Er2CoGa8 crystallizes in the tetragonal P4/mmm space group. Er(1) is bonded to four Ga(1,1); four equivalent Ga(2); and four equivalent Ga(3) atoms to form ErGa12 cuboctahedra that share corners with eight equivalent Er(1)Ga12 cuboctahedra, edges with four equivalent Ga(2)Er4Ga8 cuboctahedra, edges with eight equivalent Ga(3)Er4Ga8 cuboctahedra, faces with four equivalent Ga(2)Er4Ga8 cuboctahedra, faces with four equivalent Ga(3)Er4Ga8 cuboctahedra, and faces with five equivalent Er(1)Ga12 cuboctahedra. Co(1) is bonded in a body-centered cubic geometry to eight Ga(1,1) atoms. There are four inequivalent Ga sites. In the first Ga site, Ga(1) is bonded in a 11-coordinate geometry to two equivalent Er(1); two equivalent Co(1); two equivalent Ga(2); and five Ga(1,1) atoms. In the second Ga site, Ga(1) is bonded in a 11-coordinate geometry to two equivalent Er(1); two equivalent Co(1); two equivalent Ga(2); and five Ga(1,1) atoms. In the third Ga site, Ga(2) is bonded to four equivalent Er(1); four Ga(1,1); and four equivalent Ga(3) atoms to form distorted GaEr4Ga8 cuboctahedra that share corners with eight equivalent Ga(2)Er4Ga8 cuboctahedra, edges with four equivalent Er(1)Ga12 cuboctahedra, edges with eight equivalent Ga(3)Er4Ga8 cuboctahedra, faces with four equivalent Er(1)Ga12 cuboctahedra, faces with four equivalent Ga(3)Er4Ga8 cuboctahedra, and faces with five equivalent Ga(2)Er4Ga8 cuboctahedra. In the fourth Ga site, Ga(3) is bonded to four equivalent Er(1), four equivalent Ga(2), and four equivalent Ga(3) atoms to form distorted GaEr4Ga8 cuboctahedra that share corners with four equivalent Ga(3)Er4Ga8 cuboctahedra, edges with eight equivalent Er(1)Ga12 cuboctahedra, edges with eight equivalent Ga(2)Er4Ga8 cuboctahedra, faces with four equivalent Er(1)Ga12 cuboctahedra, faces with four equivalent Ga(2)Er4Ga8 cuboctahedra, and faces with eight equivalent Ga(3)Er4Ga8 cuboctahedra.
Er2CoGa8 crystallizes in the tetragonal P4/mmm space group. Er(1) is bonded to four Ga(1,1); four equivalent Ga(2); and four equivalent Ga(3) atoms to form ErGa12 cuboctahedra that share corners with eight equivalent Er(1)Ga12 cuboctahedra, edges with four equivalent Ga(2)Er4Ga8 cuboctahedra, edges with eight equivalent Ga(3)Er4Ga8 cuboctahedra, faces with four equivalent Ga(2)Er4Ga8 cuboctahedra, faces with four equivalent Ga(3)Er4Ga8 cuboctahedra, and faces with five equivalent Er(1)Ga12 cuboctahedra. All Er(1)-Ga(1,1) bond lengths are 2.95 Å. All Er(1)-Ga(2) bond lengths are 2.95 Å. All Er(1)-Ga(3) bond lengths are 2.98 Å. Co(1) is bonded in a body-centered cubic geometry to eight Ga(1,1) atoms. All Co(1)-Ga(1,1) bond lengths are 2.44 Å. There are four inequivalent Ga sites. In the first Ga site, Ga(1) is bonded in a 11-coordinate geometry to two equivalent Er(1); two equivalent Co(1); two equivalent Ga(2); and five Ga(1,1) atoms. Both Ga(1)-Ga(2) bond lengths are 2.94 Å. There is one shorter (2.54 Å) and four longer (2.95 Å) Ga(1)-Ga(1,1) bond lengths. In the second Ga site, Ga(1) is bonded in a 11-coordinate geometry to two equivalent Er(1); two equivalent Co(1); two equivalent Ga(2); and five Ga(1,1) atoms. Both Ga(1)-Ga(2) bond lengths are 2.94 Å. The Ga(1)-Ga(1) bond length is 2.54 Å. In the third Ga site, Ga(2) is bonded to four equivalent Er(1); four Ga(1,1); and four equivalent Ga(3) atoms to form distorted GaEr4Ga8 cuboctahedra that share corners with eight equivalent Ga(2)Er4Ga8 cuboctahedra, edges with four equivalent Er(1)Ga12 cuboctahedra, edges with eight equivalent Ga(3)Er4Ga8 cuboctahedra, faces with four equivalent Er(1)Ga12 cuboctahedra, faces with four equivalent Ga(3)Er4Ga8 cuboctahedra, and faces with five equivalent Ga(2)Er4Ga8 cuboctahedra. All Ga(2)-Ga(3) bond lengths are 2.99 Å. In the fourth Ga site, Ga(3) is bonded to four equivalent Er(1), four equivalent Ga(2), and four equivalent Ga(3) atoms to form distorted GaEr4Ga8 cuboctahedra that share corners with four equivalent Ga(3)Er4Ga8 cuboctahedra, edges with eight equivalent Er(1)Ga12 cuboctahedra, edges with eight equivalent Ga(2)Er4Ga8 cuboctahedra, faces with four equivalent Er(1)Ga12 cuboctahedra, faces with four equivalent Ga(2)Er4Ga8 cuboctahedra, and faces with eight equivalent Ga(3)Er4Ga8 cuboctahedra. All Ga(3)-Ga(3) bond lengths are 2.95 Å.