Er2YAl9 crystallizes in the hexagonal P-6m2 space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to three equivalent Al(6), three equivalent Al(7), and six equivalent Al(4) atoms to form ErAl12 cuboctahedra that share corners with three equivalent Y(1)Al12 cuboctahedra, corners with three equivalent Y(2)Al12 cuboctahedra, corners with six equivalent Er(1)Al12 cuboctahedra, edges with six equivalent Al(6)YEr3Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra, faces with three equivalent Er(3)Al12 cuboctahedra, faces with three equivalent Er(4)Al12 cuboctahedra, faces with three equivalent Al(6)YEr3Al8 cuboctahedra, and faces with three equivalent Al(7)YEr3Al8 cuboctahedra. In the second Er site, Er(2) is bonded to three equivalent Al(1), three equivalent Al(3), and six equivalent Al(5) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(2)Al12 cuboctahedra, corners with six equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with six equivalent Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Er(2)Al12 cuboctahedra,  a faceface with one Er(4)Al12 cuboctahedra, faces with three equivalent Y(1)Al12 cuboctahedra, faces with three equivalent Y(3)Al12 cuboctahedra, and faces with six equivalent Al(5)Y2Er2Al8 cuboctahedra. In the third Er site, Er(3) is bonded to three equivalent Al(2), three equivalent Al(4), and six equivalent Al(6) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(3)Al12 cuboctahedra, corners with six equivalent Al(6)YEr3Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with six equivalent Al(6)YEr3Al8 cuboctahedra,  a faceface with one Er(3)Al12 cuboctahedra,  a faceface with one Er(4)Al12 cuboctahedra, faces with three equivalent Er(1)Al12 cuboctahedra, faces with three equivalent Y(2)Al12 cuboctahedra, and faces with six equivalent Al(6)YEr3Al8 cuboctahedra. In the fourth Er site, Er(4) is bonded to three equivalent Al(1), three equivalent Al(4), and six equivalent Al(7) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(4)Al12 cuboctahedra, corners with six equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with six equivalent Al(6)YEr3Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra,  a faceface with one Er(2)Al12 cuboctahedra,  a faceface with one Er(3)Al12 cuboctahedra, faces with three equivalent Er(1)Al12 cuboctahedra, faces with three equivalent Y(1)Al12 cuboctahedra, and faces with six equivalent Al(7)YEr3Al8 cuboctahedra. There are three inequivalent Y sites. In the first Y site, Y(1) is bonded to three equivalent Al(5), three equivalent Al(7), and six equivalent Al(1) atoms to form YAl12 cuboctahedra that share corners with three equivalent Er(1)Al12 cuboctahedra, corners with three equivalent Y(3)Al12 cuboctahedra, corners with six equivalent Y(1)Al12 cuboctahedra, edges with six equivalent Al(5)Y2Er2Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra, faces with three equivalent Er(2)Al12 cuboctahedra, faces with three equivalent Er(4)Al12 cuboctahedra, faces with three equivalent Al(5)Y2Er2Al8 cuboctahedra, and faces with three equivalent Al(7)YEr3Al8 cuboctahedra. In the second Y site, Y(2) is bonded to six equivalent Al(2) and six equivalent Al(6) atoms to form YAl12 cuboctahedra that share corners with six equivalent Er(1)Al12 cuboctahedra, corners with six equivalent Y(2)Al12 cuboctahedra, edges with twelve equivalent Al(6)YEr3Al8 cuboctahedra, faces with six equivalent Er(3)Al12 cuboctahedra, and faces with six equivalent Al(6)YEr3Al8 cuboctahedra. In the third Y site, Y(3) is bonded to six equivalent Al(3) and six equivalent Al(5) atoms to form YAl12 cuboctahedra that share corners with six equivalent Y(1)Al12 cuboctahedra, corners with six equivalent Y(3)Al12 cuboctahedra, edges with twelve equivalent Al(5)Y2Er2Al8 cuboctahedra, faces with six equivalent Er(2)Al12 cuboctahedra, and faces with six equivalent Al(5)Y2Er2Al8 cuboctahedra. There are seven inequivalent Al sites. In the first Al site, Al(7) is bonded to one Er(1), two equivalent Er(4), one Y(1), two equivalent Al(1), two equivalent