MgDy2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Dy(1)Se6 octahedra, corners with four equivalent Dy(2)Se6 octahedra,  an edgeedge with one Dy(2)Se6 octahedra, and edges with two equivalent Dy(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form DySe6 octahedra that share corners with four equivalent Dy(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Dy(1)Se6 octahedra, edges with two equivalent Dy(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. In the second Dy site, Dy(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form DySe6 octahedra that share corners with four equivalent Dy(1)Se6 octahedra, corners with four equivalent Dy(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Dy(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Dy(1), and two equivalent Dy(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Dy(2), and two equivalent Dy(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Dy(2), and two equivalent Dy(1) atoms to form distorted corner-sharing SeDy3Mg trigonal pyramids.
MgDy2Se4 is Spinel-like structured and crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form MgSe4 tetrahedra that share corners with two equivalent Dy(1)Se6 octahedra, corners with four equivalent Dy(2)Se6 octahedra,  an edgeedge with one Dy(2)Se6 octahedra, and edges with two equivalent Dy(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 58-66°. The Mg(1)-Se(2) bond length is 2.59 Å. The Mg(1)-Se(3) bond length is 2.51 Å. Both Mg(1)-Se(1) bond lengths are 2.58 Å. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form DySe6 octahedra that share corners with four equivalent Dy(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Dy(1)Se6 octahedra, edges with two equivalent Dy(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-54°. Both Dy(1)-Se(1) bond lengths are 2.88 Å. Both Dy(1)-Se(2) bond lengths are 2.86 Å. Both Dy(1)-Se(3) bond lengths are 2.82 Å. In the second Dy site, Dy(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form DySe6 octahedra that share corners with four equivalent Dy(1)Se6 octahedra, corners with four equivalent Dy(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Dy(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-54°. The Dy(2)-Se(2) bond length is 2.84 Å. The Dy(2)-Se(3) bond length is 2.86 Å. There are two shorter (2.89 Å) and two longer (2.91 Å) Dy(2)-Se(1) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Dy(1), and two equivalent Dy(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Dy(2), and two equivalent Dy(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Dy(2), and two equivalent Dy(1) atoms to form distorted corner-sharing SeDy3Mg trigonal pyramids.