TaNb(Cu3Se4)2 is Sulvanite-derived structured and crystallizes in the cubic F-43m space group. Ta(1) is bonded to four equivalent Se(2) atoms to form distorted TaSe4 tetrahedra that share edges with six equivalent Cu(1)Se4 tetrahedra. Nb(1) is bonded to four equivalent Se(1) atoms to form distorted NbSe4 tetrahedra that share edges with six equivalent Cu(1)Se4 tetrahedra. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form distorted CuSe4 tetrahedra that share corners with eight equivalent Cu(1)Se4 tetrahedra,  an edgeedge with one Ta(1)Se4 tetrahedra, and  an edgeedge with one Nb(1)Se4 tetrahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Nb(1) and three equivalent Cu(1) atoms to form SeNbCu3 tetrahedra that share corners with six equivalent Se(2)TaCu3 tetrahedra and edges with three equivalent Se(1)NbCu3 tetrahedra. In the second Se site, Se(2) is bonded to one Ta(1) and three equivalent Cu(1) atoms to form SeTaCu3 tetrahedra that share corners with six equivalent Se(1)NbCu3 tetrahedra and edges with three equivalent Se(2)TaCu3 tetrahedra.
TaNb(Cu3Se4)2 is Sulvanite-derived structured and crystallizes in the cubic F-43m space group. Ta(1) is bonded to four equivalent Se(2) atoms to form distorted TaSe4 tetrahedra that share edges with six equivalent Cu(1)Se4 tetrahedra. All Ta(1)-Se(2) bond lengths are 2.47 Å. Nb(1) is bonded to four equivalent Se(1) atoms to form distorted NbSe4 tetrahedra that share edges with six equivalent Cu(1)Se4 tetrahedra. All Nb(1)-Se(1) bond lengths are 2.47 Å. Cu(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form distorted CuSe4 tetrahedra that share corners with eight equivalent Cu(1)Se4 tetrahedra,  an edgeedge with one Ta(1)Se4 tetrahedra, and  an edgeedge with one Nb(1)Se4 tetrahedra. Both Cu(1)-Se(1) bond lengths are 2.47 Å. Both Cu(1)-Se(2) bond lengths are 2.47 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one Nb(1) and three equivalent Cu(1) atoms to form SeNbCu3 tetrahedra that share corners with six equivalent Se(2)TaCu3 tetrahedra and edges with three equivalent Se(1)NbCu3 tetrahedra. In the second Se site, Se(2) is bonded to one Ta(1) and three equivalent Cu(1) atoms to form SeTaCu3 tetrahedra that share corners with six equivalent Se(1)NbCu3 tetrahedra and edges with three equivalent Se(2)TaCu3 tetrahedra.