Al(4), and four equivalent Al(7) atoms to form distorted AlYEr3Al8 cuboctahedra that share corners with two equivalent Er(2)Al12 cuboctahedra, corners with two equivalent Er(3)Al12 cuboctahedra, corners with four equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with four equivalent Al(6)YEr3Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with two equivalent Er(1)Al12 cuboctahedra, edges with two equivalent Er(4)Al12 cuboctahedra, edges with two equivalent Y(1)Al12 cuboctahedra, edges with four equivalent Al(7)YEr3Al8 cuboctahedra,  a faceface with one Er(1)Al12 cuboctahedra,  a faceface with one Y(1)Al12 cuboctahedra,  a faceface with one Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Al(6)YEr3Al8 cuboctahedra, faces with two equivalent Er(4)Al12 cuboctahedra, and faces with four equivalent Al(7)YEr3Al8 cuboctahedra. In the second Al site, Al(1) is bonded in a 10-coordinate geometry to one Er(2), one Er(4), two equivalent Y(1), two equivalent Al(1), two equivalent Al(5), and two equivalent Al(7) atoms. In the third Al site, Al(2) is bonded in a 10-coordinate geometry to two equivalent Er(3), two equivalent Y(2), two equivalent Al(2), and four equivalent Al(6) atoms. In the fourth Al site, Al(3) is bonded in a 10-coordinate geometry to two equivalent Er(2), two equivalent Y(3), two equivalent Al(3), and four equivalent Al(5) atoms. In the fifth Al site, Al(4) is bonded in a 10-coordinate geometry to one Er(3), one Er(4), two equivalent Er(1), two equivalent Al(4), two equivalent Al(6), and two equivalent Al(7) atoms. In the sixth Al site, Al(5) is bonded to two equivalent Er(2), one Y(1), one Y(3), two equivalent Al(1), two equivalent Al(3), and four equivalent Al(5) atoms to form distorted AlY2Er2Al8 cuboctahedra that share corners with two equivalent Er(2)Al12 cuboctahedra, corners with two equivalent Er(4)Al12 cuboctahedra, corners with four equivalent Al(7)YEr3Al8 cuboctahedra, corners with ten equivalent Al(5)Y2Er2Al8 cuboctahedra, edges with two equivalent Er(2)Al12 cuboctahedra, edges with two equivalent Y(1)Al12 cuboctahedra, edges with two equivalent Y(3)Al12 cuboctahedra, edges with four equivalent Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Y(1)Al12 cuboctahedra,  a faceface with one Y(3)Al12 cuboctahedra,  a faceface with one Al(7)YEr3Al8 cuboctahedra, faces with two equivalent Er(2)Al12 cuboctahedra, and faces with five equivalent Al(5)Y2Er2Al8 cuboctahedra. In the seventh Al site, Al(6) is bonded to one Er(1), two equivalent Er(3), one Y(2), two equivalent Al(2), two equivalent Al(4), and four equivalent Al(6) atoms to form distorted AlYEr3Al8 cuboctahedra that share corners with two equivalent Er(3)Al12 cuboctahedra, corners with two equivalent Er(4)Al12 cuboctahedra, corners with four equivalent Al(7)YEr3Al8 cuboctahedra, corners with ten equivalent Al(6)YEr3Al8 cuboctahedra, edges with two equivalent Er(1)Al12 cuboctahedra, edges with two equivalent Er(3)Al12 cuboctahedra, edges with two equivalent Y(2)Al12 cuboctahedra, edges with four equivalent Al(6)YEr3Al8 cuboctahedra,  a faceface with one Er(1)Al12 cuboctahedra,  a faceface with one Y(2)Al12 cuboctahedra,  a faceface with one Al(7)YEr3Al8 cuboctahedra, faces with two equivalent Er(3)Al12 cuboctahedra, and faces with five equivalent Al(6)YEr3Al8 cuboctahedra.
Er2YAl9 crystallizes in the hexagonal P-6m2 space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to three equivalent Al(6), three equivalent Al(7), and six equivalent Al(4) atoms to form ErAl12 cuboctahedra that share corners with three equivalent Y(1)Al12 cuboctahedra, corners with three equivalent Y(2)Al12 cuboctahedra, corners with six equivalent Er(1)Al12 cuboctahedra, edges with six equivalent Al(6)YEr3Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra, faces with three equivalent Er(3)Al12 cuboctahedra, faces with three equivalent Er(4)Al12 cuboctahedra, faces with three equivalent Al(6)YEr3Al8 cuboctahedra, and faces with three equivalent Al(7)YEr3Al8 cuboctahedra. All Er(1)-Al(6) bond lengths