ErCu3S3 crystallizes in the trigonal P-31m space group. Er(1) is bonded to six equivalent S(1) atoms to form ErS6 octahedra that share corners with twelve equivalent Cu(1)S4 tetrahedra, edges with three equivalent Er(1)S6 octahedra, and edges with six equivalent Cu(1)S4 tetrahedra. Cu(1) is bonded to four equivalent S(1) atoms to form CuS4 tetrahedra that share corners with four equivalent Er(1)S6 octahedra, corners with six equivalent Cu(1)S4 tetrahedra, edges with two equivalent Er(1)S6 octahedra, and edges with three equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 17-56°. S(1) is bonded in a 6-coordinate geometry to two equivalent Er(1) and four equivalent Cu(1) atoms.
ErCu3S3 crystallizes in the trigonal P-31m space group. Er(1) is bonded to six equivalent S(1) atoms to form ErS6 octahedra that share corners with twelve equivalent Cu(1)S4 tetrahedra, edges with three equivalent Er(1)S6 octahedra, and edges with six equivalent Cu(1)S4 tetrahedra. All Er(1)-S(1) bond lengths are 2.72 Å. Cu(1) is bonded to four equivalent S(1) atoms to form CuS4 tetrahedra that share corners with four equivalent Er(1)S6 octahedra, corners with six equivalent Cu(1)S4 tetrahedra, edges with two equivalent Er(1)S6 octahedra, and edges with three equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 17-56°. There are a spread of Cu(1)-S(1) bond distances ranging from 2.30-2.42 Å. S(1) is bonded in a 6-coordinate geometry to two equivalent Er(1) and four equivalent Cu(1) atoms.