Er4CrSe7 crystallizes in the monoclinic Cm space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one Se(4), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(5) atoms. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one Se(3), two equivalent Se(1), two equivalent Se(4), and two equivalent Se(6) atoms. In the third Er site, Er(3) is bonded to one Se(5), one Se(6), two equivalent Se(3), and two equivalent Se(4) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(4)Se6 octahedra,  a cornercorner with one Cr(1)Se6 octahedra, and edges with two equivalent Er(3)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. In the fourth Er site, Er(4) is bonded to one Se(2), one Se(6), two equivalent Se(1), and two equivalent Se(7) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Cr(1)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and edges with three equivalent Cr(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 3-50°. Cr(1) is bonded to one Se(1), one Se(5), two equivalent Se(2), and two equivalent Se(7) atoms to form CrSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Er(4)Se6 octahedra, edges with two equivalent Cr(1)Se6 octahedra, and edges with three equivalent Er(4)Se6 octahedra. The corner-sharing octahedral tilt angles range from 3-53°. There are seven inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Er(2), two equivalent Er(4), and one Cr(1) atom to form SeEr4Cr trigonal bipyramids that share  a cornercorner with one Se(5)Er3Cr tetrahedra, corners with four equivalent Se(6)Er4 tetrahedra, corners with two equivalent Se(3)Er5 trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(6)Er4 tetrahedra,  an edgeedge with one Se(4)Er5 trigonal bipyramid, edges with two equivalent Se(2)Er3Cr2 trigonal bipyramids, and edges with two equivalent Se(1)Er4Cr trigonal bipyramids. In the second Se site, Se(2) is bonded to one Er(4), two equivalent Er(1), and two equivalent Cr(1) atoms to form SeEr3Cr2 trigonal bipyramids that share  a cornercorner with one Se(6)Er4 tetrahedra, corners with four equivalent Se(5)Er3Cr tetrahedra, corners with two equivalent Se(3)Er5 trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(5)Er3Cr tetrahedra,  an edgeedge with one Se(3)Er5 trigonal bipyramid, edges with two equivalent Se(2)Er3Cr2 trigonal bipyramids, and edges with two equivalent Se(1)Er4Cr trigonal bipyramids. In the third Se site, Se(3) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with four equivalent Se(5)Er3Cr tetrahedra, corners with two equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with two equivalent Se(1)Er4Cr trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(5)Er3Cr tetrahedra, edges with two equivalent Se(6)Er4 tetrahedra,  an edgeedge with one Se(2)Er3Cr2 trigonal bipyramid, edges with two equivalent Se(3)Er5 trigonal bipyramids, and edges with three equivalent Se(4)Er5 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to one Er(1), two equivalent Er(2), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with four equivalent Se(6)Er4 tetrahedra, corners with two equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with two equivalent Se(1)Er4Cr trigonal bipyramids, corners with two equivalent Se(3)Er5 trigonal bipyramids,  an edgeedge with one Se(6)Er4 tetrahedra, edges with two equivalent Se(5)Er3Cr tetrahedra,  an edgeedge with one Se(1)Er4Cr trigonal bipyramid, edges with two equivalent Se(4)Er5 trigonal bipyramids, and edges with three equivalent Se(3)Er5 trigonal bipyramids. In the fifth Se site, Se(5) is bonded to one Er(3), two equivalent Er(1), and one Cr(1) atom to form distorted SeEr3Cr tetrahedra that share  a cornercorner with one Se(6)Er4 tetrahedra, corners with two equivalent Se(5)Er3Cr tetrahedra,  a cornercorner with one Se(1)Er4Cr trigonal bipyramid, corners with four equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Er5 trigonal bipyramids,  an edgeedge with one Se(2)Er3Cr2 trigonal bipyramid,  an edgeedge with one Se(3)Er5 trigonal bipyramid, and edges with two equivalent Se(4)Er5 trigonal bipyramids. In the sixth Se site, Se(6) is bonded to one Er(3), one Er(4), and two equivalent Er(2) atoms to form SeEr4 tetrahedra that share  a cornercorner with one Se(5)Er3Cr tetrahedra, corners with two equivalent Se(6)Er4 tetrahedra,  a cornercorner with one Se(2)Er3Cr2 trigonal bipyramid, corners with four equivalent Se(1)Er4Cr trigonal bipyramids, corners with four equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(1)Er4Cr trigonal bipyramid,  an edgeedge with one Se(4)Er5 trigonal bipyramid, and edges with two equivalent Se(3)Er5 trigonal bipyramids. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to two equivalent Er(4) and two equivalent Cr(1) atoms.
