MgErSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form MgSe6 octahedra that share corners with eight equivalent Er(1)Se4 tetrahedra and edges with three equivalent Mg(1)Se6 octahedra. Er(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form ErSe4 tetrahedra that share corners with eight equivalent Mg(1)Se6 octahedra and  an edgeedge with one Er(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-72°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Er(1) atom. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Er(1) atoms to form edge-sharing SeEr2Mg2 tetrahedra.
MgErSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form MgSe6 octahedra that share corners with eight equivalent Er(1)Se4 tetrahedra and edges with three equivalent Mg(1)Se6 octahedra. Both Mg(1)-Se(2) bond lengths are 2.67 Å. There are two shorter (2.70 Å) and two longer (2.92 Å) Mg(1)-Se(1) bond lengths. Er(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form ErSe4 tetrahedra that share corners with eight equivalent Mg(1)Se6 octahedra and  an edgeedge with one Er(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-72°. Both Er(1)-Se(1) bond lengths are 2.71 Å. There is one shorter (2.70 Å) and one longer (2.71 Å) Er(1)-Se(2) bond length. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Er(1) atom. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Er(1) atoms to form edge-sharing SeEr2Mg2 tetrahedra.