Cr3Al9Si crystallizes in the hexagonal P6_3/mmc space group. Cr(1) is bonded to two equivalent Cr(1), two equivalent Al(1), six equivalent Al(2), and two equivalent Si(1) atoms to form distorted CrAl8Cr2Si2 cuboctahedra that share corners with four equivalent Cr(1)Al8Cr2Si2 cuboctahedra, faces with two equivalent Si(1)Al6Cr6 cuboctahedra, and faces with six equivalent Cr(1)Al8Cr2Si2 cuboctahedra. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded in a 12-coordinate geometry to two equivalent Cr(1), two equivalent Al(1), and eight equivalent Al(2) atoms. In the second Al site, Al(2) is bonded in a 13-coordinate geometry to three equivalent Cr(1), four equivalent Al(1), five equivalent Al(2), and one Si(1) atom. Si(1) is bonded to six equivalent Cr(1) and six equivalent Al(2) atoms to form SiAl6Cr6 cuboctahedra that share faces with two equivalent Si(1)Al6Cr6 cuboctahedra and faces with six equivalent Cr(1)Al8Cr2Si2 cuboctahedra.
Cr3Al9Si crystallizes in the hexagonal P6_3/mmc space group. Cr(1) is bonded to two equivalent Cr(1), two equivalent Al(1), six equivalent Al(2), and two equivalent Si(1) atoms to form distorted CrAl8Cr2Si2 cuboctahedra that share corners with four equivalent Cr(1)Al8Cr2Si2 cuboctahedra, faces with two equivalent Si(1)Al6Cr6 cuboctahedra, and faces with six equivalent Cr(1)Al8Cr2Si2 cuboctahedra. Both Cr(1)-Cr(1) bond lengths are 2.56 Å. Both Cr(1)-Al(1) bond lengths are 2.45 Å. There are two shorter (2.68 Å) and four longer (2.69 Å) Cr(1)-Al(2) bond lengths. Both Cr(1)-Si(1) bond lengths are 2.45 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded in a 12-coordinate geometry to two equivalent Cr(1), two equivalent Al(1), and eight equivalent Al(2) atoms. Both Al(1)-Al(1) bond lengths are 2.76 Å. There are four shorter (2.78 Å) and four longer (2.94 Å) Al(1)-Al(2) bond lengths. In the second Al site, Al(2) is bonded in a 13-coordinate geometry to three equivalent Cr(1), four equivalent Al(1), five equivalent Al(2), and one Si(1) atom. There are a spread of Al(2)-Al(2) bond distances ranging from 2.77-2.94 Å. The Al(2)-Si(1) bond length is 2.64 Å. Si(1) is bonded to six equivalent Cr(1) and six equivalent Al(2) atoms to form SiAl6Cr6 cuboctahedra that share faces with two equivalent Si(1)Al6Cr6 cuboctahedra and faces with six equivalent Cr(1)Al8Cr2Si2 cuboctahedra.