Er2Al17 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 Al(3) and twelve equivalent Al(4) atoms. In the second Er site, Er(2) is bonded in a 20-coordinate geometry to two equivalent Al(1), six equivalent Al(2), six equivalent Al(3), and six equivalent Al(4) atoms. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded in a 8-coordinate geometry to one Er(2), one Al(1), three equivalent Al(2), and three equivalent Al(4) atoms. In the second Al site, Al(2) is bonded to two equivalent Er(2), two equivalent Al(1), four equivalent Al(3), and four equivalent Al(4) atoms to form AlEr2Al10 cuboctahedra that share corners with four equivalent Al(2)Er2Al10 cuboctahedra, corners with ten equivalent Al(4)Er3Al9 cuboctahedra, edges with six equivalent Al(4)Er3Al9 cuboctahedra, faces with four equivalent Al(4)Er3Al9 cuboctahedra, and faces with six equivalent Al(2)Er2Al10 cuboctahedra. In the third Al site, Al(3) is bonded in a 10-coordinate geometry to one Er(1), one Er(2), two equivalent Al(2), two equivalent Al(3), and four equivalent Al(4) atoms. In the fourth Al site, Al(4) is bonded to one Er(2), two equivalent Er(1), one Al(1), two equivalent Al(2), two equivalent Al(4), and four equivalent Al(3) atoms to form distorted AlEr3Al9 cuboctahedra that share corners with five equivalent Al(2)Er2Al10 cuboctahedra, corners with ten equivalent Al(4)Er3Al9 cuboctahedra, edges with three equivalent Al(2)Er2Al10 cuboctahedra, edges with five equivalent Al(4)Er3Al9 cuboctahedra, faces with two equivalent Al(2)Er2Al10 cuboctahedra, and faces with eight equivalent Al(4)Er3Al9 cuboctahedra.
Er2Al17 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 Al(3) and twelve equivalent Al(4) atoms. All Er(1)-Al(3) bond lengths are 3.26 Å. There are six shorter (3.34 Å) and six longer (3.59 Å) Er(1)-Al(4) bond lengths. In the second Er site, Er(2) is bonded in a 20-coordinate geometry to two equivalent Al(1), six equivalent Al(2), six equivalent Al(3), and six equivalent Al(4) atoms. Both Er(2)-Al(1) bond lengths are 3.11 Å. All Er(2)-Al(2) bond lengths are 3.51 Å. All Er(2)-Al(3) bond lengths are 3.33 Å. All Er(2)-Al(4) bond lengths are 3.43 Å. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded in a 8-coordinate geometry to one Er(2), one Al(1), three equivalent Al(2), and three equivalent Al(4) atoms. The Al(1)-Al(1) bond length is 2.63 Å. All Al(1)-Al(2) bond lengths are 2.87 Å. All Al(1)-Al(4) bond lengths are 2.99 Å. In the second Al site, Al(2) is bonded to two equivalent Er(2), two equivalent Al(1), four equivalent Al(3), and four equivalent Al(4) atoms to form AlEr2Al10 cuboctahedra that share corners with four equivalent Al(2)Er2Al10 cuboctahedra, corners with ten equivalent Al(4)Er3Al9 cuboctahedra, edges with six equivalent Al(4)Er3Al9 cuboctahedra, faces with four equivalent Al(4)Er3Al9 cuboctahedra, and faces with six equivalent Al(2)Er2Al10 cuboctahedra. All Al(2)-Al(3) bond lengths are 2.66 Å. All Al(2)-Al(4) bond lengths are 2.76 Å. In the third Al site, Al(3) is bonded in a 10-coordinate geometry to one Er(1), one Er(2), two equivalent Al(2), two equivalent Al(3), and four equivalent Al(4) atoms. There is one shorter (2.79 Å) and one longer (2.93 Å) Al(3)-Al(3) bond length. There are two shorter (2.74 Å) and two longer (2.80 Å) Al(3)-Al(4) bond lengths. In the fourth Al site, Al(4) is bonded to one Er(2), two equivalent Er(1), one Al(1), two equivalent Al(2), two equivalent Al(4), and four equivalent Al(3) atoms to form distorted AlEr3Al9 cuboctahedra that share corners with five equivalent Al(2)Er2Al10 cuboctahedra, corners with ten equivalent Al(4)Er3Al9 cuboctahedra, edges with three equivalent Al(2)Er2Al10 cuboctahedra, edges with five equivalent Al(4)Er3Al9 cuboctahedra, faces with two equivalent Al(2)Er2Al10 cuboctahedra, and faces with eight equivalent Al(4)Er3Al9 cuboctahedra. Both Al(4)-Al(4) bond lengths are 2.70 Å.