MgEr2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Er(1)S6 octahedra, corners with two equivalent Er(2)S6 octahedra,  an edgeedge with one Er(1)S6 octahedra, edges with three equivalent Er(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 5-8°. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form ErS6 octahedra that share corners with three equivalent Er(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Er(2)S6 octahedra, edges with four equivalent Er(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. In the second Er site, Er(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form ErS6 octahedra that share corners with three equivalent Er(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Er(1)S6 octahedra, edges with two equivalent Er(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Er(2) atoms to form SEr2Mg3 square pyramids that share corners with two equivalent S(4)Er3Mg2 square pyramids, edges with three equivalent S(4)Er3Mg2 square pyramids, and edges with four equivalent S(1)Er2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Er(2), and two equivalent Er(1) atoms to form SEr3Mg2 square pyramids that share corners with two equivalent S(1)Er2Mg3 square pyramids, edges with two equivalent S(4)Er3Mg2 square pyramids, and edges with three equivalent S(1)Er2Mg3 square pyramids.
MgEr2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Er(1)S6 octahedra, corners with two equivalent Er(2)S6 octahedra,  an edgeedge with one Er(1)S6 octahedra, edges with three equivalent Er(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 5-8°. Both Mg(1)-S(4) bond lengths are 2.66 Å. There is one shorter (2.58 Å) and two longer (2.62 Å) Mg(1)-S(1) bond lengths. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form ErS6 octahedra that share corners with three equivalent Er(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Er(2)S6 octahedra, edges with four equivalent Er(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 53-59°. The Er(1)-S(3) bond length is 2.68 Å. Both Er(1)-S(4) bond lengths are 2.74 Å. There are two shorter (2.69 Å) and one longer (2.79 Å) Er(1)-S(2) bond length. In the second Er site, Er(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form ErS6 octahedra that share corners with three equivalent Er(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Er(1)S6 octahedra, edges with two equivalent Er(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 53-59°. The Er(2)-S(2) bond length is 2.75 Å. The Er(2)-S(4) bond length is 2.78 Å. Both Er(2)-S(1) bond lengths are 2.76 Å. Both Er(2)-S(3) bond lengths are 2.64 Å. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Er(2) atoms to form SEr2Mg3 square pyramids that share corners with two equivalent S(4)Er3Mg2 square pyramids, edges with three equivalent S(4)Er3Mg2 square pyramids, and edges with four equivalent S(1)Er2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Er(2) and three equivalent Er(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Er(2), and two equivalent Er(1) atoms to form SEr3Mg2 square pyramids that share corners with two equivalent S(1)Er2Mg3 square pyramids, edges with two equivalent S(4)Er3Mg2 square pyramids, and edges with three equivalent S(1)Er2Mg3 square pyramids.