Sr2Mg3Al crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 12-coordinate geometry to three equivalent Mg(2), six equivalent Mg(1), and three equivalent Al(1) atoms. In the second Sr site, Sr(2) is bonded in a 12-coordinate geometry to three equivalent Mg(1), six equivalent Mg(2), and three equivalent Al(1) atoms. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Sr(2), four equivalent Sr(1), four equivalent Mg(1), and two equivalent Al(1) atoms to form distorted MgSr6Mg4Al2 cuboctahedra that share corners with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, corners with four equivalent Al(1)Sr6Mg6 cuboctahedra, corners with ten equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with two equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, edges with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, faces with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, faces with six equivalent Al(1)Sr6Mg6 cuboctahedra, and faces with eight equivalent Mg(1)Sr6Mg4Al2 cuboctahedra. In the second Mg site, Mg(2) is bonded to two equivalent Sr(1), four equivalent Sr(2), four equivalent Mg(2), and two equivalent Al(1) atoms to form MgSr6Mg4Al2 cuboctahedra that share corners with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, corners with four equivalent Al(1)Sr6Mg6 cuboctahedra, corners with ten equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, edges with two equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, faces with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, faces with six equivalent Al(1)Sr6Mg6 cuboctahedra, and faces with eight equivalent Mg(2)Sr6Mg4Al2 cuboctahedra. Al(1) is bonded to three equivalent Sr(1), three equivalent Sr(2), three equivalent Mg(1), and three equivalent Mg(2) atoms to form AlSr6Mg6 cuboctahedra that share corners with three equivalent Al(1)Sr6Mg6 cuboctahedra, corners with six equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, corners with six equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with six equivalent Al(1)Sr6Mg6 cuboctahedra,  a faceface with one Al(1)Sr6Mg6 cuboctahedra, faces with nine equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, and faces with nine equivalent Mg(2)Sr6Mg4Al2 cuboctahedra.
Sr2Mg3Al crystallizes in the hexagonal P6_3/mmc space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 12-coordinate geometry to three equivalent Mg(2), six equivalent Mg(1), and three equivalent Al(1) atoms. All Sr(1)-Mg(2) bond lengths are 3.70 Å. All Sr(1)-Mg(1) bond lengths are 3.69 Å. All Sr(1)-Al(1) bond lengths are 3.64 Å. In the second Sr site, Sr(2) is bonded in a 12-coordinate geometry to three equivalent Mg(1), six equivalent Mg(2), and three equivalent Al(1) atoms. All Sr(2)-Mg(1) bond lengths are 3.67 Å. All Sr(2)-Mg(2) bond lengths are 3.71 Å. All Sr(2)-Al(1) bond lengths are 3.67 Å. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent Sr(2), four equivalent Sr(1), four equivalent Mg(1), and two equivalent Al(1) atoms to form distorted MgSr6Mg4Al2 cuboctahedra that share corners with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, corners with four equivalent Al(1)Sr6Mg6 cuboctahedra, corners with ten equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with two equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, edges with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, faces with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, faces with six equivalent Al(1)Sr6Mg6 cuboctahedra, and faces with eight equivalent Mg(1)Sr6Mg4Al2 cuboctahedra. There are two shorter (3.10 Å) and two longer (3.14 Å) Mg(1)-Mg(1) bond lengths. Both Mg(1)-Al(1) bond lengths are 3.06 Å. In the second Mg site, Mg(2) is bonded to two equivalent Sr(1), four equivalent Sr(2), four equivalent Mg(2), and two equivalent Al(1) atoms to form MgSr6Mg4Al2 cuboctahedra that share corners with four equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, corners with four equivalent Al(1)Sr6Mg6 cuboctahedra, corners with ten equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, edges with two equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, faces with four equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, faces with six equivalent Al(1)Sr6Mg6 cuboctahedra, and faces with eight equivalent Mg(2)Sr6Mg4Al2 cuboctahedra. All Mg(2)-Mg(2) bond lengths are 3.12 Å. Both Mg(2)-Al(1) bond lengths are 3.27 Å. Al(1) is bonded to three equivalent Sr(1), three equivalent Sr(2), three equivalent Mg(1), and three equivalent Mg(2) atoms to form AlSr6Mg6 cuboctahedra that share corners with three equivalent Al(1)Sr6Mg6 cuboctahedra, corners with six equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, corners with six equivalent Mg(2)Sr6Mg4Al2 cuboctahedra, edges with six equivalent Al(1)Sr6Mg6 cuboctahedra,  a faceface with one Al(1)Sr6Mg6 cuboctahedra, faces with nine equivalent Mg(1)Sr6Mg4Al2 cuboctahedra, and faces with nine equivalent Mg(2)Sr6Mg4Al2 cuboctahedra.