MgNb2O5 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with four equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Nb(2)O6 octahedra, and edges with four equivalent Nb(1)O6 octahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(5) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with four equivalent Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with four equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-38°. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-40°. There are five inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a T-shaped geometry to one Mg(1) and two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Nb(1) and two equivalent Nb(2) atoms. In the fifth O site, O(5) is bonded to two equivalent Mg(1), one Nb(2), and two equivalent Nb(1) atoms to form distorted corner-sharing OMg2Nb3 trigonal bipyramids.
MgNb2O5 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(2), two equivalent O(1), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with four equivalent Mg(1)O5 trigonal bipyramids, edges with two equivalent Nb(2)O6 octahedra, and edges with four equivalent Nb(1)O6 octahedra. The Mg(1)-O(2) bond length is 1.99 Å. Both Mg(1)-O(1) bond lengths are 2.10 Å. Both Mg(1)-O(5) bond lengths are 2.09 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(5) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with four equivalent Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with four equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-38°. The Nb(1)-O(3) bond length is 1.92 Å. The Nb(1)-O(4) bond length is 2.40 Å. Both Nb(1)-O(1) bond lengths are 2.08 Å. Both Nb(1)-O(5) bond lengths are 2.06 Å. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), two equivalent O(2), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-40°. The Nb(2)-O(3) bond length is 1.91 Å. The Nb(2)-O(5) bond length is 2.43 Å. Both Nb(2)-O(2) bond lengths are 2.08 Å. Both Nb(2)-O(4) bond lengths are 2.07 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1) and two equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a T-shaped geometry to one Mg(1) and two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one Nb(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Nb(1) and two equivalent Nb(2) atoms. In the fifth O site, O(5) is bonded to two equivalent Mg(1), one Nb(2), and two equivalent Nb(1) atoms to form distorted corner-sharing OMg2Nb3 trigonal bipyramids.