MgEr2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Er(1)Se6 octahedra, corners with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with three equivalent Er(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 6-11°. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with three equivalent Er(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Er(2)Se6 octahedra, edges with four equivalent Er(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. 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 two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. 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 three equivalent Mg(1) and two equivalent Er(2) atoms to form SeEr2Mg3 square pyramids that share corners with two equivalent Se(4)Er3Mg2 square pyramids, edges with three equivalent Se(4)Er3Mg2 square pyramids, and edges with four equivalent Se(1)Er2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Er(2), and two equivalent Er(1) atoms to form SeEr3Mg2 square pyramids that share corners with two equivalent Se(1)Er2Mg3 square pyramids, edges with two equivalent Se(4)Er3Mg2 square pyramids, and edges with three equivalent Se(1)Er2Mg3 square pyramids.
MgEr2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Er(1)Se6 octahedra, corners with two equivalent Er(2)Se6 octahedra,  an edgeedge with one Er(1)Se6 octahedra, edges with three equivalent Er(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 6-11°. Both Mg(1)-Se(4) bond lengths are 2.81 Å. All Mg(1)-Se(1) bond lengths are 2.75 Å. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with three equivalent Er(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Er(2)Se6 octahedra, edges with four equivalent Er(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 52-59°. The Er(1)-Se(3) bond length is 2.83 Å. Both Er(1)-Se(4) bond lengths are 2.86 Å. There are two shorter (2.82 Å) and one longer (2.92 Å) Er(1)-Se(2) 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 two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Er(1)Se6 octahedra, edges with two equivalent Er(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 52-59°. The Er(2)-Se(2) bond length is 2.89 Å. The Er(2)-Se(4) bond length is 2.90 Å. Both Er(2)-Se(1) bond lengths are 2.89 Å. Both Er(2)-Se(3) bond lengths are 2.78 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Er(2) atoms to form SeEr2Mg3 square pyramids that share corners with two equivalent Se(4)Er3Mg2 square pyramids, edges with three equivalent Se(4)Er3Mg2 square pyramids, and edges with four equivalent Se(1)Er2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Er(2), and two equivalent Er(1) atoms to form SeEr3Mg2 square pyramids that share corners with two equivalent Se(1)Er2Mg3 square pyramids, edges with two equivalent Se(4)Er3Mg2 square pyramids, and edges with three equivalent Se(1)Er2Mg3 square pyramids.