MgNb2Al2O8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(1), one O(4), one O(6), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Al(1)O6 octahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Al(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 6-34°. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-33°. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(5), one O(7), and two equivalent O(4) atoms to form AlO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Al(1)O6 octahedra,  an edgeedge with one 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 2-19°. In the second Al site, Al(2) is bonded in a 6-coordinate geometry to one O(1), one O(4), one O(6), one O(8), and two equivalent O(3) atoms. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Nb(2), and one Al(2) atom to form OMgNb2Al trigonal pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the third O site, O(3) is bonded to one Nb(2), one Al(1), and two equivalent Al(2) atoms to form distorted ONbAl3 trigonal pyramids that share corners with two equivalent O(3)NbAl3 trigonal pyramids, edges with two equivalent O(4)MgNbAl3 square pyramids, and edges with two equivalent O(1)MgNb2Al trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Nb(1), one Al(2), and two equivalent Al(1) atoms to form distorted OMgNbAl3 square pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Nb(2), and one Al(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Al(2) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Al(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(2), and one Al(2) atom.
MgNb2Al2O8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(1), one O(4), one O(6), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Al(1)O6 octahedra. The Mg(1)-O(1) bond length is 2.03 Å. The Mg(1)-O(4) bond length is 2.02 Å. The Mg(1)-O(6) bond length is 2.02 Å. Both Mg(1)-O(5) bond lengths are 1.98 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Al(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 6-34°. The Nb(1)-O(4) bond length is 2.34 Å. The Nb(1)-O(6) bond length is 2.05 Å. The Nb(1)-O(7) bond length is 2.04 Å. The Nb(1)-O(8) bond length is 2.14 Å. Both Nb(1)-O(2) bond lengths are 1.99 Å. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-33°. The Nb(2)-O(3) bond length is 2.33 Å. The Nb(2)-O(5) bond length is 2.24 Å. The Nb(2)-O(7) bond length is 2.04 Å. The Nb(2)-O(8) bond length is 2.01 Å. Both Nb(2)-O(1) bond lengths are 1.99 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(5), one O(7), and two equivalent O(4) atoms to form AlO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Al(1)O6 octahedra,  an edgeedge with one 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 2-19°. The Al(1)-O(2) bond length is 2.06 Å. The Al(1)-O(3) bond length is 1.93 Å. The Al(1)-O(5) bond length is 1.94 Å. The Al(1)-O(7) bond length is 1.88 Å. Both Al(1)-O(4) bond lengths are 1.93 Å. In the second Al site, Al(2) is bonded in a 6-coordinate geometry to one O(1), one O(4), one O(6), one O(8), and two equivalent O(3) atoms. The Al(2)-O(1) bond length is 2.23 Å. The Al(2)-O(4) bond length is 2.33 Å. The Al(2)-O(6) bond length is 1.79 Å. The Al(2)-O(8) bond length is 1.82 Å. Both Al(2)-O(3) bond lengths are 2.01 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Nb(2), and one Al(2) atom to form OMgNb2Al trigonal pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the third O site, O(3) is bonded to one Nb(2), one Al(1), and two equivalent Al(2) atoms to form distorted ONbAl3 trigonal pyramids that share corners with two equivalent O(3)NbAl3 trigonal pyramids, edges with two equivalent O(4)MgNbAl3 square pyramids, and edges with two equivalent O(1)MgNb2Al trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Nb(1), one Al(2), and two equivalent Al(1) atoms to form distorted OMgNbAl3 square pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Nb(2), and one Al(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Al(2) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Al(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(2), and one Al(2) atom.