MgTm2Se4 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 Tm(1)Se6 octahedra, corners with two equivalent Tm(2)Se6 octahedra,  an edgeedge with one Tm(1)Se6 octahedra, edges with three equivalent Tm(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 5-8°. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form TmSe6 octahedra that share corners with three equivalent Tm(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tm(2)Se6 octahedra, edges with four equivalent Tm(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. In the second Tm site, Tm(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form TmSe6 octahedra that share corners with three equivalent Tm(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tm(1)Se6 octahedra, edges with two equivalent Tm(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Tm(2) atoms to form SeTm2Mg3 square pyramids that share corners with two equivalent Se(4)Tm3Mg2 square pyramids, edges with three equivalent Se(4)Tm3Mg2 square pyramids, and edges with four equivalent Se(1)Tm2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Tm(2) and three equivalent Tm(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Tm(1) and two equivalent Tm(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form SeTm3Mg2 square pyramids that share corners with two equivalent Se(1)Tm2Mg3 square pyramids, edges with two equivalent Se(4)Tm3Mg2 square pyramids, and edges with three equivalent Se(1)Tm2Mg3 square pyramids.
MgTm2Se4 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 Tm(1)Se6 octahedra, corners with two equivalent Tm(2)Se6 octahedra,  an edgeedge with one Tm(1)Se6 octahedra, edges with three equivalent Tm(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 5-8°. Both Mg(1)-Se(4) bond lengths are 2.78 Å. There is one shorter (2.72 Å) and two longer (2.73 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form TmSe6 octahedra that share corners with three equivalent Tm(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tm(2)Se6 octahedra, edges with four equivalent Tm(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. The Tm(1)-Se(3) bond length is 2.81 Å. Both Tm(1)-Se(4) bond lengths are 2.85 Å. There are two shorter (2.80 Å) and one longer (2.91 Å) Tm(1)-Se(2) bond length. In the second Tm site, Tm(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form TmSe6 octahedra that share corners with three equivalent Tm(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Tm(1)Se6 octahedra, edges with two equivalent Tm(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. The Tm(2)-Se(2) bond length is 2.89 Å. The Tm(2)-Se(4) bond length is 2.90 Å. Both Tm(2)-Se(1) bond lengths are 2.87 Å. Both Tm(2)-Se(3) bond lengths are 2.75 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Tm(2) atoms to form SeTm2Mg3 square pyramids that share corners with two equivalent Se(4)Tm3Mg2 square pyramids, edges with three equivalent Se(4)Tm3Mg2 square pyramids, and edges with four equivalent Se(1)Tm2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Tm(2) and three equivalent Tm(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Tm(1) and two equivalent Tm(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form SeTm3Mg2 square pyramids that share corners with two equivalent Se(1)Tm2Mg3 square pyramids, edges with two equivalent Se(4)Tm3Mg2 square pyramids, and edges with three equivalent Se(1)Tm2Mg3 square pyramids.