Er3SmSe6 crystallizes in the monoclinic P2_1/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(5), two equivalent Se(1), and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with three equivalent Er(3)Se6 octahedra, corners with two equivalent Er(2)Se7 pentagonal bipyramids, and edges with four equivalent Er(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-60°. In the second Er site, Er(2) is bonded to one Se(1), one Se(6), two equivalent Se(3), and three equivalent Se(4) atoms to form distorted ErSe7 pentagonal bipyramids that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and edges with four equivalent Er(2)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 37-50°. In the third Er site, Er(3) is bonded to one Se(4), two equivalent Se(6), and three equivalent Se(5) atoms to form ErSe6 octahedra that share corners with three equivalent Er(1)Se6 octahedra,  a cornercorner with one Er(2)Se7 pentagonal bipyramid, edges with four equivalent Er(3)Se6 octahedra, and edges with two equivalent Er(2)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 56-60°. Sm(1) is bonded in a 8-coordinate geometry to one Se(1), two equivalent Se(2), two equivalent Se(6), and three equivalent Se(3) atoms. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(2), two equivalent Er(1), and one Sm(1) atom to form distorted SeSmEr3 trigonal pyramids that share corners with two equivalent Se(2)Sm2Er3 square pyramids,  a cornercorner with one Se(3)Sm3Er2 trigonal bipyramid, corners with three equivalent Se(6)Sm2Er3 trigonal bipyramids, corners with two equivalent Se(1)SmEr3 trigonal pyramids, corners with three equivalent Se(4)Er4 trigonal pyramids, edges with three equivalent Se(2)Sm2Er3 square pyramids, and edges with two equivalent Se(3)Sm3Er2 trigonal bipyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and two equivalent Sm(1) atoms to form SeSm2Er3 square pyramids that share corners with two equivalent Se(6)Sm2Er3 trigonal bipyramids, corners with four equivalent Se(3)Sm3Er2 trigonal bipyramids, corners with two equivalent Se(1)SmEr3 trigonal pyramids, edges with four equivalent Se(2)Sm2Er3 square pyramids,  an edgeedge with one Se(6)Sm2Er3 trigonal bipyramid,  an edgeedge with one Se(3)Sm3Er2 trigonal bipyramid, and edges with three equivalent Se(1)SmEr3 trigonal pyramids. In the third Se site, Se(3) is bonded to two equivalent Er(2) and three equivalent Sm(1) atoms to form distorted SeSm3Er2 trigonal bipyramids that share corners with four equivalent Se(2)Sm2Er3 square pyramids, corners with two equivalent Se(6)Sm2Er3 trigonal bipyramids,  a cornercorner with one Se(1)SmEr3 trigonal pyramid, corners with four equivalent Se(4)Er4 trigonal pyramids,  an edgeedge with one Se(2)Sm2Er3 square pyramid, edges with three equivalent Se(6)Sm2Er3 trigonal bipyramids, edges with four equivalent Se(3)Sm3Er2 trigonal bipyramids,  an edgeedge with one Se(4)Er4 trigonal pyramid, and edges with two equivalent Se(1)SmEr3 trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Er(3) and three equivalent Er(2) atoms to form distorted SeEr4 trigonal pyramids that share  a cornercorner with one Se(6)Sm2Er3 trigonal bipyramid, corners with four equivalent Se(3)Sm3Er2 trigonal bipyramids, corners with two equivalent Se(4)Er4 trigonal pyramids, corners with three equivalent Se(1)SmEr3 trigonal pyramids,  an edgeedge with one Se(3)Sm3Er2 trigonal bipyramid, edges with two equivalent Se(6)Sm2Er3 trigonal bipyramids, and edges with two equivalent Se(4)Er4 trigonal pyramids. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Er(1) and three equivalent Er(3) atoms. In the sixth Se site, Se(6) is bonded to one Er(2), two equivalent Er(3), and two equivalent Sm(1) atoms to form SeSm2Er3 trigonal bipyramids that share corners with two equivalent Se(2)Sm2Er3 square pyramids, corners with two equivalent Se(3)Sm3Er2 trigonal bipyramids,  a cornercorner with one Se(4)Er4 trigonal pyramid, corners with three equivalent Se(1)SmEr3 trigonal pyramids,  an edgeedge with one Se(2)Sm2Er3 square pyramid, edges with two equivalent Se(6)Sm2Er3 trigonal bipyramids, edges with three equivalent Se(3)Sm3Er2 trigonal bipyramids, and edges with two equivalent Se(4)Er4 trigonal pyramids.
