Al18Cr2Mg3 crystallizes in the cubic Fd-3m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 14-coordinate geometry to two equivalent Mg(2) and twelve equivalent Al(1) atoms. In the second Mg site, Mg(2) is bonded in a 16-coordinate geometry to four equivalent Mg(1) and twelve equivalent Al(1) atoms. Cr(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form CrAl12 cuboctahedra that share corners with six equivalent Cr(1)Al12 cuboctahedra, edges with eighteen equivalent Al(1)Mg3Al8Cr cuboctahedra, and faces with six equivalent Al(1)Mg3Al8Cr cuboctahedra. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one Mg(2), two equivalent Mg(1), one Cr(1), three equivalent Al(2), and five equivalent Al(1) atoms to form AlMg3Al8Cr cuboctahedra that share corners with fifteen equivalent Al(1)Mg3Al8Cr cuboctahedra, edges with two equivalent Al(1)Mg3Al8Cr cuboctahedra, edges with three equivalent Cr(1)Al12 cuboctahedra,  a faceface with one Cr(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(1)Mg3Al8Cr cuboctahedra. In the second Al site, Al(2) is bonded in a 12-coordinate geometry to two equivalent Cr(1), four equivalent Al(2), and six equivalent Al(1) atoms.
Al18Cr2Mg3 crystallizes in the cubic Fd-3m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 14-coordinate geometry to two equivalent Mg(2) and twelve equivalent Al(1) atoms. Both Mg(1)-Mg(2) bond lengths are 3.12 Å. All Mg(1)-Al(1) bond lengths are 3.09 Å. In the second Mg site, Mg(2) is bonded in a 16-coordinate geometry to four equivalent Mg(1) and twelve equivalent Al(1) atoms. All Mg(2)-Al(1) bond lengths are 3.20 Å. Cr(1) is bonded to six equivalent Al(1) and six equivalent Al(2) atoms to form CrAl12 cuboctahedra that share corners with six equivalent Cr(1)Al12 cuboctahedra, edges with eighteen equivalent Al(1)Mg3Al8Cr cuboctahedra, and faces with six equivalent Al(1)Mg3Al8Cr cuboctahedra. All Cr(1)-Al(1) bond lengths are 2.77 Å. All Cr(1)-Al(2) bond lengths are 2.55 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one Mg(2), two equivalent Mg(1), one Cr(1), three equivalent Al(2), and five equivalent Al(1) atoms to form AlMg3Al8Cr cuboctahedra that share corners with fifteen equivalent Al(1)Mg3Al8Cr cuboctahedra, edges with two equivalent Al(1)Mg3Al8Cr cuboctahedra, edges with three equivalent Cr(1)Al12 cuboctahedra,  a faceface with one Cr(1)Al12 cuboctahedra, and faces with fifteen equivalent Al(1)Mg3Al8Cr cuboctahedra. There is one shorter (2.70 Å) and two longer (2.81 Å) Al(1)-Al(2) bond lengths. There are three shorter (2.73 Å) and two longer (2.89 Å) Al(1)-Al(1) bond lengths. In the second Al site, Al(2) is bonded in a 12-coordinate geometry to two equivalent Cr(1), four equivalent Al(2), and six equivalent Al(1) atoms. All Al(2)-Al(2) bond lengths are 2.79 Å.