Tm2Se3 crystallizes in the orthorhombic Fddd space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing TmSe6 octahedra. The corner-sharing octahedral tilt angles are 4°. In the second Tm site, Tm(2) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing TmSe6 octahedra. The corner-sharing octahedral tilt angles range from 3-4°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent Tm(1) and two equivalent Tm(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to two equivalent Tm(1) and two equivalent Tm(2) atoms.
Tm2Se3 crystallizes in the orthorhombic Fddd space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing TmSe6 octahedra. The corner-sharing octahedral tilt angles are 4°. Both Tm(1)-Se(1) bond lengths are 2.82 Å. There are two shorter (2.80 Å) and two longer (2.84 Å) Tm(1)-Se(2) bond lengths. In the second Tm site, Tm(2) is bonded to two equivalent Se(1) and four equivalent Se(2) atoms to form a mixture of corner and edge-sharing TmSe6 octahedra. The corner-sharing octahedral tilt angles range from 3-4°. Both Tm(2)-Se(1) bond lengths are 2.84 Å. All Tm(2)-Se(2) bond lengths are 2.81 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a rectangular see-saw-like geometry to two equivalent Tm(1) and two equivalent Tm(2) atoms. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to two equivalent Tm(1) and two equivalent Tm(2) atoms.