BaAl5O9 crystallizes in the hexagonal P6_3/mmc space group. Ba(1) is bonded in a 9-coordinate geometry to three equivalent O(1) and six equivalent O(2) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form AlO6 octahedra that share corners with four equivalent Al(2)O4 tetrahedra and edges with four equivalent Al(1)O6 octahedra. In the second Al site, Al(2) is bonded to one O(1) and three equivalent O(2) atoms to form AlO4 tetrahedra that share corners with six equivalent Al(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 57°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to three equivalent Ba(1) and two equivalent Al(2) atoms. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ba(1), one Al(2), and two equivalent Al(1) atoms. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Al(1) atoms.
BaAl5O9 crystallizes in the hexagonal P6_3/mmc space group. Ba(1) is bonded in a 9-coordinate geometry to three equivalent O(1) and six equivalent O(2) atoms. All Ba(1)-O(1) bond lengths are 3.30 Å. All Ba(1)-O(2) bond lengths are 2.79 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to two equivalent O(3) and four equivalent O(2) atoms to form AlO6 octahedra that share corners with four equivalent Al(2)O4 tetrahedra and edges with four equivalent Al(1)O6 octahedra. Both Al(1)-O(3) bond lengths are 1.88 Å. All Al(1)-O(2) bond lengths are 1.95 Å. In the second Al site, Al(2) is bonded to one O(1) and three equivalent O(2) atoms to form AlO4 tetrahedra that share corners with six equivalent Al(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 57°. The Al(2)-O(1) bond length is 1.75 Å. All Al(2)-O(2) bond lengths are 1.78 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a linear geometry to three equivalent Ba(1) and two equivalent Al(2) atoms. In the second O site, O(2) is bonded in a 4-coordinate geometry to one Ba(1), one Al(2), and two equivalent Al(1) atoms. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to three equivalent Al(1) atoms.