(2)-Se(3) bond lengths are 2.80 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Yb(1), one Er(1), and two equivalent Er(2) atoms to form distorted SeYb2Er3 trigonal bipyramids that share corners with two equivalent Se(2)Yb2Er3 square pyramids, corners with six equivalent Se(3)Yb3Er2 square pyramids, edges with two equivalent Se(3)Yb3Er2 square pyramids, edges with three equivalent Se(2)Yb2Er3 square pyramids, and edges with two equivalent Se(1)Yb2Er3 trigonal bipyramids. In the second Se site, Se(2) is bonded to two equivalent Yb(1), one Er(2), and two equivalent Er(1) atoms to form SeYb2Er3 square pyramids that share corners with two equivalent Se(3)Yb3Er2 square pyramids, corners with two equivalent Se(1)Yb2Er3 trigonal bipyramids, edges with two equivalent Se(2)Yb2Er3 square pyramids, edges with three equivalent Se(3)Yb3Er2 square pyramids, and edges with three equivalent Se(1)Yb2Er3 trigonal bipyramids. In the third Se site, Se(3) is bonded to three equivalent Yb(1) and two equivalent Er(2) atoms to form SeYb3Er2 square pyramids that share corners with two equivalent Se(2)Yb2Er3 square pyramids, corners with six equivalent Se(1)Yb2Er3 trigonal bipyramids, edges with three equivalent Se(2)Yb2Er3 square pyramids, edges with four equivalent Se(3)Yb3Er2 square pyramids, and edges with two equivalent Se(1)Yb2Er3 trigonal bipyramids. In the fourth Se site, Se(4) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms.