MgDy2Se4 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 Dy(1)Se6 octahedra, corners with two equivalent Dy(2)Se6 octahedra,  an edgeedge with one Dy(1)Se6 octahedra, edges with three equivalent Dy(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 3-4°. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form DySe6 octahedra that share corners with three equivalent Dy(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Dy(2)Se6 octahedra, edges with four equivalent Dy(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 54-57°. In the second Dy site, Dy(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form DySe6 octahedra that share corners with three equivalent Dy(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Dy(1)Se6 octahedra, edges with two equivalent Dy(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 54-57°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Dy(2) atoms to form SeDy2Mg3 square pyramids that share corners with two equivalent Se(4)Dy3Mg2 square pyramids, edges with three equivalent Se(4)Dy3Mg2 square pyramids, and edges with four equivalent Se(1)Dy2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Dy(2) and three equivalent Dy(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Dy(1) and two equivalent Dy(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Dy(2), and two equivalent Dy(1) atoms to form SeDy3Mg2 square pyramids that share corners with two equivalent Se(1)Dy2Mg3 square pyramids, edges with two equivalent Se(4)Dy3Mg2 square pyramids, and edges with three equivalent Se(1)Dy2Mg3 square pyramids.
MgDy2Se4 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 Dy(1)Se6 octahedra, corners with two equivalent Dy(2)Se6 octahedra,  an edgeedge with one Dy(1)Se6 octahedra, edges with three equivalent Dy(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 3-4°. Both Mg(1)-Se(4) bond lengths are 2.77 Å. There is one shorter (2.67 Å) and two longer (2.74 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form DySe6 octahedra that share corners with three equivalent Dy(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Dy(2)Se6 octahedra, edges with four equivalent Dy(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 54-57°. The Dy(1)-Se(3) bond length is 2.83 Å. Both Dy(1)-Se(4) bond lengths are 2.89 Å. There are two shorter (2.84 Å) and one longer (2.95 Å) Dy(1)-Se(2) bond length. In the second Dy site, Dy(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form DySe6 octahedra that share corners with three equivalent Dy(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Dy(1)Se6 octahedra, edges with two equivalent Dy(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 54-57°. The Dy(2)-Se(2) bond length is 2.92 Å. The Dy(2)-Se(4) bond length is 2.96 Å. Both Dy(2)-Se(1) bond lengths are 2.93 Å. Both Dy(2)-Se(3) bond lengths are 2.77 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Dy(2) atoms to form SeDy2Mg3 square pyramids that share corners with two equivalent Se(4)Dy3Mg2 square pyramids, edges with three equivalent Se(4)Dy3Mg2 square pyramids, and edges with four equivalent Se(1)Dy2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Dy(2) and three equivalent Dy(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Dy(1) and two equivalent Dy(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Dy(2), and two equivalent Dy(1) atoms to form SeDy3Mg2 square pyramids that share corners with two equivalent Se(1)Dy2Mg3 square pyramids, edges with two equivalent Se(4)Dy3Mg2 square pyramids, and edges with three equivalent Se(1)Dy2Mg3 square pyramids.