CuAsSe2 crystallizes in the trigonal R3m space group. Cu(1) is bonded to one Se(2) and three equivalent Se(1) atoms to form CuSe4 tetrahedra that share corners with six equivalent Cu(1)Se4 tetrahedra and corners with six equivalent As(1)Se4 tetrahedra. As(1) is bonded to one Se(1) and three equivalent Se(2) atoms to form AsSe4 tetrahedra that share corners with six equivalent Cu(1)Se4 tetrahedra and corners with six equivalent As(1)Se4 tetrahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Cu(1) and one As(1) atom to form SeCu3As tetrahedra that share corners with six equivalent Se(1)Cu3As tetrahedra and corners with six equivalent Se(2)CuAs3 tetrahedra. In the second Se site, Se(2) is bonded to one Cu(1) and three equivalent As(1) atoms to form SeCuAs3 tetrahedra that share corners with six equivalent Se(1)Cu3As tetrahedra and corners with six equivalent Se(2)CuAs3 tetrahedra.
CuAsSe2 crystallizes in the trigonal R3m space group. Cu(1) is bonded to one Se(2) and three equivalent Se(1) atoms to form CuSe4 tetrahedra that share corners with six equivalent Cu(1)Se4 tetrahedra and corners with six equivalent As(1)Se4 tetrahedra. The Cu(1)-Se(2) bond length is 2.45 Å. All Cu(1)-Se(1) bond lengths are 2.58 Å. As(1) is bonded to one Se(1) and three equivalent Se(2) atoms to form AsSe4 tetrahedra that share corners with six equivalent Cu(1)Se4 tetrahedra and corners with six equivalent As(1)Se4 tetrahedra. The As(1)-Se(1) bond length is 2.40 Å. All As(1)-Se(2) bond lengths are 2.59 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Cu(1) and one As(1) atom to form SeCu3As tetrahedra that share corners with six equivalent Se(1)Cu3As tetrahedra and corners with six equivalent Se(2)CuAs3 tetrahedra. In the second Se site, Se(2) is bonded to one Cu(1) and three equivalent As(1) atoms to form SeCuAs3 tetrahedra that share corners with six equivalent Se(1)Cu3As tetrahedra and corners with six equivalent Se(2)CuAs3 tetrahedra.