Er2Co17 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 18-coordinate geometry to six equivalent Co(2) and twelve equivalent Co(3) atoms. In the second Er site, Er(2) is bonded in a 20-coordinate geometry to two equivalent Co(4), six equivalent Co(1), six equivalent Co(2), and six equivalent Co(3) atoms. There are four inequivalent Co sites. In the first Co site, Co(4) is bonded in a 14-coordinate geometry to one Er(2), one Co(4), three equivalent Co(1), three equivalent Co(3), and six equivalent Co(2) atoms. In the second Co site, Co(1) is bonded to two equivalent Er(2), two equivalent Co(4), four equivalent Co(2), and four equivalent Co(3) atoms to form CoEr2Co10 cuboctahedra that share corners with four equivalent Co(1)Er2Co10 cuboctahedra, corners with ten equivalent Co(3)Er3Co9 cuboctahedra, edges with six equivalent Co(3)Er3Co9 cuboctahedra, faces with four equivalent Co(3)Er3Co9 cuboctahedra, and faces with six equivalent Co(1)Er2Co10 cuboctahedra. In the third Co site, Co(2) is bonded in a 12-coordinate geometry to one Er(1), one Er(2), two equivalent Co(1), two equivalent Co(2), two equivalent Co(4), and four equivalent Co(3) atoms. In the fourth Co site, Co(3) is bonded to one Er(2), two equivalent Er(1), one Co(4), two equivalent Co(1), two equivalent Co(3), and four equivalent Co(2) atoms to form distorted CoEr3Co9 cuboctahedra that share corners with five equivalent Co(1)Er2Co10 cuboctahedra, corners with ten equivalent Co(3)Er3Co9 cuboctahedra, edges with three equivalent Co(1)Er2Co10 cuboctahedra, edges with five equivalent Co(3)Er3Co9 cuboctahedra, faces with two equivalent Co(1)Er2Co10 cuboctahedra, and faces with eight equivalent Co(3)Er3Co9 cuboctahedra.
Er2Co17 crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 18-coordinate geometry to six equivalent Co(2) and twelve equivalent Co(3) atoms. All Er(1)-Co(2) bond lengths are 2.91 Å. There are six shorter (3.01 Å) and six longer (3.19 Å) Er(1)-Co(3) bond lengths. In the second Er site, Er(2) is bonded in a 20-coordinate geometry to two equivalent Co(4), six equivalent Co(1), six equivalent Co(2), and six equivalent Co(3) atoms. Both Er(2)-Co(4) bond lengths are 2.88 Å. All Er(2)-Co(1) bond lengths are 3.11 Å. All Er(2)-Co(2) bond lengths are 2.94 Å. All Er(2)-Co(3) bond lengths are 3.03 Å. There are four inequivalent Co sites. In the first Co site, Co(4) is bonded in a 14-coordinate geometry to one Er(2), one Co(4), three equivalent Co(1), three equivalent Co(3), and six equivalent Co(2) atoms. The Co(4)-Co(4) bond length is 2.28 Å. All Co(4)-Co(1) bond lengths are 2.53 Å. All Co(4)-Co(3) bond lengths are 2.59 Å. All Co(4)-Co(2) bond lengths are 2.66 Å. In the second Co site, Co(1) is bonded to two equivalent Er(2), two equivalent Co(4), four equivalent Co(2), and four equivalent Co(3) atoms to form CoEr2Co10 cuboctahedra that share corners with four equivalent Co(1)Er2Co10 cuboctahedra, corners with ten equivalent Co(3)Er3Co9 cuboctahedra, edges with six equivalent Co(3)Er3Co9 cuboctahedra, faces with four equivalent Co(3)Er3Co9 cuboctahedra, and faces with six equivalent Co(1)Er2Co10 cuboctahedra. All Co(1)-Co(2) bond lengths are 2.37 Å. All Co(1)-Co(3) bond lengths are 2.39 Å. In the third Co site, Co(2) is bonded in a 12-coordinate geometry to one Er(1), one Er(2), two equivalent Co(1), two equivalent Co(2), two equivalent Co(4), and four equivalent Co(3) atoms. There is one shorter (2.36 Å) and one longer (2.44 Å) Co(2)-Co(2) bond length. There are two shorter (2.47 Å) and two longer (2.53 Å) Co(2)-Co(3) bond lengths. In the fourth Co site, Co(3) is bonded to one Er(2), two equivalent Er(1), one Co(4), two equivalent Co(1), two equivalent Co(3), and four equivalent Co(2) atoms to form distorted CoEr3Co9 cuboctahedra that share corners with five equivalent Co(1)Er2Co10 cuboctahedra, corners with ten equivalent Co(3)Er3Co9 cuboctahedra, edges with three equivalent Co(1)Er2Co10 cuboctahedra, edges with five equivalent Co(3)Er3Co9 cuboctahedra, faces with two equivalent Co(1)Er2Co10 cuboctahedra, and faces with eight equivalent Co(3)Er3Co9 cuboctahedra. Both Co(3)-Co(3) bond lengths are 2.37 Å.