Rb3MgTbV2O8 crystallizes in the triclinic P1 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(1), one O(2), one O(4), one O(6), and one O(8) atom. In the second Rb site, Rb(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(4), two equivalent O(5), and two equivalent O(7) atoms. In the third Rb site, Rb(3) is bonded in a 6-coordinate geometry to one O(7), one O(8), two equivalent O(1), and two equivalent O(2) atoms. Mg(1) is bonded to one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one V(2)O4 tetrahedra,  an edgeedge with one V(1)O4 tetrahedra, and  an edgeedge with one V(2)O4 tetrahedra. Tb(1) is bonded in a 5-coordinate geometry to one O(3), one O(4), one O(5), one O(6), and one O(7) atom. There are two inequivalent V sites. In the first V site, V(1) is bonded to one O(1), one O(3), one O(5), and one O(7) atom to form VO4 tetrahedra that share  an edgeedge with one Mg(1)O5 trigonal bipyramid. In the second V site, V(2) is bonded to one O(2), one O(4), one O(6), and one O(8) atom to form VO4 tetrahedra that share  a cornercorner with one Mg(1)O5 trigonal bipyramid and  an edgeedge with one Mg(1)O5 trigonal bipyramid. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one V(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one V(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one V(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Rb(1), one Rb(2), one Mg(1), one Tb(1), and one V(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Rb(2), one Mg(1), one Tb(1), and one V(1) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Rb(1), one Mg(1), one Tb(1), and one V(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one Rb(3), two equivalent Rb(2), one Tb(1), and one V(1) atom. In the eighth O site, O(8) is bonded in a distorted bent 150 degrees geometry to one Rb(1), one Rb(3), one Mg(1), and one V(2) atom.
Rb3MgTbV2O8 crystallizes in the triclinic P1 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(1), one O(2), one O(4), one O(6), and one O(8) atom. The Rb(1)-O(1) bond length is 2.69 Å. The Rb(1)-O(2) bond length is 2.92 Å. The Rb(1)-O(4) bond length is 3.01 Å. The Rb(1)-O(6) bond length is 3.08 Å. The Rb(1)-O(8) bond length is 2.85 Å. In the second Rb site, Rb(2) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(4), two equivalent O(5), and two equivalent O(7) atoms. The Rb(2)-O(1) bond length is 3.18 Å. The Rb(2)-O(2) bond length is 2.92 Å. The Rb(2)-O(4) bond length is 3.39 Å. There is one shorter (2.99 Å) and one longer (3.14 Å) Rb(2)-O(5) bond length. There is one shorter (2.95 Å) and one longer (3.12 Å) Rb(2)-O(7) bond length. In the third Rb site, Rb(3) is bonded in a 6-coordinate geometry to one O(7), one O(8), two equivalent O(1), and two equivalent O(2) atoms. The Rb(3)-O(7) bond length is 3.27 Å. The Rb(3)-O(8) bond length is 3.18 Å. There is one shorter (3.00 Å) and one longer (3.44 Å) Rb(3)-O(1) bond length. There is one shorter (2.96 Å) and one longer (3.13 Å) Rb(3)-O(2) bond length. Mg(1) is bonded to one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one V(2)O4 tetrahedra,  an edgeedge with one V(1)O4 tetrahedra, and  an edgeedge with one V(2)O4 tetrahedra. The Mg(1)-O(3) bond length is 2.03 Å. The Mg(1)-O(4) bond length is 2.03 Å. The Mg(1)-O(5) bond length is 2.13 Å. The Mg(1)-O(6) bond length is 2.11 Å. The Mg(1)-O(8) bond length is 2.01 Å. Tb(1) is bonded in a 5-coordinate geometry to one O(3), one O(4), one O(5), one O(6), and one O(7) atom. The Tb(1)-O(3) bond length is 2.24 Å. The Tb(1)-O(4) bond length is 2.32 Å. The Tb(1)-O(5) bond length is 2.29 Å. The Tb(1)-O(6) bond length is 2.27 Å. The Tb(1)-O(7) bond length is 2.22 Å. There are two inequivalent V sites. In the first V site, V(1) is bonded to one O(1), one O(3), one O(5), and one O(7) atom to form VO4 tetrahedra that share  an edgeedge with one Mg(1)O5 trigonal bipyramid. The V(1)-O(1) bond length is 1.73 Å. The V(1)-O(3) bond length is 1.85 Å. The V(1)-O(5) bond length is 1.92 Å. The V(1)-O(7) bond length is 1.84 Å. In the second V site, V(2) is bonded to one O(2), one O(4), one O(6), and one O(8) atom to form VO4 tetrahedra that share  a cornercorner with one Mg(1)O5 trigonal bipyramid and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The V(2)-O(2) bond length is 1.74 Å. The V(2)-O(4) bond length is 1.93 Å. The V(2)-O(6) bond length is 1.90 Å. The V(2)-O(8) bond length is 1.80 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one V(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Rb(1), one Rb(2), two equivalent Rb(3), and one V(2) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one V(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Rb(1), one Rb(2), one Mg(1), one Tb(1), and one V(2) atom. In the fifth O site, O(5) is bonded in a 3-coordinate geometry to two equivalent Rb(2), one Mg(1), one Tb(1), and one V(1) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Rb(1), one Mg(1), one Tb(1), and one V(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to one Rb(3), two equivalent Rb(2), one Tb(1), and one V(1) atom. In the eighth O site, O(8) is bonded in a distorted bent 150 degrees geometry to one Rb(1), one Rb(3), one Mg(1), and one V(2) atom.