Al4Si crystallizes in the trigonal R-3m space group. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded to three equivalent Al(2), six equivalent Al(1), and three equivalent Si(1) atoms to form distorted AlAl9Si3 cuboctahedra that share corners with three equivalent Al(2)Al12 cuboctahedra, corners with six equivalent Si(1)Al6 cuboctahedra, corners with nine equivalent Al(1)Al9Si3 cuboctahedra, edges with six equivalent Si(1)Al6 cuboctahedra, edges with nine equivalent Al(2)Al12 cuboctahedra, edges with nine equivalent Al(1)Al9Si3 cuboctahedra, faces with six equivalent Al(2)Al12 cuboctahedra, and faces with six equivalent Al(1)Al9Si3 cuboctahedra. In the second Al site, Al(2) is bonded to three equivalent Al(1) and nine equivalent Al(2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra, corners with three equivalent Si(1)Al6 cuboctahedra, corners with six equivalent Al(2)Al12 cuboctahedra, edges with three equivalent Si(1)Al6 cuboctahedra, edges with nine equivalent Al(1)Al9Si3 cuboctahedra, edges with twelve equivalent Al(2)Al12 cuboctahedra, faces with six equivalent Al(1)Al9Si3 cuboctahedra, and faces with twelve equivalent Al(2)Al12 cuboctahedra. In the third Al site, Al(2) is bonded to three equivalent Al(1) and nine Al(2,2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra; corners with three equivalent Si(1)Al6 cuboctahedra; corners with six equivalent Al(2)Al12 cuboctahedra; edges with three equivalent Si(1)Al6 cuboctahedra; edges with nine equivalent Al(1)Al9Si3 cuboctahedra; edges with twelve Al(2,2)Al12 cuboctahedra; faces with six equivalent Al(1)Al9Si3 cuboctahedra; and faces with twelve Al(2,2)Al12 cuboctahedra. In the fourth Al site, Al(2) is bonded to three equivalent Al(1) and nine Al(2,2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra; corners with three equivalent Si(1)Al6 cuboctahedra; corners with six equivalent Al(2)Al12 cuboctahedra; edges with three equivalent Si(1)Al6 cuboctahedra; edges with nine equivalent Al(1)Al9Si3 cuboctahedra; edges with twelve Al(2,2)Al12 cuboctahedra; faces with six equivalent Al(1)Al9Si3 cuboctahedra; and faces with twelve Al(2,2)Al12 cuboctahedra. Si(1) is bonded to six equivalent Al(1) atoms to form distorted SiAl6 cuboctahedra that share corners with six Al(2,2)Al12 cuboctahedra; corners with twelve equivalent Al(1)Al9Si3 cuboctahedra; edges with six Al(2,2)Al12 cuboctahedra; edges with six equivalent Si(1)Al6 cuboctahedra; and edges with twelve equivalent Al(1)Al9Si3 cuboctahedra.
Al4Si crystallizes in the trigonal R-3m space group. There are four inequivalent Al sites. In the first Al site, Al(1) is bonded to three equivalent Al(2), six equivalent Al(1), and three equivalent Si(1) atoms to form distorted AlAl9Si3 cuboctahedra that share corners with three equivalent Al(2)Al12 cuboctahedra, corners with six equivalent Si(1)Al6 cuboctahedra, corners with nine equivalent Al(1)Al9Si3 cuboctahedra, edges with six equivalent Si(1)Al6 cuboctahedra, edges with nine equivalent Al(2)Al12 cuboctahedra, edges with nine equivalent Al(1)Al9Si3 cuboctahedra, faces with six equivalent Al(2)Al12 cuboctahedra, and faces with six equivalent Al(1)Al9Si3 cuboctahedra. All Al(1)-Al(2) bond lengths are 2.84 Å. All Al(1)-Al(1) bond lengths are 2.85 Å. All Al(1)-Si(1) bond lengths are 2.73 Å. In the second Al site, Al(2) is bonded to three equivalent Al(1) and nine equivalent Al(2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra, corners with three equivalent Si(1)Al6 cuboctahedra, corners with six equivalent Al(2)Al12 cuboctahedra, edges with three equivalent Si(1)Al6 cuboctahedra, edges with nine equivalent Al(1)Al9Si3 cuboctahedra, edges with twelve equivalent Al(2)Al12 cuboctahedra, faces with six equivalent Al(1)Al9Si3 cuboctahedra, and faces with twelve equivalent Al(2)Al12 cuboctahedra. There are six shorter (2.85 Å) and three longer (2.88 Å) Al(2)-Al(2) bond lengths. In the third Al site, Al(2) is bonded to three equivalent Al(1) and nine Al(2,2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra; corners with three equivalent Si(1)Al6 cuboctahedra; corners with six equivalent Al(2)Al12 cuboctahedra; edges with three equivalent Si(1)Al6 cuboctahedra; edges with nine equivalent Al(1)Al9Si3 cuboctahedra; edges with twelve Al(2,2)Al12 cuboctahedra; faces with six equivalent Al(1)Al9Si3 cuboctahedra; and faces with twelve Al(2,2)Al12 cuboctahedra. All Al(2)-Al(1) bond lengths are 2.84 Å. There are six shorter (2.85 Å) and three longer (2.88 Å) Al(2)-Al(2,2) bond lengths. In the fourth Al site, Al(2) is bonded to three equivalent Al(1) and nine Al(2,2) atoms to form AlAl12 cuboctahedra that share corners with three equivalent Al(1)Al9Si3 cuboctahedra; corners with three equivalent Si(1)Al6 cuboctahedra; corners with six equivalent Al(2)Al12 cuboctahedra; edges with three equivalent Si(1)Al6 cuboctahedra; edges with nine equivalent Al(1)Al9Si3 cuboctahedra; edges with twelve Al(2,2)Al12 cuboctahedra; faces with six equivalent Al(1)Al9Si3 cuboctahedra; and faces with twelve Al(2,2)Al12 cuboctahedra. All Al(2)-Al(1) bond lengths are 2.84 Å. There are six shorter (2.85 Å) and three longer (2.88 Å) Al(2)-Al(2,2) bond lengths. Si(1) is bonded to six equivalent Al(1) atoms to form distorted SiAl6 cuboctahedra that share corners with six Al(2,2)Al12 cuboctahedra; corners with twelve equivalent Al(1)Al9Si3 cuboctahedra; edges with six Al(2,2)Al12 cuboctahedra; edges with six equivalent Si(1)Al6 cuboctahedra; and edges with twelve equivalent Al(1)Al9Si3 cuboctahedra.