Cu5Er crystallizes in the cubic F-43m space group. Er(1) is bonded in a 16-coordinate geometry to four equivalent Cu(1) and twelve equivalent Cu(2) atoms. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to four equivalent Er(1) and twelve equivalent Cu(2) atoms to form distorted CuEr4Cu12 tetrahedra that share corners with twelve equivalent Cu(2)Er3Cu9 cuboctahedra, faces with twenty-eight equivalent Cu(2)Er3Cu9 cuboctahedra, and faces with twelve equivalent Cu(1)Er4Cu12 tetrahedra. In the second Cu site, Cu(2) is bonded to three equivalent Er(1), three equivalent Cu(1), and six equivalent Cu(2) atoms to form distorted CuEr3Cu9 cuboctahedra that share corners with eighteen equivalent Cu(2)Er3Cu9 cuboctahedra, corners with three equivalent Cu(1)Er4Cu12 tetrahedra, edges with six equivalent Cu(2)Er3Cu9 cuboctahedra, faces with eighteen equivalent Cu(2)Er3Cu9 cuboctahedra, and faces with seven equivalent Cu(1)Er4Cu12 tetrahedra.
Cu5Er crystallizes in the cubic F-43m space group. Er(1) is bonded in a 16-coordinate geometry to four equivalent Cu(1) and twelve equivalent Cu(2) atoms. All Er(1)-Cu(1) bond lengths are 3.00 Å. All Er(1)-Cu(2) bond lengths are 2.87 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to four equivalent Er(1) and twelve equivalent Cu(2) atoms to form distorted CuEr4Cu12 tetrahedra that share corners with twelve equivalent Cu(2)Er3Cu9 cuboctahedra, faces with twenty-eight equivalent Cu(2)Er3Cu9 cuboctahedra, and faces with twelve equivalent Cu(1)Er4Cu12 tetrahedra. All Cu(1)-Cu(2) bond lengths are 2.87 Å. In the second Cu site, Cu(2) is bonded to three equivalent Er(1), three equivalent Cu(1), and six equivalent Cu(2) atoms to form distorted CuEr3Cu9 cuboctahedra that share corners with eighteen equivalent Cu(2)Er3Cu9 cuboctahedra, corners with three equivalent Cu(1)Er4Cu12 tetrahedra, edges with six equivalent Cu(2)Er3Cu9 cuboctahedra, faces with eighteen equivalent Cu(2)Er3Cu9 cuboctahedra, and faces with seven equivalent Cu(1)Er4Cu12 tetrahedra. There are three shorter (2.43 Å) and three longer (2.47 Å) Cu(2)-Cu(2) bond lengths.