ErCuS2 crystallizes in the orthorhombic P2_12_12_1 space group. Er(1) is bonded to three equivalent S(1) and three equivalent S(2) atoms to form ErS6 octahedra that share corners with four equivalent Er(1)S6 octahedra, corners with six equivalent Cu(1)S4 tetrahedra, edges with four equivalent Er(1)S6 octahedra, and edges with three equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-52°. Cu(1) is bonded to one S(1) and three equivalent S(2) atoms to form CuS4 tetrahedra that share corners with six equivalent Er(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, edges with three equivalent Er(1)S6 octahedra, and edges with two equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 13-58°. There are two inequivalent S sites. In the first S site, S(1) is bonded in a rectangular see-saw-like geometry to three equivalent Er(1) and one Cu(1) atom. In the second S site, S(2) is bonded to three equivalent Er(1) and three equivalent Cu(1) atoms to form distorted edge-sharing SEr3Cu3 octahedra.
ErCuS2 crystallizes in the orthorhombic P2_12_12_1 space group. Er(1) is bonded to three equivalent S(1) and three equivalent S(2) atoms to form ErS6 octahedra that share corners with four equivalent Er(1)S6 octahedra, corners with six equivalent Cu(1)S4 tetrahedra, edges with four equivalent Er(1)S6 octahedra, and edges with three equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-52°. There are two shorter (2.63 Å) and one longer (2.68 Å) Er(1)-S(1) bond length. There are a spread of Er(1)-S(2) bond distances ranging from 2.76-2.78 Å. Cu(1) is bonded to one S(1) and three equivalent S(2) atoms to form CuS4 tetrahedra that share corners with six equivalent Er(1)S6 octahedra, corners with two equivalent Cu(1)S4 tetrahedra, edges with three equivalent Er(1)S6 octahedra, and edges with two equivalent Cu(1)S4 tetrahedra. The corner-sharing octahedral tilt angles range from 13-58°. The Cu(1)-S(1) bond length is 2.29 Å. There are a spread of Cu(1)-S(2) bond distances ranging from 2.36-2.40 Å. There are two inequivalent S sites. In the first S site, S(1) is bonded in a rectangular see-saw-like geometry to three equivalent Er(1) and one Cu(1) atom. In the second S site, S(2) is bonded to three equivalent Er(1) and three equivalent Cu(1) atoms to form distorted edge-sharing SEr3Cu3 octahedra.