HfCuTlSe3 crystallizes in the orthorhombic Cmcm space group. Hf(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form HfSe6 octahedra that share corners with two equivalent Hf(1)Se6 octahedra, edges with two equivalent Hf(1)Se6 octahedra, and edges with four equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 43°. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form CuSe4 tetrahedra that share corners with two equivalent Cu(1)Se4 tetrahedra and edges with four equivalent Hf(1)Se6 octahedra. Tl(1) is bonded in a 6-coordinate geometry to two equivalent Se(2) and four equivalent Se(1) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to two equivalent Hf(1), one Cu(1), and two equivalent Tl(1) atoms. In the second Se site, Se(2) is bonded in a 4-coordinate geometry to two equivalent Hf(1), two equivalent Cu(1), and two equivalent Tl(1) atoms.
HfCuTlSe3 crystallizes in the orthorhombic Cmcm space group. Hf(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form HfSe6 octahedra that share corners with two equivalent Hf(1)Se6 octahedra, edges with two equivalent Hf(1)Se6 octahedra, and edges with four equivalent Cu(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 43°. Both Hf(1)-Se(2) bond lengths are 2.72 Å. All Hf(1)-Se(1) bond lengths are 2.71 Å. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form CuSe4 tetrahedra that share corners with two equivalent Cu(1)Se4 tetrahedra and edges with four equivalent Hf(1)Se6 octahedra. Both Cu(1)-Se(1) bond lengths are 2.43 Å. Both Cu(1)-Se(2) bond lengths are 2.45 Å. Tl(1) is bonded in a 6-coordinate geometry to two equivalent Se(2) and four equivalent Se(1) atoms. Both Tl(1)-Se(2) bond lengths are 3.37 Å. All Tl(1)-Se(1) bond lengths are 3.37 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to two equivalent Hf(1), one Cu(1), and two equivalent Tl(1) atoms. In the second Se site, Se(2) is bonded in a 4-coordinate geometry to two equivalent Hf(1), two equivalent Cu(1), and two equivalent Tl(1) atoms.