Ba3MgNb2MnO9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one Mg(1), one O(4), two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms. In the second Ba site, Ba(2) is bonded to two equivalent O(2), two equivalent O(4), four equivalent O(1), and four equivalent O(3) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(2)O12 cuboctahedra, faces with two equivalent Mn(1)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. Mg(1) is bonded in a square co-planar geometry to two equivalent Ba(1) and four equivalent O(1) atoms. Nb(1) is bonded to one O(2), one O(4), two equivalent O(1), and two equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and faces with three equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. Mn(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MnO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra and faces with two equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 5-9°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ba(1), one Ba(2), one Mg(1), one Nb(1), and one Mn(1) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to one Ba(2), three equivalent Ba(1), one Nb(1), and one Mn(1) atom. In the third O site, O(3) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Nb(1) atoms to form a mixture of distorted corner and face-sharing OBa4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 0-57°. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Nb(1) atoms.
Ba3MgNb2MnO9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 9-coordinate geometry to one Mg(1), one O(4), two equivalent O(1), two equivalent O(3), and three equivalent O(2) atoms. The Ba(1)-Mg(1) bond length is 2.91 Å. The Ba(1)-O(4) bond length is 3.17 Å. Both Ba(1)-O(1) bond lengths are 2.76 Å. Both Ba(1)-O(3) bond lengths are 2.74 Å. There is one shorter (2.91 Å) and two longer (2.97 Å) Ba(1)-O(2) bond lengths. In the second Ba site, Ba(2) is bonded to two equivalent O(2), two equivalent O(4), four equivalent O(1), and four equivalent O(3) atoms to form BaO12 cuboctahedra that share corners with six equivalent Ba(2)O12 cuboctahedra, faces with two equivalent Mn(1)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. Both Ba(2)-O(2) bond lengths are 3.07 Å. Both Ba(2)-O(4) bond lengths are 2.95 Å. All Ba(2)-O(1) bond lengths are 3.05 Å. All Ba(2)-O(3) bond lengths are 3.09 Å. Mg(1) is bonded in a square co-planar geometry to two equivalent Ba(1) and four equivalent O(1) atoms. All Mg(1)-O(1) bond lengths are 2.00 Å. Nb(1) is bonded to one O(2), one O(4), two equivalent O(1), and two equivalent O(3) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and faces with three equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-9°. The Nb(1)-O(2) bond length is 2.01 Å. The Nb(1)-O(4) bond length is 2.19 Å. Both Nb(1)-O(1) bond lengths are 2.11 Å. Both Nb(1)-O(3) bond lengths are 2.07 Å. Mn(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form MnO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra and faces with two equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 5-9°. Both Mn(1)-O(2) bond lengths are 2.63 Å. All Mn(1)-O(1) bond lengths are 2.11 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ba(1), one Ba(2), one Mg(1), one Nb(1), and one Mn(1) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to one Ba(2), three equivalent Ba(1), one Nb(1), and one Mn(1) atom. In the third O site, O(3) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Nb(1) atoms to form a mixture of distorted corner and face-sharing OBa4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 0-57°. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Nb(1) atoms.