are 2.96 Å. All Er(1)-Al(7) bond lengths are 2.96 Å. All Er(1)-Al(4) bond lengths are 3.06 Å. In the second Er site, Er(2) is bonded to three equivalent Al(1), three equivalent Al(3), and six equivalent Al(5) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(2)Al12 cuboctahedra, corners with six equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with six equivalent Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Er(2)Al12 cuboctahedra,  a faceface with one Er(4)Al12 cuboctahedra, faces with three equivalent Y(1)Al12 cuboctahedra, faces with three equivalent Y(3)Al12 cuboctahedra, and faces with six equivalent Al(5)Y2Er2Al8 cuboctahedra. All Er(2)-Al(1) bond lengths are 3.08 Å. All Er(2)-Al(3) bond lengths are 3.07 Å. All Er(2)-Al(5) bond lengths are 3.05 Å. In the third Er site, Er(3) is bonded to three equivalent Al(2), three equivalent Al(4), and six equivalent Al(6) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(3)Al12 cuboctahedra, corners with six equivalent Al(6)YEr3Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with six equivalent Al(6)YEr3Al8 cuboctahedra,  a faceface with one Er(3)Al12 cuboctahedra,  a faceface with one Er(4)Al12 cuboctahedra, faces with three equivalent Er(1)Al12 cuboctahedra, faces with three equivalent Y(2)Al12 cuboctahedra, and faces with six equivalent Al(6)YEr3Al8 cuboctahedra. All Er(3)-Al(2) bond lengths are 3.07 Å. All Er(3)-Al(4) bond lengths are 3.08 Å. All Er(3)-Al(6) bond lengths are 3.05 Å. In the fourth Er site, Er(4) is bonded to three equivalent Al(1), three equivalent Al(4), and six equivalent Al(7) atoms to form ErAl12 cuboctahedra that share corners with six equivalent Er(4)Al12 cuboctahedra, corners with six equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with six equivalent Al(6)YEr3Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra,  a faceface with one Er(2)Al12 cuboctahedra,  a faceface with one Er(3)Al12 cuboctahedra, faces with three equivalent Er(1)Al12 cuboctahedra, faces with three equivalent Y(1)Al12 cuboctahedra, and faces with six equivalent Al(7)YEr3Al8 cuboctahedra. All Er(4)-Al(1) bond lengths are 3.07 Å. All Er(4)-Al(4) bond lengths are 3.08 Å. All Er(4)-Al(7) bond lengths are 3.05 Å. There are three inequivalent Y sites. In the first Y site, Y(1) is bonded to three equivalent Al(5), three equivalent Al(7), and six equivalent Al(1) atoms to form YAl12 cuboctahedra that share corners with three equivalent Er(1)Al12 cuboctahedra, corners with three equivalent Y(3)Al12 cuboctahedra, corners with six equivalent Y(1)Al12 cuboctahedra, edges with six equivalent Al(5)Y2Er2Al8 cuboctahedra, edges with six equivalent Al(7)YEr3Al8 cuboctahedra, faces with three equivalent Er(2)Al12 cuboctahedra, faces with three equivalent Er(4)Al12 cuboctahedra, faces with three equivalent Al(5)Y2Er2Al8 cuboctahedra, and faces with three equivalent Al(7)YEr3Al8 cuboctahedra. All Y(1)-Al(5) bond lengths are 2.97 Å. All Y(1)-Al(7) bond lengths are 2.98 Å. All Y(1)-Al(1) bond lengths are 3.06 Å. In the second Y site, Y(2) is bonded to six equivalent Al(2) and six equivalent Al(6) atoms to form YAl12 cuboctahedra that share corners with six equivalent Er(1)Al12 cuboctahedra, corners with six equivalent Y(2)Al12 cuboctahedra, edges with twelve equivalent Al(6)YEr3Al8 cuboctahedra, faces with six equivalent Er(3)Al12 cuboctahedra, and faces with six equivalent Al(6)YEr3Al8 cuboctahedra. All Y(2)-Al(2) bond lengths are 3.06 Å. All Y(2)-Al(6) bond lengths are 2.98 Å. In the third Y site, Y(3) is bonded to six equivalent Al(3) and six equivalent Al(5) atoms to form YAl12 cuboctahedra that share corners with six equivalent Y(1)Al12 cuboctahedra, corners with six equivalent Y(3)Al12 cuboctahedra, edges with twelve equivalent Al(5)Y2Er2Al8 cuboctahedra, faces with six equivalent Er(2)Al12 cuboctahedra, and faces with six equivalent Al(5)Y2Er2Al8 cuboctahedra. All Y(3)-Al(3) bond lengths are 3.06 Å. All Y(3)-Al(5) bond lengths are 2.97 Å. There are seven inequivalent Al sites. In the first Al site, Al(7) is bonded to one Er(1), two equivalent Er(4), one Y(1), two equivalent Al(1), two equivalent Al(4), and four equivalent Al(7) atoms to form distorted AlYEr3Al8 cuboctahedra that share corners with two equivalent Er(2)Al12 cuboctahedra, corners with two equivalent Er(3)Al12 cuboctahedra, corners with four equivalent Al(5)Y2Er2Al8 cuboctahedra, corners with four equivalent Al(6)YEr3Al8 cuboctahedra, corners with six equivalent Al(7)YEr3Al8 cuboctahedra, edges with two equivalent Er(1)Al12 cuboctahedra, edges with two equivalent Er(4)Al12 cuboctahedra, edges with two equivalent Y(1)Al12 cuboctahedra, edges with four equivalent Al(7)YEr3Al8 cuboctahedra,  a faceface with one Er(1)Al12 cuboctahedra,  a faceface with one Y(1)Al12 cuboctahedra,  a faceface with one Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Al(6)YEr3Al8 cuboctahedra, faces with two equivalent Er(4)Al12 cuboctahedra, and faces with four equivalent Al(7)YEr3Al8 cuboctahedra. Both Al(7)-Al(1) bond lengths are 2.93 Å. Both Al(7)-Al(4) bond lengths are 2.92 Å. There are two shorter (3.04 Å) and two longer (3.06 Å) Al(7)-Al(7) bond lengths. In the second Al site, Al(1) is bonded in a 10-coordinate geometry to one Er(2), one Er(4), two equivalent Y(1), two equivalent Al(1), two equivalent Al(5), and two equivalent Al(7) atoms. Both Al(1)-Al(1) bond lengths are 2.75 Å. Both Al(1)-Al(5) bond lengths are 2.92 Å. In the third Al site, Al(2) is bonded in a 10-coordinate geometry to two equivalent Er(3), two equivalent Y(2), two equivalent Al(2), and four equivalent Al(6) atoms. Both Al(2)-Al(2) bond lengths are 2.75 Å. All Al(2)-Al(6) bond lengths are 2.93 Å. In the fourth Al site, Al(3) is bonded in a 10-coordinate geometry to two equivalent Er(2), two equivalent Y(3), two equivalent Al(3), and four equivalent Al(5) atoms. Both Al(3)-Al(3) bond lengths are 2.75 Å. All Al(3)-Al(5) bond lengths are 2.92 Å. In the fifth Al site, Al(4) is bonded in a 10-coordinate geometry to one Er(3), one Er(4), two equivalent Er(1), two equivalent Al(4), two equivalent Al(6), and two equivalent Al(7) atoms. Both Al(4)-Al(4) bond lengths are 2.75 Å. Both Al(4)-Al(6) bond lengths are 2.92 Å. In the sixth Al site, Al(5) is bonded to two equivalent Er(2), one Y(1), one Y(3), two equivalent Al(1), two equivalent Al(3), and four equivalent Al(5) atoms to form distorted AlY2Er2Al8 cuboctahedra that share corners with two equivalent Er(2)Al12 cuboctahedra, corners with two equivalent Er(4)Al12 cuboctahedra, corners with four equivalent Al(7)YEr3Al8 cuboctahedra, corners with ten equivalent Al(5)Y2Er2Al8 cuboctahedra, edges with two equivalent Er(2)Al12 cuboctahedra, edges with two equivalent Y(1)Al12 cuboctahedra, edges with two equivalent Y(3)Al12 cuboctahedra, edges with four equivalent Al(5)Y2Er2Al8 cuboctahedra,  a faceface with one Y(1)Al12 cuboctahedra,  a faceface with one Y(3)Al12 cuboctahedra,  a faceface with one Al(7)YEr3Al8 cuboctahedra, faces with two equivalent Er(2)Al12 cuboctahedra, and faces with five equivalent Al(5)Y2Er2Al8 cuboctahedra. All Al(5)-Al(5) bond lengths are 3.05 Å. In the seventh Al site, Al(6) is bonded to one Er(1), two equivalent Er(3), one Y(2), two equivalent Al(2), two equivalent Al(4), and four equivalent Al(6) atoms to form distorted AlYEr3Al8 cuboctahedra that share corners with two equivalent Er(3)Al12 cuboctahedra, corners with two equivalent Er(4)Al12 cuboctahedra, corners with four equivalent Al(7)YEr3Al8 cuboctahedra, corners with ten equivalent Al(6)YEr3Al8 cuboctahedra, edges with two equivalent Er(1)Al12 cuboctahedra, edges with two equivalent Er(3)Al12 cuboctahedra, edges with two equivalent Y(2)Al12 cuboctahedra, edges with four equivalent Al(6)YEr3Al8 cuboctahedra,  a faceface with one Er(1)Al12 cuboctahedra,  a faceface with one Y(2)Al12 cuboctahedra,  a faceface with one Al(7)YEr3Al8 cuboctahedra, faces with two equivalent Er(3)Al12 cuboctahedra, and faces with five equivalent Al(6)YEr3Al8 cuboctahedra. There are two shorter (3.04 Å) and two longer (3.06 Å) Al(6)-Al(6) bond lengths.