Er4CrSe7 crystallizes in the monoclinic Cm space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one Se(4), two equivalent Se(2), two equivalent Se(3), and two equivalent Se(5) atoms. The Er(1)-Se(4) bond length is 3.20 Å. Both Er(1)-Se(2) bond lengths are 2.98 Å. Both Er(1)-Se(3) bond lengths are 2.98 Å. Both Er(1)-Se(5) bond lengths are 2.86 Å. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one Se(3), two equivalent Se(1), two equivalent Se(4), and two equivalent Se(6) atoms. The Er(2)-Se(3) bond length is 3.09 Å. Both Er(2)-Se(1) bond lengths are 3.03 Å. Both Er(2)-Se(4) bond lengths are 2.96 Å. Both Er(2)-Se(6) bond lengths are 2.83 Å. In the third Er site, Er(3) is bonded to one Se(5), one Se(6), two equivalent Se(3), and two equivalent Se(4) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(4)Se6 octahedra,  a cornercorner with one Cr(1)Se6 octahedra, and edges with two equivalent Er(3)Se6 octahedra. The corner-sharing octahedral tilt angles range from 50-53°. The Er(3)-Se(5) bond length is 2.79 Å. The Er(3)-Se(6) bond length is 2.80 Å. Both Er(3)-Se(3) bond lengths are 2.83 Å. Both Er(3)-Se(4) bond lengths are 2.82 Å. In the fourth Er site, Er(4) is bonded to one Se(2), one Se(6), two equivalent Se(1), and two equivalent Se(7) atoms to form ErSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Cr(1)Se6 octahedra, edges with two equivalent Er(4)Se6 octahedra, and edges with three equivalent Cr(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 3-50°. The Er(4)-Se(2) bond length is 2.81 Å. The Er(4)-Se(6) bond length is 2.82 Å. Both Er(4)-Se(1) bond lengths are 2.90 Å. Both Er(4)-Se(7) bond lengths are 2.78 Å. Cr(1) is bonded to one Se(1), one Se(5), two equivalent Se(2), and two equivalent Se(7) atoms to form CrSe6 octahedra that share  a cornercorner with one Er(3)Se6 octahedra, corners with two equivalent Er(4)Se6 octahedra, edges with two equivalent Cr(1)Se6 octahedra, and edges with three equivalent Er(4)Se6 octahedra. The corner-sharing octahedral tilt angles range from 3-53°. The Cr(1)-Se(1) bond length is 2.71 Å. The Cr(1)-Se(5) bond length is 2.60 Å. Both Cr(1)-Se(2) bond lengths are 2.73 Å. Both Cr(1)-Se(7) bond lengths are 2.67 Å. There are seven inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Er(2), two equivalent Er(4), and one Cr(1) atom to form SeEr4Cr trigonal bipyramids that share  a cornercorner with one Se(5)Er3Cr tetrahedra, corners with four equivalent Se(6)Er4 tetrahedra, corners with two equivalent Se(3)Er5 trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(6)Er4 tetrahedra,  an edgeedge with one Se(4)Er5 trigonal bipyramid, edges with two equivalent Se(2)Er3Cr2 trigonal bipyramids, and edges with two equivalent Se(1)Er4Cr trigonal bipyramids. In the second Se site, Se(2) is bonded to one Er(4), two equivalent Er(1), and two equivalent Cr(1) atoms to form SeEr3Cr2 trigonal bipyramids that share  a cornercorner with one Se(6)Er4 tetrahedra, corners with four equivalent Se(5)Er3Cr tetrahedra, corners with two equivalent Se(3)Er5 trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(5)Er3Cr tetrahedra,  an edgeedge with one Se(3)Er5 trigonal bipyramid, edges with two equivalent Se(2)Er3Cr2 trigonal bipyramids, and edges with two equivalent Se(1)Er4Cr trigonal bipyramids. In the third Se site, Se(3) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with four equivalent Se(5)Er3Cr tetrahedra, corners with two equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with two equivalent Se(1)Er4Cr trigonal bipyramids, corners with two equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(5)Er3Cr tetrahedra, edges with two equivalent Se(6)Er4 tetrahedra,  an edgeedge with one Se(2)Er3Cr2 trigonal bipyramid, edges with two equivalent Se(3)Er5 trigonal bipyramids, and edges with three equivalent Se(4)Er5 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to one Er(1), two equivalent Er(2), and two equivalent Er(3) atoms to form distorted SeEr5 trigonal bipyramids that share corners with four equivalent Se(6)Er4 tetrahedra, corners with two equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with two equivalent Se(1)Er4Cr trigonal bipyramids, corners with two equivalent Se(3)Er5 trigonal bipyramids,  an edgeedge with one Se(6)Er4 tetrahedra, edges with two equivalent Se(5)Er3Cr tetrahedra,  an edgeedge with one Se(1)Er4Cr trigonal bipyramid, edges with two equivalent Se(4)Er5 trigonal bipyramids, and edges with three equivalent Se(3)Er5 trigonal bipyramids. In the fifth Se site, Se(5) is bonded to one Er(3), two equivalent Er(1), and one Cr(1) atom to form distorted SeEr3Cr tetrahedra that share  a cornercorner with one Se(6)Er4 tetrahedra, corners with two equivalent Se(5)Er3Cr tetrahedra,  a cornercorner with one Se(1)Er4Cr trigonal bipyramid, corners with four equivalent Se(2)Er3Cr2 trigonal bipyramids, corners with four equivalent Se(3)Er5 trigonal bipyramids,  an edgeedge with one Se(2)Er3Cr2 trigonal bipyramid,  an edgeedge with one Se(3)Er5 trigonal bipyramid, and edges with two equivalent Se(4)Er5 trigonal bipyramids. In the sixth Se site, Se(6) is bonded to one Er(3), one Er(4), and two equivalent Er(2) atoms to form SeEr4 tetrahedra that share  a cornercorner with one Se(5)Er3Cr tetrahedra, corners with two equivalent Se(6)Er4 tetrahedra,  a cornercorner with one Se(2)Er3Cr2 trigonal bipyramid, corners with four equivalent Se(1)Er4Cr trigonal bipyramids, corners with four equivalent Se(4)Er5 trigonal bipyramids,  an edgeedge with one Se(1)Er4Cr trigonal bipyramid,  an edgeedge with one Se(4)Er5 trigonal bipyramid, and edges with two equivalent Se(3)Er5 trigonal bipyramids. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to two equivalent Er(4) and two equivalent Cr(1) atoms.