MgLu2Se4 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 Lu(1)Se6 octahedra, corners with four equivalent Lu(2)Se6 octahedra,  an edgeedge with one Lu(2)Se6 octahedra, and edges with two equivalent Lu(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 60-67°. There are two inequivalent Lu sites. In the first Lu site, Lu(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form LuSe6 octahedra that share corners with four equivalent Lu(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Lu(1)Se6 octahedra, edges with two equivalent Lu(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-53°. In the second Lu site, Lu(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form LuSe6 octahedra that share corners with four equivalent Lu(1)Se6 octahedra, corners with four equivalent Lu(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Lu(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-53°. 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 Lu(1), and two equivalent Lu(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Lu(2), and two equivalent Lu(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Lu(2), and two equivalent Lu(1) atoms to form distorted corner-sharing SeLu3Mg tetrahedra.
MgLu2Se4 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 Lu(1)Se6 octahedra, corners with four equivalent Lu(2)Se6 octahedra,  an edgeedge with one Lu(2)Se6 octahedra, and edges with two equivalent Lu(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 60-67°. The Mg(1)-Se(2) bond length is 2.61 Å. The Mg(1)-Se(3) bond length is 2.52 Å. Both Mg(1)-Se(1) bond lengths are 2.59 Å. There are two inequivalent Lu sites. In the first Lu site, Lu(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form LuSe6 octahedra that share corners with four equivalent Lu(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Lu(1)Se6 octahedra, edges with two equivalent Lu(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-53°. Both Lu(1)-Se(1) bond lengths are 2.86 Å. Both Lu(1)-Se(2) bond lengths are 2.83 Å. Both Lu(1)-Se(3) bond lengths are 2.77 Å. In the second Lu site, Lu(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form LuSe6 octahedra that share corners with four equivalent Lu(1)Se6 octahedra, corners with four equivalent Lu(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Lu(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-53°. The Lu(2)-Se(2) bond length is 2.79 Å. The Lu(2)-Se(3) bond length is 2.80 Å. There are two shorter (2.86 Å) and two longer (2.89 Å) Lu(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 Lu(1), and two equivalent Lu(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Lu(2), and two equivalent Lu(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Lu(2), and two equivalent Lu(1) atoms to form distorted corner-sharing SeLu3Mg tetrahedra.