Rb3MgNbO8 crystallizes in the monoclinic C2 space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(4) atoms. In the second Rb site, Rb(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms. Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share faces with two equivalent Nb(1)O8 hexagonal bipyramids. Nb(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO8 hexagonal bipyramids that share faces with two equivalent Mg(1)O6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Rb(2), one Mg(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Rb(2), one Nb(1), and one O(3) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Rb(1), one Rb(2), one Mg(1), one Nb(1), and one O(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Rb(1), two equivalent Rb(2), one Mg(1), and one Nb(1) atom.
Rb3MgNbO8 crystallizes in the monoclinic C2 space group. There are two inequivalent Rb sites. In the first Rb site, Rb(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(4) atoms. Both Rb(1)-O(3) bond lengths are 2.72 Å. Both Rb(1)-O(4) bond lengths are 2.78 Å. In the second Rb site, Rb(2) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms. The Rb(2)-O(3) bond length is 2.98 Å. There is one shorter (2.92 Å) and one longer (3.04 Å) Rb(2)-O(1) bond length. There is one shorter (2.82 Å) and one longer (2.96 Å) Rb(2)-O(2) bond length. There is one shorter (2.89 Å) and one longer (3.27 Å) Rb(2)-O(4) bond length. Mg(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share faces with two equivalent Nb(1)O8 hexagonal bipyramids. Both Mg(1)-O(1) bond lengths are 2.08 Å. Both Mg(1)-O(3) bond lengths are 2.17 Å. Both Mg(1)-O(4) bond lengths are 2.09 Å. Nb(1) is bonded to two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO8 hexagonal bipyramids that share faces with two equivalent Mg(1)O6 octahedra. Both Nb(1)-O(1) bond lengths are 2.09 Å. Both Nb(1)-O(2) bond lengths are 2.05 Å. Both Nb(1)-O(3) bond lengths are 2.28 Å. Both Nb(1)-O(4) bond lengths are 2.01 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Rb(2), one Mg(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a 4-coordinate geometry to two equivalent Rb(2), one Nb(1), and one O(3) atom. The O(2)-O(3) bond length is 1.49 Å. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Rb(1), one Rb(2), one Mg(1), one Nb(1), and one O(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Rb(1), two equivalent Rb(2), one Mg(1), and one Nb(1) atom.