MgYb2Nb2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Yb(2)O6 pentagonal pyramids, and  an edgeedge with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. There are two inequivalent Yb sites. In the first Yb site, Yb(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. In the second Yb site, Yb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted YbO6 pentagonal pyramids that share corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles are 37°. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share edges with two equivalent Nb(2)O6 octahedra, edges with two equivalent Yb(2)O6 pentagonal pyramids, and  an edgeedge with one Mg(1)O4 tetrahedra. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Yb(2)O6 pentagonal pyramids, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Yb(2)O6 pentagonal pyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1), one Yb(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Yb(2), and one Nb(2) atom. In the third O site, O(3) is bonded to one Yb(1), one Yb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OYb2Nb2 trigonal pyramids. In the fourth O site, O(4) is bonded to one Yb(1), one Yb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OYb2Nb2 trigonal pyramids.
MgYb2Nb2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(1) and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Yb(2)O6 pentagonal pyramids, and  an edgeedge with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles are 52°. Both Mg(1)-O(1) bond lengths are 2.01 Å. Both Mg(1)-O(2) bond lengths are 1.98 Å. There are two inequivalent Yb sites. In the first Yb site, Yb(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms. Both Yb(1)-O(1) bond lengths are 2.31 Å. Both Yb(1)-O(3) bond lengths are 2.44 Å. Both Yb(1)-O(4) bond lengths are 2.30 Å. In the second Yb site, Yb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted YbO6 pentagonal pyramids that share corners with two equivalent Nb(2)O6 octahedra, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles are 37°. Both Yb(2)-O(2) bond lengths are 2.31 Å. Both Yb(2)-O(3) bond lengths are 2.26 Å. Both Yb(2)-O(4) bond lengths are 2.32 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share edges with two equivalent Nb(2)O6 octahedra, edges with two equivalent Yb(2)O6 pentagonal pyramids, and  an edgeedge with one Mg(1)O4 tetrahedra. Both Nb(1)-O(1) bond lengths are 1.91 Å. Both Nb(1)-O(3) bond lengths are 2.21 Å. Both Nb(1)-O(4) bond lengths are 2.00 Å. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Yb(2)O6 pentagonal pyramids, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Yb(2)O6 pentagonal pyramids. Both Nb(2)-O(2) bond lengths are 1.91 Å. Both Nb(2)-O(3) bond lengths are 1.99 Å. Both Nb(2)-O(4) bond lengths are 2.25 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1), one Yb(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Yb(2), and one Nb(2) atom. In the third O site, O(3) is bonded to one Yb(1), one Yb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OYb2Nb2 trigonal pyramids. In the fourth O site, O(4) is bonded to one Yb(1), one Yb(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge and corner-sharing OYb2Nb2 trigonal pyramids.