MgTm2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Tm(1)S6 octahedra, corners with two equivalent Tm(2)S6 octahedra,  an edgeedge with one Tm(1)S6 octahedra, edges with three equivalent Tm(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 4-5°. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form TmS6 octahedra that share corners with three equivalent Tm(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tm(2)S6 octahedra, edges with four equivalent Tm(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 55-58°. In the second Tm site, Tm(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form TmS6 octahedra that share corners with three equivalent Tm(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tm(1)S6 octahedra, edges with two equivalent Tm(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 55-58°. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Tm(2) atoms to form STm2Mg3 square pyramids that share corners with two equivalent S(4)Tm3Mg2 square pyramids, edges with three equivalent S(4)Tm3Mg2 square pyramids, and edges with four equivalent S(1)Tm2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Tm(2) and three equivalent Tm(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Tm(1) and two equivalent Tm(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form STm3Mg2 square pyramids that share corners with two equivalent S(1)Tm2Mg3 square pyramids, edges with two equivalent S(4)Tm3Mg2 square pyramids, and edges with three equivalent S(1)Tm2Mg3 square pyramids.
MgTm2S4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent S(4) and three equivalent S(1) atoms to form MgS5 square pyramids that share corners with two equivalent Tm(1)S6 octahedra, corners with two equivalent Tm(2)S6 octahedra,  an edgeedge with one Tm(1)S6 octahedra, edges with three equivalent Tm(2)S6 octahedra, and edges with four equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 4-5°. Both Mg(1)-S(4) bond lengths are 2.62 Å. There is one shorter (2.55 Å) and two longer (2.60 Å) Mg(1)-S(1) bond lengths. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to one S(3), two equivalent S(4), and three equivalent S(2) atoms to form TmS6 octahedra that share corners with three equivalent Tm(2)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tm(2)S6 octahedra, edges with four equivalent Tm(1)S6 octahedra, and  an edgeedge with one Mg(1)S5 square pyramid. The corner-sharing octahedral tilt angles range from 55-58°. The Tm(1)-S(3) bond length is 2.66 Å. Both Tm(1)-S(4) bond lengths are 2.73 Å. There are two shorter (2.68 Å) and one longer (2.78 Å) Tm(1)-S(2) bond length. In the second Tm site, Tm(2) is bonded to one S(2), one S(4), two equivalent S(1), and two equivalent S(3) atoms to form TmS6 octahedra that share corners with three equivalent Tm(1)S6 octahedra, corners with two equivalent Mg(1)S5 square pyramids, edges with two equivalent Tm(1)S6 octahedra, edges with two equivalent Tm(2)S6 octahedra, and edges with three equivalent Mg(1)S5 square pyramids. The corner-sharing octahedral tilt angles range from 55-58°. The Tm(2)-S(2) bond length is 2.76 Å. The Tm(2)-S(4) bond length is 2.81 Å. Both Tm(2)-S(1) bond lengths are 2.76 Å. Both Tm(2)-S(3) bond lengths are 2.61 Å. There are four inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Mg(1) and two equivalent Tm(2) atoms to form STm2Mg3 square pyramids that share corners with two equivalent S(4)Tm3Mg2 square pyramids, edges with three equivalent S(4)Tm3Mg2 square pyramids, and edges with four equivalent S(1)Tm2Mg3 square pyramids. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Tm(2) and three equivalent Tm(1) atoms. In the third S site, S(3) is bonded in a distorted T-shaped geometry to one Tm(1) and two equivalent Tm(2) atoms. In the fourth S site, S(4) is bonded to two equivalent Mg(1), one Tm(2), and two equivalent Tm(1) atoms to form STm3Mg2 square pyramids that share corners with two equivalent S(1)Tm2Mg3 square pyramids, edges with two equivalent S(4)Tm3Mg2 square pyramids, and edges with three equivalent S(1)Tm2Mg3 square pyramids.