Rb3NbO8 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 Nb(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Rb3NbO trigonal bipyramids, and edges with two equivalent Nb(1)O8 hexagonal bipyramids. In the second Rb site, Rb(2) is bonded in a 12-coordinate geometry to four equivalent O(1) and eight equivalent O(2) atoms. Nb(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted NbO8 hexagonal bipyramids that share corners with four equivalent Rb(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Rb3NbO 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 to one Rb(1), two equivalent Rb(2), one Nb(1), and one O(2) atom to form distorted ORb3NbO trigonal bipyramids that share  a cornercorner with one Rb(1)O8 hexagonal bipyramid,  a cornercorner with one Nb(1)O8 hexagonal bipyramid, corners with ten equivalent O(1)Rb3NbO trigonal bipyramids, and  an edgeedge with one O(1)Rb3NbO trigonal bipyramid. In the second O site, O(2) is bonded in a 7-coordinate geometry to one Rb(1), four equivalent Rb(2), one Nb(1), and one O(1) atom.
Rb3NbO8 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 Nb(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Rb3NbO trigonal bipyramids, and edges with two equivalent Nb(1)O8 hexagonal bipyramids. All Rb(1)-O(1) bond lengths are 2.85 Å. All Rb(1)-O(2) bond lengths are 2.96 Å. In the second Rb site, Rb(2) is bonded in a 12-coordinate geometry to four equivalent O(1) and eight equivalent O(2) atoms. All Rb(2)-O(1) bond lengths are 2.81 Å. There are four shorter (3.14 Å) and four longer (3.34 Å) Rb(2)-O(2) bond lengths. Nb(1) is bonded to four equivalent O(1) and four equivalent O(2) atoms to form distorted NbO8 hexagonal bipyramids that share corners with four equivalent Rb(1)O8 hexagonal bipyramids, corners with four equivalent O(1)Rb3NbO trigonal bipyramids, and edges with two equivalent Rb(1)O8 hexagonal bipyramids. All Nb(1)-O(1) bond lengths are 2.03 Å. All Nb(1)-O(2) bond lengths are 2.08 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded to one Rb(1), two equivalent Rb(2), one Nb(1), and one O(2) atom to form distorted ORb3NbO trigonal bipyramids that share  a cornercorner with one Rb(1)O8 hexagonal bipyramid,  a cornercorner with one Nb(1)O8 hexagonal bipyramid, corners with ten equivalent O(1)Rb3NbO trigonal bipyramids, and  an edgeedge with one O(1)Rb3NbO trigonal bipyramid. The O(1)-O(2) bond length is 1.50 Å. In the second O site, O(2) is bonded in a 7-coordinate geometry to one Rb(1), four equivalent Rb(2), one Nb(1), and one O(1) atom.