MgTmSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form MgSe6 octahedra that share corners with eight equivalent Tm(1)Se4 tetrahedra and edges with three equivalent Mg(1)Se6 octahedra. Tm(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form TmSe4 tetrahedra that share corners with eight equivalent Mg(1)Se6 octahedra and  an edgeedge with one Tm(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-72°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Tm(1) atom. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Tm(1) atoms to form edge-sharing SeTm2Mg2 tetrahedra.
MgTmSe3 crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form MgSe6 octahedra that share corners with eight equivalent Tm(1)Se4 tetrahedra and edges with three equivalent Mg(1)Se6 octahedra. Both Mg(1)-Se(2) bond lengths are 2.67 Å. There are two shorter (2.70 Å) and two longer (2.92 Å) Mg(1)-Se(1) bond lengths. Tm(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form TmSe4 tetrahedra that share corners with eight equivalent Mg(1)Se6 octahedra and  an edgeedge with one Tm(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles range from 56-72°. Both Tm(1)-Se(1) bond lengths are 2.69 Å. There is one shorter (2.68 Å) and one longer (2.70 Å) Tm(1)-Se(2) bond length. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted trigonal planar geometry to two equivalent Mg(1) and one Tm(1) atom. In the second Se site, Se(2) is bonded to two equivalent Mg(1) and two equivalent Tm(1) atoms to form edge-sharing SeTm2Mg2 tetrahedra.