Rb3CrO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a 12-coordinate geometry to four equivalent O(2) and eight equivalent O(1) atoms. In the second Rb site, Rb(2) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form RbO8 hexagonal bipyramids that share corners with four equivalent Cr(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3CrO trigonal bipyramids, and edges with two equivalent Cr(1)O8 hexagonal bipyramids. Cr(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted CrO8 hexagonal bipyramids that share corners with four equivalent Rb(2)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3CrO trigonal bipyramids, and edges with two equivalent Rb(2)O8 hexagonal bipyramids. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 7-coordinate geometry to one Rb(2), four equivalent Rb(1), one Cr(1), and one O(2) atom. In the second O site, O(2) is bonded to one Rb(2), two equivalent Rb(1), one Cr(1), and one O(1) atom to form distorted ORb3CrO trigonal bipyramids that share  a cornercorner with one Rb(2)O8 hexagonal bipyramid,  a cornercorner with one Cr(1)O8 hexagonal bipyramid, corners with ten equivalent O(2)Rb3CrO trigonal bipyramids, and  an edgeedge with one O(2)Rb3CrO trigonal bipyramid.
Rb3CrO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a 12-coordinate geometry to four equivalent O(2) and eight equivalent O(1) atoms. All Rb(1)-O(2) bond lengths are 2.85 Å. There are four shorter (3.17 Å) and four longer (3.33 Å) Rb(1)-O(1) bond lengths. In the second Rb site, Rb(2) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form RbO8 hexagonal bipyramids that share corners with four equivalent Cr(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3CrO trigonal bipyramids, and edges with two equivalent Cr(1)O8 hexagonal bipyramids. All Rb(2)-O(1) bond lengths are 3.03 Å. All Rb(2)-O(2) bond lengths are 2.90 Å. Cr(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted CrO8 hexagonal bipyramids that share corners with four equivalent Rb(2)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3CrO trigonal bipyramids, and edges with two equivalent Rb(2)O8 hexagonal bipyramids. All Cr(1)-O(1) bond lengths are 2.01 Å. All Cr(1)-O(2) bond lengths are 1.91 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 7-coordinate geometry to one Rb(2), four equivalent Rb(1), one Cr(1), and one O(2) atom. The O(1)-O(2) bond length is 1.45 Å. In the second O site, O(2) is bonded to one Rb(2), two equivalent Rb(1), one Cr(1), and one O(1) atom to form distorted ORb3CrO trigonal bipyramids that share  a cornercorner with one Rb(2)O8 hexagonal bipyramid,  a cornercorner with one Cr(1)O8 hexagonal bipyramid, corners with ten equivalent O(2)Rb3CrO trigonal bipyramids, and  an edgeedge with one O(2)Rb3CrO trigonal bipyramid.