MgTm2Se4 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 Tm(1)Se6 octahedra, corners with four equivalent Tm(2)Se6 octahedra,  an edgeedge with one Tm(2)Se6 octahedra, and edges with two equivalent Tm(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 59-67°. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TmSe6 octahedra that share corners with four equivalent Tm(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tm(1)Se6 octahedra, edges with two equivalent Tm(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-53°. In the second Tm site, Tm(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TmSe6 octahedra that share corners with four equivalent Tm(1)Se6 octahedra, corners with four equivalent Tm(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tm(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 Tm(1), and two equivalent Tm(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tm(2), and two equivalent Tm(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form distorted corner-sharing SeTm3Mg trigonal pyramids.
MgTm2Se4 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 Tm(1)Se6 octahedra, corners with four equivalent Tm(2)Se6 octahedra,  an edgeedge with one Tm(2)Se6 octahedra, and edges with two equivalent Tm(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 59-67°. The Mg(1)-Se(2) bond length is 2.61 Å. The Mg(1)-Se(3) bond length is 2.53 Å. Both Mg(1)-Se(1) bond lengths are 2.59 Å. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form TmSe6 octahedra that share corners with four equivalent Tm(2)Se6 octahedra, corners with two equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tm(1)Se6 octahedra, edges with two equivalent Tm(2)Se6 octahedra, and edges with two equivalent Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 50-53°. Both Tm(1)-Se(1) bond lengths are 2.88 Å. Both Tm(1)-Se(2) bond lengths are 2.85 Å. Both Tm(1)-Se(3) bond lengths are 2.80 Å. In the second Tm site, Tm(2) is bonded to one Se(2), one Se(3), and four equivalent Se(1) atoms to form TmSe6 octahedra that share corners with four equivalent Tm(1)Se6 octahedra, corners with four equivalent Tm(2)Se6 octahedra, corners with four equivalent Mg(1)Se4 tetrahedra, edges with two equivalent Tm(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 46-53°. The Tm(2)-Se(2) bond length is 2.81 Å. The Tm(2)-Se(3) bond length is 2.83 Å. There are two shorter (2.89 Å) and two longer (2.90 Å) Tm(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 Tm(1), and two equivalent Tm(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Tm(2), and two equivalent Tm(1) atoms. In the third Se site, Se(3) is bonded to one Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form distorted corner-sharing SeTm3Mg trigonal pyramids.