Tm2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 8-coordinate geometry to two equivalent Se(2), three equivalent Se(1), and three equivalent Se(3) atoms. In the second Tm site, Tm(2) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. In the third Tm site, Tm(3) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Tm(3), two equivalent Tm(2), and three equivalent Tm(1) atoms to form distorted SeTm6 octahedra that share corners with three equivalent Se(1)Tm6 octahedra, corners with seven equivalent Se(3)Tm5 square pyramids, corners with five equivalent Se(2)Tm5 trigonal bipyramids, edges with two equivalent Se(1)Tm6 octahedra,  an edgeedge with one Se(3)Tm5 square pyramid, edges with three equivalent Se(2)Tm5 trigonal bipyramids, faces with two equivalent Se(1)Tm6 octahedra, faces with two equivalent Se(3)Tm5 square pyramids, and  a faceface with one Se(2)Tm5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 20-48°. In the second Se site, Se(2) is bonded to one Tm(2), two equivalent Tm(1), and two equivalent Tm(3) atoms to form distorted SeTm5 trigonal bipyramids that share corners with five equivalent Se(1)Tm6 octahedra, corners with four equivalent Se(3)Tm5 square pyramids, corners with four equivalent Se(2)Tm5 trigonal bipyramids, edges with three equivalent Se(1)Tm6 octahedra, edges with four equivalent Se(3)Tm5 square pyramids,  an edgeedge with one Se(2)Tm5 trigonal bipyramid,  a faceface with one Se(1)Tm6 octahedra, and  a faceface with one Se(2)Tm5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 25-50°. In the third Se site, Se(3) is bonded to one Tm(2), one Tm(3), and three equivalent Tm(1) atoms to form distorted SeTm5 square pyramids that share corners with seven equivalent Se(1)Tm6 octahedra, corners with five equivalent Se(3)Tm5 square pyramids, corners with four equivalent Se(2)Tm5 trigonal bipyramids,  an edgeedge with one Se(1)Tm6 octahedra, edges with four equivalent Se(2)Tm5 trigonal bipyramids, faces with two equivalent Se(1)Tm6 octahedra, and  a faceface with one Se(3)Tm5 square pyramid. The corner-sharing octahedral tilt angles range from 16-54°.
Tm2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 8-coordinate geometry to two equivalent Se(2), three equivalent Se(1), and three equivalent Se(3) atoms. There is one shorter (2.82 Å) and one longer (3.13 Å) Tm(1)-Se(2) bond length. There are a spread of Tm(1)-Se(1) bond distances ranging from 2.84-3.29 Å. There are a spread of Tm(1)-Se(3) bond distances ranging from 2.82-3.08 Å. In the second Tm site, Tm(2) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. Both Tm(2)-Se(2) bond lengths are 2.84 Å. Both Tm(2)-Se(3) bond lengths are 3.13 Å. There are two shorter (2.84 Å) and two longer (3.07 Å) Tm(2)-Se(1) bond lengths. In the third Tm site, Tm(3) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Both Tm(3)-Se(1) bond lengths are 3.24 Å. Both Tm(3)-Se(3) bond lengths are 2.86 Å. There are two shorter (2.85 Å) and two longer (3.04 Å) Tm(3)-Se(2) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Tm(3), two equivalent Tm(2), and three equivalent Tm(1) atoms to form distorted SeTm6 octahedra that share corners with three equivalent Se(1)Tm6 octahedra, corners with seven equivalent Se(3)Tm5 square pyramids, corners with five equivalent Se(2)Tm5 trigonal bipyramids, edges with two equivalent Se(1)Tm6 octahedra,  an edgeedge with one Se(3)Tm5 square pyramid, edges with three equivalent Se(2)Tm5 trigonal bipyramids, faces with two equivalent Se(1)Tm6 octahedra, faces with two equivalent Se(3)Tm5 square pyramids, and  a faceface with one Se(2)Tm5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 20-48°. In the second Se site, Se(2) is bonded to one Tm(2), two equivalent Tm(1), and two equivalent Tm(3) atoms to form distorted SeTm5 trigonal bipyramids that share corners with five equivalent Se(1)Tm6 octahedra, corners with four equivalent Se(3)Tm5 square pyramids, corners with four equivalent Se(2)Tm5 trigonal bipyramids, edges with three equivalent Se(1)Tm6 octahedra, edges with four equivalent Se(3)Tm5 square pyramids,  an edgeedge with one Se(2)Tm5 trigonal bipyramid,  a faceface with one Se(1)Tm6 octahedra, and  a faceface with one Se(2)Tm5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 25-50°. In the third Se site, Se(3) is bonded to one Tm(2), one Tm(3), and three equivalent Tm(1) atoms to form distorted SeTm5 square pyramids that share corners with seven equivalent Se(1)Tm6 octahedra, corners with five equivalent Se(3)Tm5 square pyramids, corners with four equivalent Se(2)Tm5 trigonal bipyramids,  an edgeedge with one Se(1)Tm6 octahedra, edges with four equivalent Se(2)Tm5 trigonal bipyramids, faces with two equivalent Se(1)Tm6 octahedra, and  a faceface with one Se(3)Tm5 square pyramid. The corner-sharing octahedral tilt angles range from 16-54°.