Er8CoGa3 crystallizes in the hexagonal P6_3mc space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to six equivalent Ga(1) atoms to form distorted ErGa6 octahedra that share corners with six equivalent Er(3)Ga3Co tetrahedra, corners with six equivalent Er(2)Ga4Co trigonal pyramids, faces with two equivalent Er(1)Ga6 octahedra, and faces with six equivalent Er(2)Ga4Co trigonal pyramids. In the second Er site, Er(2) is bonded to one Co(1) and four equivalent Ga(1) atoms to form distorted ErGa4Co trigonal pyramids that share corners with two equivalent Er(1)Ga6 octahedra, corners with three equivalent Er(3)Ga3Co tetrahedra, corners with six equivalent Er(2)Ga4Co trigonal pyramids,  an edgeedge with one Er(3)Ga3Co tetrahedra, edges with four equivalent Er(2)Ga4Co trigonal pyramids, and faces with two equivalent Er(1)Ga6 octahedra. The corner-sharing octahedral tilt angles range from 53-58°. In the third Er site, Er(3) is bonded to one Co(1) and three equivalent Ga(1) atoms to form distorted ErGa3Co tetrahedra that share corners with six equivalent Er(1)Ga6 octahedra, corners with nine equivalent Er(2)Ga4Co trigonal pyramids, and edges with three equivalent Er(2)Ga4Co trigonal pyramids. The corner-sharing octahedral tilt angles range from 16-46°. In the fourth Er site, Er(4) is bonded in a 3-coordinate geometry to one Co(1) and four equivalent Ga(1) atoms. Co(1) is bonded in a 7-coordinate geometry to one Er(3), three equivalent Er(2), and three equivalent Er(4) atoms. Ga(1) is bonded in a 11-coordinate geometry to one Er(3), two equivalent Er(1), four equivalent Er(2), and four equivalent Er(4) atoms.
Er8CoGa3 crystallizes in the hexagonal P6_3mc space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to six equivalent Ga(1) atoms to form distorted ErGa6 octahedra that share corners with six equivalent Er(3)Ga3Co tetrahedra, corners with six equivalent Er(2)Ga4Co trigonal pyramids, faces with two equivalent Er(1)Ga6 octahedra, and faces with six equivalent Er(2)Ga4Co trigonal pyramids. There are three shorter (3.20 Å) and three longer (3.35 Å) Er(1)-Ga(1) bond lengths. In the second Er site, Er(2) is bonded to one Co(1) and four equivalent Ga(1) atoms to form distorted ErGa4Co trigonal pyramids that share corners with two equivalent Er(1)Ga6 octahedra, corners with three equivalent Er(3)Ga3Co tetrahedra, corners with six equivalent Er(2)Ga4Co trigonal pyramids,  an edgeedge with one Er(3)Ga3Co tetrahedra, edges with four equivalent Er(2)Ga4Co trigonal pyramids, and faces with two equivalent Er(1)Ga6 octahedra. The corner-sharing octahedral tilt angles range from 53-58°. The Er(2)-Co(1) bond length is 2.77 Å. There are a spread of Er(2)-Ga(1) bond distances ranging from 2.90-3.77 Å. In the third Er site, Er(3) is bonded to one Co(1) and three equivalent Ga(1) atoms to form distorted ErGa3Co tetrahedra that share corners with six equivalent Er(1)Ga6 octahedra, corners with nine equivalent Er(2)Ga4Co trigonal pyramids, and edges with three equivalent Er(2)Ga4Co trigonal pyramids. The corner-sharing octahedral tilt angles range from 16-46°. The Er(3)-Co(1) bond length is 2.64 Å. All Er(3)-Ga(1) bond lengths are 3.03 Å. In the fourth Er site, Er(4) is bonded in a 3-coordinate geometry to one Co(1) and four equivalent Ga(1) atoms. The Er(4)-Co(1) bond length is 2.73 Å. There are two shorter (3.13 Å) and two longer (3.65 Å) Er(4)-Ga(1) bond lengths. Co(1) is bonded in a 7-coordinate geometry to one Er(3), three equivalent Er(2), and three equivalent Er(4) atoms. Ga(1) is bonded in a 11-coordinate geometry to one Er(3), two equivalent Er(1), four equivalent Er(2), and four equivalent Er(4) atoms.