Ba2MgNb4O12 crystallizes in the monoclinic Pm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one O(3), two equivalent O(10), two equivalent O(12), two equivalent O(5), and two equivalent O(8) atoms. In the second Ba site, Ba(2) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), two equivalent O(11), and two equivalent O(9) atoms. Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form MgO5 square pyramids that share corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(4)O6 octahedra. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(10), one O(5), one O(6), one O(7), one O(8), and two equivalent O(1) atoms. In the second Nb site, Nb(2) is bonded to one O(5), one O(6), one O(7), one O(9), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(4)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles are 8°. In the third Nb site, Nb(3) is bonded to one O(10), one O(11), one O(12), one O(8), and two equivalent O(3) atoms to form distorted corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles range from 21-39°. In the fourth Nb site, Nb(4) is bonded to one O(11), one O(12), one O(7), one O(9), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with two equivalent Nb(4)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles range from 25-39°. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1) and two equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Ba(2), one Mg(1), and two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(2), and two equivalent Nb(3) atoms. In the fourth O site, O(4) is bonded to one Ba(2), one Mg(1), and two equivalent Nb(4) atoms to form distorted OBaMgNb2 trigonal pyramids that share corners with four equivalent O(9)Ba2Nb2 tetrahedra and corners with two equivalent O(4)BaMgNb2 trigonal pyramids. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Nb(1), one Nb(2), and one Nb(4) atom. In the eighth O site, O(8) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded to two equivalent Ba(2), one Nb(2), and one Nb(4) atom to form distorted OBa2Nb2 tetrahedra that share corners with two equivalent O(9)Ba2Nb2 tetrahedra and corners with four equivalent O(4)BaMgNb2 trigonal pyramids. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(1), and one Nb(3) atom. In the eleventh O site, O(11) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ba(2), one Nb(3), and one Nb(4) atom. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(3), and one Nb(4) atom.
Ba2MgNb4O12 crystallizes in the monoclinic Pm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one O(3), two equivalent O(10), two equivalent O(12), two equivalent O(5), and two equivalent O(8) atoms. The Ba(1)-O(3) bond length is 3.06 Å. Both Ba(1)-O(10) bond lengths are 2.89 Å. Both Ba(1)-O(12) bond lengths are 3.00 Å. Both Ba(1)-O(5) bond lengths are 2.81 Å. Both Ba(1)-O(8) bond lengths are 2.98 Å. In the second Ba site, Ba(2) is bonded in a 7-coordinate geometry to one O(2), one O(3), one O(4), two equivalent O(11), and two equivalent O(9) atoms. The Ba(2)-O(2) bond length is 2.94 Å. The Ba(2)-O(3) bond length is 2.75 Å. The Ba(2)-O(4) bond length is 2.83 Å. Both Ba(2)-O(11) bond lengths are 2.72 Å. Both Ba(2)-O(9) bond lengths are 2.61 Å. Mg(1) is bonded to one O(1), one O(2), one O(4), and two equivalent O(7) atoms to form MgO5 square pyramids that share corners with two equivalent Mg(1)O5 square pyramids, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Nb(4)O6 octahedra. The Mg(1)-O(1) bond length is 2.08 Å. The Mg(1)-O(2) bond length is 2.04 Å. The Mg(1)-O(4) bond length is 2.10 Å. Both Mg(1)-O(7) bond lengths are 2.04 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 6-coordinate geometry to one O(10), one O(5), one O(6), one O(7), one O(8), and two equivalent O(1) atoms. The Nb(1)-O(10) bond length is 2.13 Å. The Nb(1)-O(5) bond length is 2.16 Å. The Nb(1)-O(6) bond length is 2.04 Å. The Nb(1)-O(7) bond length is 2.68 Å. The Nb(1)-O(8) bond length is 2.00 Å. Both Nb(1)-O(1) bond lengths are 2.06 Å. In the second Nb site, Nb(2) is bonded to one O(5), one O(6), one O(7), one O(9), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(4)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles are 8°. The Nb(2)-O(5) bond length is 1.95 Å. The Nb(2)-O(6) bond length is 1.89 Å. The Nb(2)-O(7) bond length is 2.34 Å. The Nb(2)-O(9) bond length is 2.01 Å. Both Nb(2)-O(2) bond lengths are 2.02 Å. In the third Nb site, Nb(3) is bonded to one O(10), one O(11), one O(12), one O(8), and two equivalent O(3) atoms to form distorted corner-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles range from 21-39°. The Nb(3)-O(10) bond length is 1.94 Å. The Nb(3)-O(11) bond length is 2.07 Å. The Nb(3)-O(12) bond length is 2.36 Å. The Nb(3)-O(8) bond length is 2.06 Å. Both Nb(3)-O(3) bond lengths are 2.05 Å. In the fourth Nb site, Nb(4) is bonded to one O(11), one O(12), one O(7), one O(9), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with two equivalent Nb(4)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, and edges with two equivalent Mg(1)O5 square pyramids. The corner-sharing octahedral tilt angles range from 25-39°. The Nb(4)-O(11) bond length is 2.03 Å. The Nb(4)-O(12) bond length is 1.89 Å. The Nb(4)-O(7) bond length is 2.30 Å. The Nb(4)-O(9) bond length is 2.12 Å. Both Nb(4)-O(4) bond lengths are 2.07 Å. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1) and two equivalent Nb(1) atoms. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Ba(2), one Mg(1), and two equivalent Nb(2) atoms. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Ba(1), one Ba(2), and two equivalent Nb(3) atoms. In the fourth O site, O(4) is bonded to one Ba(2), one Mg(1), and two equivalent Nb(4) atoms to form distorted OBaMgNb2 trigonal pyramids that share corners with four equivalent O(9)Ba2Nb2 tetrahedra and corners with two equivalent O(4)BaMgNb2 trigonal pyramids. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the seventh O site, O(7) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one Nb(1), one Nb(2), and one Nb(4) atom. In the eighth O site, O(8) is bonded in a distorted water-like geometry to two equivalent Ba(1), one Nb(1), and one Nb(3) atom. In the ninth O site, O(9) is bonded to two equivalent Ba(2), one Nb(2), and one Nb(4) atom to form distorted OBa2Nb2 tetrahedra that share corners with two equivalent O(9)Ba2Nb2 tetrahedra and corners with four equivalent O(4)BaMgNb2 trigonal pyramids. In the tenth O site, O(10) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(1), and one Nb(3) atom. In the eleventh O site, O(11) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ba(2), one Nb(3), and one Nb(4) atom. In the twelfth O site, O(12) is bonded in a 4-coordinate geometry to two equivalent Ba(1), one Nb(3), and one Nb(4) atom.