MgTb2Nb2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), one O(6), and one O(8) atom to form MgO5 trigonal bipyramids that share corners with two equivalent Tb(1)O7 pentagonal bipyramids, corners with two equivalent Nb(2)O5 trigonal bipyramids,  an edgeedge with one Tb(1)O7 pentagonal bipyramid, edges with two equivalent Tb(2)O7 pentagonal bipyramids, and edges with two equivalent Nb(1)O5 trigonal bipyramids. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with five equivalent Nb(2)O5 trigonal bipyramids, edges with three equivalent Tb(2)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Nb(1)O5 trigonal bipyramids. In the second Tb site, Tb(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form distorted TbO7 pentagonal bipyramids that share corners with five equivalent Nb(1)O5 trigonal bipyramids, edges with three equivalent Tb(1)O7 pentagonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and edges with two equivalent Nb(2)O5 trigonal bipyramids. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(2), one O(4), one O(5), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Tb(2)O7 pentagonal bipyramids, edges with two equivalent Tb(1)O7 pentagonal bipyramids,  an edgeedge with one Nb(2)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O5 trigonal bipyramids. In the second Nb site, Nb(2) is bonded to one O(3), one O(4), one O(6), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Tb(1)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Tb(2)O7 pentagonal bipyramids, and  an edgeedge with one Nb(1)O5 trigonal bipyramid. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a T-shaped geometry to one Mg(1), one Tb(2), and one Nb(1) atom. In the second O site, O(2) is bonded to one Mg(1), one Tb(1), one Tb(2), and one Nb(1) atom to form distorted OTb2MgNb trigonal pyramids that share corners with four equivalent O(8)Tb2MgNb2 square pyramids, corners with two equivalent O(5)Tb2MgNb trigonal pyramids, and  an edgeedge with one O(5)Tb2MgNb trigonal pyramid. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Tb(1), one Tb(2), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Tb(1), one Tb(2), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Tb(1), one Tb(2), and one Nb(1) atom to form distorted OTb2MgNb trigonal pyramids that share corners with two equivalent O(2)Tb2MgNb trigonal pyramids, edges with two equivalent O(8)Tb2MgNb2 square pyramids, and  an edgeedge with one O(2)Tb2MgNb trigonal pyramid. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Tb(1), one Tb(2), and one Nb(2) atom. In the eighth O site, O(8) is bonded to one Mg(1), one Tb(1), one Tb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OTb2MgNb2 square pyramids.
MgTb2Nb2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), one O(6), and one O(8) atom to form MgO5 trigonal bipyramids that share corners with two equivalent Tb(1)O7 pentagonal bipyramids, corners with two equivalent Nb(2)O5 trigonal bipyramids,  an edgeedge with one Tb(1)O7 pentagonal bipyramid, edges with two equivalent Tb(2)O7 pentagonal bipyramids, and edges with two equivalent Nb(1)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 2.08 Å. The Mg(1)-O(2) bond length is 2.00 Å. The Mg(1)-O(5) bond length is 2.04 Å. The Mg(1)-O(6) bond length is 1.98 Å. The Mg(1)-O(8) bond length is 1.99 Å. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TbO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with five equivalent Nb(2)O5 trigonal bipyramids, edges with three equivalent Tb(2)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Nb(1)O5 trigonal bipyramids. The Tb(1)-O(2) bond length is 2.27 Å. The Tb(1)-O(3) bond length is 2.28 Å. The Tb(1)-O(4) bond length is 2.44 Å. The Tb(1)-O(5) bond length is 2.47 Å. The Tb(1)-O(6) bond length is 2.45 Å. The Tb(1)-O(7) bond length is 2.31 Å. The Tb(1)-O(8) bond length is 2.60 Å. In the second Tb site, Tb(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form distorted TbO7 pentagonal bipyramids that share corners with five equivalent Nb(1)O5 trigonal bipyramids, edges with three equivalent Tb(1)O7 pentagonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and edges with two equivalent Nb(2)O5 trigonal bipyramids. The Tb(2)-O(1) bond length is 2.36 Å. The Tb(2)-O(2) bond length is 2.38 Å. The Tb(2)-O(3) bond length is 2.25 Å. The Tb(2)-O(4) bond length is 2.44 Å. The Tb(2)-O(5) bond length is 2.33 Å. The Tb(2)-O(7) bond length is 2.39 Å. The Tb(2)-O(8) bond length is 2.43 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(2), one O(4), one O(5), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Tb(2)O7 pentagonal bipyramids, edges with two equivalent Tb(1)O7 pentagonal bipyramids,  an edgeedge with one Nb(2)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The Nb(1)-O(1) bond length is 1.87 Å. The Nb(1)-O(2) bond length is 2.07 Å. The Nb(1)-O(4) bond length is 2.04 Å. The Nb(1)-O(5) bond length is 2.01 Å. The Nb(1)-O(8) bond length is 2.29 Å. In the second Nb site, Nb(2) is bonded to one O(3), one O(4), one O(6), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Tb(1)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Tb(2)O7 pentagonal bipyramids, and  an edgeedge with one Nb(1)O5 trigonal bipyramid. The Nb(2)-O(3) bond length is 2.02 Å. The Nb(2)-O(4) bond length is 2.40 Å. The Nb(2)-O(6) bond length is 1.90 Å. The Nb(2)-O(7) bond length is 1.93 Å. The Nb(2)-O(8) bond length is 2.11 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a T-shaped geometry to one Mg(1), one Tb(2), and one Nb(1) atom. In the second O site, O(2) is bonded to one Mg(1), one Tb(1), one Tb(2), and one Nb(1) atom to form distorted OTb2MgNb trigonal pyramids that share corners with four equivalent O(8)Tb2MgNb2 square pyramids, corners with two equivalent O(5)Tb2MgNb trigonal pyramids, and  an edgeedge with one O(5)Tb2MgNb trigonal pyramid. In the third O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Tb(1), one Tb(2), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Tb(1), one Tb(2), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Tb(1), one Tb(2), and one Nb(1) atom to form distorted OTb2MgNb trigonal pyramids that share corners with two equivalent O(2)Tb2MgNb trigonal pyramids, edges with two equivalent O(8)Tb2MgNb2 square pyramids, and  an edgeedge with one O(2)Tb2MgNb trigonal pyramid. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Tb(1), one Tb(2), and one Nb(2) atom. In the eighth O site, O(8) is bonded to one Mg(1), one Tb(1), one Tb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OTb2MgNb2 square pyramids.