ErCuSe2 crystallizes in the trigonal P3m1 space group. Er(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with six equivalent Cu(1)Se4 tetrahedra, edges with six equivalent Er(1)Se6 octahedra, and edges with three equivalent Cu(1)Se4 tetrahedra. Cu(1) is bonded to one Se(2) and three equivalent Se(1) atoms to form CuSe4 tetrahedra that share corners with six equivalent Er(1)Se6 octahedra, corners with six equivalent Cu(1)Se4 tetrahedra, and edges with three equivalent Er(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 19-57°. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Er(1) and three equivalent Cu(1) atoms to form distorted SeEr3Cu3 octahedra that share corners with six equivalent Se(2)Er3Cu trigonal pyramids, edges with six equivalent Se(1)Er3Cu3 octahedra, and edges with three equivalent Se(2)Er3Cu trigonal pyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and one Cu(1) atom to form distorted SeEr3Cu trigonal pyramids that share corners with six equivalent Se(1)Er3Cu3 octahedra, corners with six equivalent Se(2)Er3Cu trigonal pyramids, and edges with three equivalent Se(1)Er3Cu3 octahedra. The corner-sharing octahedral tilt angles range from 4-73°.
ErCuSe2 crystallizes in the trigonal P3m1 space group. Er(1) is bonded to three equivalent Se(1) and three equivalent Se(2) atoms to form ErSe6 octahedra that share corners with six equivalent Cu(1)Se4 tetrahedra, edges with six equivalent Er(1)Se6 octahedra, and edges with three equivalent Cu(1)Se4 tetrahedra. All Er(1)-Se(1) bond lengths are 2.91 Å. All Er(1)-Se(2) bond lengths are 2.77 Å. Cu(1) is bonded to one Se(2) and three equivalent Se(1) atoms to form CuSe4 tetrahedra that share corners with six equivalent Er(1)Se6 octahedra, corners with six equivalent Cu(1)Se4 tetrahedra, and edges with three equivalent Er(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 19-57°. The Cu(1)-Se(2) bond length is 2.43 Å. All Cu(1)-Se(1) bond lengths are 2.45 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Er(1) and three equivalent Cu(1) atoms to form distorted SeEr3Cu3 octahedra that share corners with six equivalent Se(2)Er3Cu trigonal pyramids, edges with six equivalent Se(1)Er3Cu3 octahedra, and edges with three equivalent Se(2)Er3Cu trigonal pyramids. In the second Se site, Se(2) is bonded to three equivalent Er(1) and one Cu(1) atom to form distorted SeEr3Cu trigonal pyramids that share corners with six equivalent Se(1)Er3Cu3 octahedra, corners with six equivalent Se(2)Er3Cu trigonal pyramids, and edges with three equivalent Se(1)Er3Cu3 octahedra. The corner-sharing octahedral tilt angles range from 4-73°.