Er3SmSe6 crystallizes in the monoclinic P2_1/m space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(5), two equivalent Se(1), and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with three equivalent Er(3)Se6 octahedra, corners with two equivalent Er(2)Se7 pentagonal bipyramids, and edges with four equivalent Er(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 56-60°. The Er(1)-Se(5) bond length is 2.80 Å. Both Er(1)-Se(1) bond lengths are 2.75 Å. There is one shorter (2.82 Å) and two longer (2.87 Å) Er(1)-Se(2) bond lengths. In the second Er site, Er(2) is bonded to one Se(1), one Se(6), two equivalent Se(3), and three equivalent Se(4) atoms to form distorted ErSe7 pentagonal bipyramids that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(3)Se6 octahedra, and edges with four equivalent Er(2)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 37-50°. The Er(2)-Se(1) bond length is 2.80 Å. The Er(2)-Se(6) bond length is 2.96 Å. Both Er(2)-Se(3) bond lengths are 3.03 Å. There are two shorter (2.79 Å) and one longer (3.04 Å) Er(2)-Se(4) bond length. In the third Er site, Er(3) is bonded to one Se(4), two equivalent Se(6), and three equivalent Se(5) atoms to form ErSe6 octahedra that share corners with three equivalent Er(1)Se6 octahedra,  a cornercorner with one Er(2)Se7 pentagonal bipyramid, edges with four equivalent Er(3)Se6 octahedra, and edges with two equivalent Er(2)Se7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 56-60°. The Er(3)-Se(4) bond length is 2.77 Å. Both Er(3)-Se(6) bond lengths are 2.86 Å. There are two shorter (2.81 Å) and one longer (2.87 Å) Er(3)-Se(5) bond length. Sm(1) is bonded in a 8-coordinate geometry to one Se(1), two equivalent Se(2), two equivalent Se(6), and three equivalent Se(3) atoms. The Sm(1)-Se(1) bond length is 3.07 Å. Both Sm(1)-Se(2) bond lengths are 3.13 Å. Both Sm(1)-Se(6) bond lengths are 3.12 Å. There are two shorter (2.97 Å) and one longer (3.06 Å) Sm(1)-Se(3) bond length. There are six inequivalent Se sites. In the first Se site, Se(1) is bonded to one Er(2), two equivalent Er(1), and one Sm(1) atom to form distorted SeSmEr3 trigonal pyramids that share corners with two equivalent Se(2)Sm2Er3 square pyramids,  a cornercorner with one Se(3)Sm3Er2 trigonal bipyramid, corners with three equivalent Se(6)Sm2Er3 trigonal bipyramids, corners with two equivalent Se(1)SmEr3 trigonal pyramids, corners with three equivalent Se(4)Er4 trigonal pyramids, edges with three equivalent Se(2)Sm2Er3 square pyramids, and edges with two equivalent Se(3)Sm3Er2 trigonal bipyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and two equivalent Sm(1) atoms to form SeSm2Er3 square pyramids that share corners with two equivalent Se(6)Sm2Er3 trigonal bipyramids, corners with four equivalent Se(3)Sm3Er2 trigonal bipyramids, corners with two equivalent Se(1)SmEr3 trigonal pyramids, edges with four equivalent Se(2)Sm2Er3 square pyramids,  an edgeedge with one Se(6)Sm2Er3 trigonal bipyramid,  an edgeedge with one Se(3)Sm3Er2 trigonal bipyramid, and edges with three equivalent Se(1)SmEr3 trigonal pyramids. In the third Se site, Se(3) is bonded to two equivalent Er(2) and three equivalent Sm(1) atoms to form distorted SeSm3Er2 trigonal bipyramids that share corners with four equivalent Se(2)Sm2Er3 square pyramids, corners with two equivalent Se(6)Sm2Er3 trigonal bipyramids,  a cornercorner with one Se(1)SmEr3 trigonal pyramid, corners with four equivalent Se(4)Er4 trigonal pyramids,  an edgeedge with one Se(2)Sm2Er3 square pyramid, edges with three equivalent Se(6)Sm2Er3 trigonal bipyramids, edges with four equivalent Se(3)Sm3Er2 trigonal bipyramids,  an edgeedge with one Se(4)Er4 trigonal pyramid, and edges with two equivalent Se(1)SmEr3 trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Er(3) and three equivalent Er(2) atoms to form distorted SeEr4 trigonal pyramids that share  a cornercorner with one Se(6)Sm2Er3 trigonal bipyramid, corners with four equivalent Se(3)Sm3Er2 trigonal bipyramids, corners with two equivalent Se(4)Er4 trigonal pyramids, corners with three equivalent Se(1)SmEr3 trigonal pyramids,  an edgeedge with one Se(3)Sm3Er2 trigonal bipyramid, edges with two equivalent Se(6)Sm2Er3 trigonal bipyramids, and edges with two equivalent Se(4)Er4 trigonal pyramids. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Er(1) and three equivalent Er(3) atoms. In the sixth Se site, Se(6) is bonded to one Er(2), two equivalent Er(3), and two equivalent Sm(1) atoms to form SeSm2Er3 trigonal bipyramids that share corners with two equivalent Se(2)Sm2Er3 square pyramids, corners with two equivalent Se(3)Sm3Er2 trigonal bipyramids,  a cornercorner with one Se(4)Er4 trigonal pyramid, corners with three equivalent Se(1)SmEr3 trigonal pyramids,  an edgeedge with one Se(2)Sm2Er3 square pyramid, edges with two equivalent Se(6)Sm2Er3 trigonal bipyramids, edges with three equivalent Se(3)Sm3Er2 trigonal bipyramids, and edges with two equivalent Se(4)Er4 trigonal pyramids.