MgDy2Nb2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form MgO5 trigonal bipyramids that share corners with two equivalent Dy(1)O7 pentagonal bipyramids, corners with two equivalent Nb(1)O5 trigonal bipyramids,  an edgeedge with one Dy(1)O7 pentagonal bipyramid, edges with two equivalent Dy(2)O7 pentagonal bipyramids, and edges with two equivalent Nb(2)O5 trigonal bipyramids. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted DyO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with five equivalent Nb(1)O5 trigonal bipyramids, edges with three equivalent Dy(2)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Nb(2)O5 trigonal bipyramids. In the second Dy site, Dy(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted DyO7 pentagonal bipyramids that share corners with five equivalent Nb(2)O5 trigonal bipyramids, edges with three equivalent Dy(1)O7 pentagonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and edges with two equivalent Nb(1)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(3), one O(4), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Dy(1)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Dy(2)O7 pentagonal bipyramids, and  an edgeedge with one Nb(2)O5 trigonal bipyramid. In the second Nb site, Nb(2) is bonded to one O(1), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Dy(2)O7 pentagonal bipyramids, edges with two equivalent Dy(1)O7 pentagonal bipyramids,  an edgeedge with one Nb(1)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O5 trigonal bipyramids. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Dy(1), one Dy(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted corner and edge-sharing ODy2MgNb2 square pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Dy(1), one Dy(2), and one Nb(1) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mg(1), one Dy(1), and one Nb(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Dy(1), one Dy(2), and one Nb(1) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Dy(1), one Dy(2), and one Nb(2) atom to form distorted ODy2MgNb trigonal pyramids that share corners with four equivalent O(1)Dy2MgNb2 square pyramids, corners with two equivalent O(6)Dy2MgNb trigonal pyramids, and  an edgeedge with one O(6)Dy2MgNb trigonal pyramid. In the sixth O site, O(6) is bonded to one Mg(1), one Dy(1), one Dy(2), and one Nb(2) atom to form distorted ODy2MgNb trigonal pyramids that share corners with two equivalent O(5)Dy2MgNb trigonal pyramids, edges with two equivalent O(1)Dy2MgNb2 square pyramids, and  an edgeedge with one O(5)Dy2MgNb trigonal pyramid. In the seventh O site, O(7) is bonded in a T-shaped geometry to one Mg(1), one Dy(2), and one Nb(2) atom. In the eighth O site, O(8) is bonded in a rectangular see-saw-like geometry to one Dy(1), one Dy(2), one Nb(1), and one Nb(2) atom.
MgDy2Nb2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(3), one O(5), one O(6), and one O(7) atom to form MgO5 trigonal bipyramids that share corners with two equivalent Dy(1)O7 pentagonal bipyramids, corners with two equivalent Nb(1)O5 trigonal bipyramids,  an edgeedge with one Dy(1)O7 pentagonal bipyramid, edges with two equivalent Dy(2)O7 pentagonal bipyramids, and edges with two equivalent Nb(2)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 1.99 Å. The Mg(1)-O(3) bond length is 1.98 Å. The Mg(1)-O(5) bond length is 2.00 Å. The Mg(1)-O(6) bond length is 2.04 Å. The Mg(1)-O(7) bond length is 2.07 Å. There are two inequivalent Dy sites. In the first Dy site, Dy(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form distorted DyO7 pentagonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids, corners with five equivalent Nb(1)O5 trigonal bipyramids, edges with three equivalent Dy(2)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O5 trigonal bipyramid, and edges with two equivalent Nb(2)O5 trigonal bipyramids. The Dy(1)-O(1) bond length is 2.57 Å. The Dy(1)-O(2) bond length is 2.31 Å. The Dy(1)-O(3) bond length is 2.43 Å. The Dy(1)-O(4) bond length is 2.27 Å. The Dy(1)-O(5) bond length is 2.25 Å. The Dy(1)-O(6) bond length is 2.46 Å. The Dy(1)-O(8) bond length is 2.44 Å. In the second Dy site, Dy(2) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted DyO7 pentagonal bipyramids that share corners with five equivalent Nb(2)O5 trigonal bipyramids, edges with three equivalent Dy(1)O7 pentagonal bipyramids, edges with two equivalent Mg(1)O5 trigonal bipyramids, and edges with two equivalent Nb(1)O5 trigonal bipyramids. The Dy(2)-O(1) bond length is 2.43 Å. The Dy(2)-O(2) bond length is 2.37 Å. The Dy(2)-O(4) bond length is 2.24 Å. The Dy(2)-O(5) bond length is 2.37 Å. The Dy(2)-O(6) bond length is 2.32 Å. The Dy(2)-O(7) bond length is 2.34 Å. The Dy(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(3), one O(4), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Dy(1)O7 pentagonal bipyramids, corners with two equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Dy(2)O7 pentagonal bipyramids, and  an edgeedge with one Nb(2)O5 trigonal bipyramid. The Nb(1)-O(1) bond length is 2.11 Å. The Nb(1)-O(2) bond length is 1.93 Å. The Nb(1)-O(3) bond length is 1.90 Å. The Nb(1)-O(4) bond length is 2.02 Å. The Nb(1)-O(8) bond length is 2.39 Å. In the second Nb site, Nb(2) is bonded to one O(1), one O(5), one O(6), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with five equivalent Dy(2)O7 pentagonal bipyramids, edges with two equivalent Dy(1)O7 pentagonal bipyramids,  an edgeedge with one Nb(1)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The Nb(2)-O(1) bond length is 2.29 Å. The Nb(2)-O(5) bond length is 2.06 Å. The Nb(2)-O(6) bond length is 2.01 Å. The Nb(2)-O(7) bond length is 1.87 Å. The Nb(2)-O(8) bond length is 2.04 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Dy(1), one Dy(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted corner and edge-sharing ODy2MgNb2 square pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Dy(1), one Dy(2), and one Nb(1) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Mg(1), one Dy(1), and one Nb(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal non-coplanar geometry to one Dy(1), one Dy(2), and one Nb(1) atom. In the fifth O site, O(5) is bonded to one Mg(1), one Dy(1), one Dy(2), and one Nb(2) atom to form distorted ODy2MgNb trigonal pyramids that share corners with four equivalent O(1)Dy2MgNb2 square pyramids, corners with two equivalent O(6)Dy2MgNb trigonal pyramids, and  an edgeedge with one O(6)Dy2MgNb trigonal pyramid. In the sixth O site, O(6) is bonded to one Mg(1), one Dy(1), one Dy(2), and one Nb(2) atom to form distorted ODy2MgNb trigonal pyramids that share corners with two equivalent O(5)Dy2MgNb trigonal pyramids, edges with two equivalent O(1)Dy2MgNb2 square pyramids, and  an edgeedge with one O(5)Dy2MgNb trigonal pyramid. In the seventh O site, O(7) is bonded in a T-shaped geometry to one Mg(1), one Dy(2), and one Nb(2) atom. In the eighth O site, O(8) is bonded in a rectangular see-saw-like geometry to one Dy(1), one Dy(2), one Nb(1), and one Nb(2) atom.