Rb3TaO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form RbO8 hexagonal bipyramids that share corners with four equivalent Ta(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3TaO trigonal bipyramids, and edges with two equivalent Ta(1)O8 hexagonal bipyramids. In the second Rb site, Rb(2) is bonded in a 12-coordinate geometry to four equivalent O(2) and eight equivalent O(1) atoms. Ta(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted TaO8 hexagonal bipyramids that share corners with four equivalent Rb(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3TaO trigonal bipyramids, and edges with two equivalent Rb(1)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(1), four equivalent Rb(2), one Ta(1), and one O(2) atom. In the second O site, O(2) is bonded to one Rb(1), two equivalent Rb(2), one Ta(1), and one O(1) atom to form distorted ORb3TaO trigonal bipyramids that share  a cornercorner with one Rb(1)O8 hexagonal bipyramid,  a cornercorner with one Ta(1)O8 hexagonal bipyramid, corners with ten equivalent O(2)Rb3TaO trigonal bipyramids, and  an edgeedge with one O(2)Rb3TaO trigonal bipyramid.
Rb3TaO8 crystallizes in the tetragonal I-42m space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form RbO8 hexagonal bipyramids that share corners with four equivalent Ta(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3TaO trigonal bipyramids, and edges with two equivalent Ta(1)O8 hexagonal bipyramids. All Rb(1)-O(1) bond lengths are 2.96 Å. All Rb(1)-O(2) bond lengths are 2.86 Å. In the second Rb site, Rb(2) is bonded in a 12-coordinate geometry to four equivalent O(2) and eight equivalent O(1) atoms. All Rb(2)-O(2) bond lengths are 2.81 Å. There are four shorter (3.15 Å) and four longer (3.33 Å) Rb(2)-O(1) bond lengths. Ta(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted TaO8 hexagonal bipyramids that share corners with four equivalent Rb(1)O8 hexagonal bipyramids, corners with four equivalent O(2)Rb3TaO trigonal bipyramids, and edges with two equivalent Rb(1)O8 hexagonal bipyramids. All Ta(1)-O(1) bond lengths are 2.06 Å. All Ta(1)-O(2) bond lengths are 2.01 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 7-coordinate geometry to one Rb(1), four equivalent Rb(2), one Ta(1), and one O(2) atom. The O(1)-O(2) bond length is 1.51 Å. In the second O site, O(2) is bonded to one Rb(1), two equivalent Rb(2), one Ta(1), and one O(1) atom to form distorted ORb3TaO trigonal bipyramids that share  a cornercorner with one Rb(1)O8 hexagonal bipyramid,  a cornercorner with one Ta(1)O8 hexagonal bipyramid, corners with ten equivalent O(2)Rb3TaO trigonal bipyramids, and  an edgeedge with one O(2)Rb3TaO trigonal bipyramid.