Er7CoI12 is trigonal omega-derived structured and crystallizes in the trigonal R-3 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to six equivalent I(1) atoms to form edge-sharing ErI6 octahedra. In the second Er site, Er(2) is bonded to one Co(1), two equivalent I(1), and three equivalent I(2) atoms to form distorted ErCoI5 octahedra that share corners with three equivalent Er(2)CoI5 octahedra,  an edgeedge with one Er(1)I6 octahedra, and edges with five equivalent Er(2)CoI5 octahedra. The corner-sharing octahedral tilt angles range from 0-10°. Co(1) is bonded in an octahedral geometry to six equivalent Er(2) atoms. There are two inequivalent I sites. In the first I site, I(1) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the second I site, I(2) is bonded in a 3-coordinate geometry to three equivalent Er(2) atoms.
Er7CoI12 is trigonal omega-derived structured and crystallizes in the trigonal R-3 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to six equivalent I(1) atoms to form edge-sharing ErI6 octahedra. All Er(1)-I(1) bond lengths are 3.00 Å. In the second Er site, Er(2) is bonded to one Co(1), two equivalent I(1), and three equivalent I(2) atoms to form distorted ErCoI5 octahedra that share corners with three equivalent Er(2)CoI5 octahedra,  an edgeedge with one Er(1)I6 octahedra, and edges with five equivalent Er(2)CoI5 octahedra. The corner-sharing octahedral tilt angles range from 0-10°. The Er(2)-Co(1) bond length is 2.55 Å. There is one shorter (3.17 Å) and one longer (3.19 Å) Er(2)-I(1) bond length. There are a spread of Er(2)-I(2) bond distances ranging from 3.08-3.42 Å. Co(1) is bonded in an octahedral geometry to six equivalent Er(2) atoms. There are two inequivalent I sites. In the first I site, I(1) is bonded in a distorted T-shaped geometry to one Er(1) and two equivalent Er(2) atoms. In the second I site, I(2) is bonded in a 3-coordinate geometry to three equivalent Er(2) atoms.