MgTmS3 is Ilmenite-like structured and crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form distorted MgS6 octahedra that share corners with four equivalent Tm(1)S6 pentagonal pyramids, edges with three equivalent Mg(1)S6 octahedra, and edges with four equivalent Tm(1)S6 pentagonal pyramids. Tm(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form distorted TmS6 pentagonal pyramids that share corners with four equivalent Mg(1)S6 octahedra, edges with four equivalent Mg(1)S6 octahedra,  an edgeedge with one Tm(1)S6 pentagonal pyramid, and  a faceface with one Tm(1)S6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-47°. There are two inequivalent S sites. In the first S site, S(1) is bonded in a 4-coordinate geometry to two equivalent Mg(1) and two equivalent Tm(1) atoms. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Tm(1) atoms.
MgTmS3 is Ilmenite-like structured and crystallizes in the monoclinic C2/m space group. Mg(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form distorted MgS6 octahedra that share corners with four equivalent Tm(1)S6 pentagonal pyramids, edges with three equivalent Mg(1)S6 octahedra, and edges with four equivalent Tm(1)S6 pentagonal pyramids. Both Mg(1)-S(2) bond lengths are 2.56 Å. There are two shorter (2.61 Å) and two longer (2.82 Å) Mg(1)-S(1) bond lengths. Tm(1) is bonded to two equivalent S(2) and four equivalent S(1) atoms to form distorted TmS6 pentagonal pyramids that share corners with four equivalent Mg(1)S6 octahedra, edges with four equivalent Mg(1)S6 octahedra,  an edgeedge with one Tm(1)S6 pentagonal pyramid, and  a faceface with one Tm(1)S6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-47°. There is one shorter (2.65 Å) and one longer (2.66 Å) Tm(1)-S(2) bond length. There are two shorter (2.64 Å) and two longer (2.88 Å) Tm(1)-S(1) bond lengths. There are two inequivalent S sites. In the first S site, S(1) is bonded in a 4-coordinate geometry to two equivalent Mg(1) and two equivalent Tm(1) atoms. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Tm(1) atoms.