Ba2MgNb4O12 crystallizes in the triclinic P-1 space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form distorted BaO6 octahedra that share corners with two equivalent Nb(2)O5 trigonal bipyramids, corners with four equivalent Nb(1)O5 trigonal bipyramids, and edges with two equivalent Nb(1)O5 trigonal bipyramids. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to two equivalent Mg(1), two equivalent O(1), two equivalent O(2), and two equivalent O(5) atoms. Mg(1) is bonded in a distorted square co-planar geometry to two equivalent Ba(2), two equivalent O(4), and two equivalent O(5) atoms. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), and two equivalent O(6) atoms to form distorted NbO5 trigonal bipyramids that share corners with two equivalent Ba(1)O6 octahedra, corners with two equivalent Nb(2)O5 trigonal bipyramids,  an edgeedge with one Ba(1)O6 octahedra, and  an edgeedge with one Nb(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 48-56°. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Ba(1)O6 octahedra, corners with two equivalent Nb(1)O5 trigonal bipyramids, and  an edgeedge with one Nb(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 65°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(2), and one Nb(1) atom. In the second O site, O(2) is bonded in a distorted water-like geometry to one Ba(2) and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ba(1), one Nb(1), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Ba(2), one Mg(1), and two equivalent Nb(2) atoms. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Ba(1) and two equivalent Nb(1) atoms.
Ba2MgNb4O12 crystallizes in the triclinic P-1 space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(6) atoms to form distorted BaO6 octahedra that share corners with two equivalent Nb(2)O5 trigonal bipyramids, corners with four equivalent Nb(1)O5 trigonal bipyramids, and edges with two equivalent Nb(1)O5 trigonal bipyramids. Both Ba(1)-O(1) bond lengths are 2.72 Å. Both Ba(1)-O(3) bond lengths are 2.76 Å. Both Ba(1)-O(6) bond lengths are 2.80 Å. In the second Ba site, Ba(2) is bonded in a 8-coordinate geometry to two equivalent Mg(1), two equivalent O(1), two equivalent O(2), and two equivalent O(5) atoms. Both Ba(2)-Mg(1) bond lengths are 3.04 Å. Both Ba(2)-O(1) bond lengths are 3.34 Å. Both Ba(2)-O(2) bond lengths are 2.69 Å. Both Ba(2)-O(5) bond lengths are 2.81 Å. Mg(1) is bonded in a distorted square co-planar geometry to two equivalent Ba(2), two equivalent O(4), and two equivalent O(5) atoms. Both Mg(1)-O(4) bond lengths are 2.07 Å. Both Mg(1)-O(5) bond lengths are 1.95 Å. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(3), one O(4), and two equivalent O(6) atoms to form distorted NbO5 trigonal bipyramids that share corners with two equivalent Ba(1)O6 octahedra, corners with two equivalent Nb(2)O5 trigonal bipyramids,  an edgeedge with one Ba(1)O6 octahedra, and  an edgeedge with one Nb(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 48-56°. The Nb(1)-O(1) bond length is 1.82 Å. The Nb(1)-O(3) bond length is 2.01 Å. The Nb(1)-O(4) bond length is 2.05 Å. There is one shorter (1.99 Å) and one longer (2.05 Å) Nb(1)-O(6) bond length. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), and two equivalent O(5) atoms to form distorted NbO5 trigonal bipyramids that share  a cornercorner with one Ba(1)O6 octahedra, corners with two equivalent Nb(1)O5 trigonal bipyramids, and  an edgeedge with one Nb(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 65°. The Nb(2)-O(2) bond length is 1.81 Å. The Nb(2)-O(3) bond length is 1.96 Å. The Nb(2)-O(4) bond length is 2.03 Å. There is one shorter (2.03 Å) and one longer (2.18 Å) Nb(2)-O(5) bond length. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 1-coordinate geometry to one Ba(1), one Ba(2), and one Nb(1) atom. In the second O site, O(2) is bonded in a distorted water-like geometry to one Ba(2) and one Nb(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Ba(1), one Nb(1), and one Nb(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to one Mg(1), one Nb(1), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Ba(2), one Mg(1), and two equivalent Nb(2) atoms. In the sixth O site, O(6) is bonded in a distorted trigonal non-coplanar geometry to one Ba(1) and two equivalent Nb(1) atoms.