Lu2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Lu sites. In the first Lu site, Lu(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. In the second Lu site, Lu(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. In the third Lu site, Lu(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Lu(2), two equivalent Lu(1), and two equivalent Lu(3) atoms to form distorted SeLu5 square pyramids that share corners with five equivalent Se(2)Lu6 octahedra, corners with four equivalent Se(1)Lu5 square pyramids, corners with four equivalent Se(3)Lu5 square pyramids, edges with three equivalent Se(2)Lu6 octahedra,  an edgeedge with one Se(1)Lu5 square pyramid, edges with four equivalent Se(3)Lu5 square pyramids,  a faceface with one Se(2)Lu6 octahedra, and  a faceface with one Se(1)Lu5 square pyramid. The corner-sharing octahedral tilt angles range from 24-48°. In the second Se site, Se(2) is bonded to one Lu(3), two equivalent Lu(2), and three equivalent Lu(1) atoms to form distorted SeLu6 octahedra that share corners with three equivalent Se(2)Lu6 octahedra, corners with five equivalent Se(1)Lu5 square pyramids, corners with seven equivalent Se(3)Lu5 square pyramids, edges with two equivalent Se(2)Lu6 octahedra,  an edgeedge with one Se(3)Lu5 square pyramid, edges with three equivalent Se(1)Lu5 square pyramids, faces with two equivalent Se(2)Lu6 octahedra,  a faceface with one Se(1)Lu5 square pyramid, and faces with two equivalent Se(3)Lu5 square pyramids. The corner-sharing octahedral tilt angles range from 18-50°. In the third Se site, Se(3) is bonded to one Lu(2), one Lu(3), and three equivalent Lu(1) atoms to form distorted SeLu5 square pyramids that share corners with seven equivalent Se(2)Lu6 octahedra, corners with four equivalent Se(1)Lu5 square pyramids, corners with five equivalent Se(3)Lu5 square pyramids,  an edgeedge with one Se(2)Lu6 octahedra, edges with four equivalent Se(1)Lu5 square pyramids, faces with two equivalent Se(2)Lu6 octahedra, and  a faceface with one Se(3)Lu5 square pyramid. The corner-sharing octahedral tilt angles range from 16-53°.
Lu2Se3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Lu sites. In the first Lu site, Lu(1) is bonded in a 8-coordinate geometry to two equivalent Se(1), three equivalent Se(2), and three equivalent Se(3) atoms. There is one shorter (2.81 Å) and one longer (3.10 Å) Lu(1)-Se(1) bond length. There are a spread of Lu(1)-Se(2) bond distances ranging from 2.83-3.24 Å. There are a spread of Lu(1)-Se(3) bond distances ranging from 2.82-3.05 Å. In the second Lu site, Lu(2) is bonded in a 8-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and four equivalent Se(2) atoms. Both Lu(2)-Se(1) bond lengths are 2.83 Å. Both Lu(2)-Se(3) bond lengths are 3.06 Å. There are two shorter (2.82 Å) and two longer (3.13 Å) Lu(2)-Se(2) bond lengths. In the third Lu site, Lu(3) is bonded in a 8-coordinate geometry to two equivalent Se(2), two equivalent Se(3), and four equivalent Se(1) atoms. Both Lu(3)-Se(2) bond lengths are 3.20 Å. Both Lu(3)-Se(3) bond lengths are 2.83 Å. There are two shorter (2.83 Å) and two longer (3.06 Å) Lu(3)-Se(1) bond lengths. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Lu(2), two equivalent Lu(1), and two equivalent Lu(3) atoms to form distorted SeLu5 square pyramids that share corners with five equivalent Se(2)Lu6 octahedra, corners with four equivalent Se(1)Lu5 square pyramids, corners with four equivalent Se(3)Lu5 square pyramids, edges with three equivalent Se(2)Lu6 octahedra,  an edgeedge with one Se(1)Lu5 square pyramid, edges with four equivalent Se(3)Lu5 square pyramids,  a faceface with one Se(2)Lu6 octahedra, and  a faceface with one Se(1)Lu5 square pyramid. The corner-sharing octahedral tilt angles range from 24-48°. In the second Se site, Se(2) is bonded to one Lu(3), two equivalent Lu(2), and three equivalent Lu(1) atoms to form distorted SeLu6 octahedra that share corners with three equivalent Se(2)Lu6 octahedra, corners with five equivalent Se(1)Lu5 square pyramids, corners with seven equivalent Se(3)Lu5 square pyramids, edges with two equivalent Se(2)Lu6 octahedra,  an edgeedge with one Se(3)Lu5 square pyramid, edges with three equivalent Se(1)Lu5 square pyramids, faces with two equivalent Se(2)Lu6 octahedra,  a faceface with one Se(1)Lu5 square pyramid, and faces with two equivalent Se(3)Lu5 square pyramids. The corner-sharing octahedral tilt angles range from 18-50°. In the third Se site, Se(3) is bonded to one Lu(2), one Lu(3), and three equivalent Lu(1) atoms to form distorted SeLu5 square pyramids that share corners with seven equivalent Se(2)Lu6 octahedra, corners with four equivalent Se(1)Lu5 square pyramids, corners with five equivalent Se(3)Lu5 square pyramids,  an edgeedge with one Se(2)Lu6 octahedra, edges with four equivalent Se(1)Lu5 square pyramids, faces with two equivalent Se(2)Lu6 octahedra, and  a faceface with one Se(3)Lu5 square pyramid. The corner-sharing octahedral tilt angles range from 16-53°.