Rb3AlO3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. In the second Rb site, Rb(2) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(1) atoms. In the third Rb site, Rb(3) is bonded to four equivalent O(1) atoms to form distorted RbO4 tetrahedra that share corners with four equivalent Al(1)O4 tetrahedra and edges with two equivalent Rb(3)O4 tetrahedra. Al(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form AlO4 tetrahedra that share corners with four equivalent Rb(3)O4 tetrahedra and  an edgeedge with one Al(1)O4 tetrahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to two equivalent Rb(1), two equivalent Rb(2), two equivalent Rb(3), and one Al(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), two equivalent Rb(2), and two equivalent Al(1) atoms.
Rb3AlO3 crystallizes in the monoclinic C2/m space group. There are three inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. The Rb(1)-O(2) bond length is 2.72 Å. There are two shorter (2.90 Å) and two longer (2.95 Å) Rb(1)-O(1) bond lengths. In the second Rb site, Rb(2) is bonded in a 6-coordinate geometry to two equivalent O(2) and four equivalent O(1) atoms. Both Rb(2)-O(2) bond lengths are 3.32 Å. There are two shorter (2.83 Å) and two longer (2.97 Å) Rb(2)-O(1) bond lengths. In the third Rb site, Rb(3) is bonded to four equivalent O(1) atoms to form distorted RbO4 tetrahedra that share corners with four equivalent Al(1)O4 tetrahedra and edges with two equivalent Rb(3)O4 tetrahedra. There are two shorter (2.82 Å) and two longer (2.96 Å) Rb(3)-O(1) bond lengths. Al(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form AlO4 tetrahedra that share corners with four equivalent Rb(3)O4 tetrahedra and  an edgeedge with one Al(1)O4 tetrahedra. Both Al(1)-O(1) bond lengths are 1.76 Å. Both Al(1)-O(2) bond lengths are 1.83 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to two equivalent Rb(1), two equivalent Rb(2), two equivalent Rb(3), and one Al(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Rb(1), two equivalent Rb(2), and two equivalent Al(1) atoms.