Ba2Mg(NiO3)2 crystallizes in the monoclinic C2/m space group. Ba(1) is bonded in a 7-coordinate geometry to three equivalent O(2) and four equivalent O(1) atoms. Mg(1) is bonded in a square co-planar geometry to four equivalent O(1) atoms. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form face-sharing NiO6 octahedra. In the second Ni site, Ni(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form distorted face-sharing NiO6 octahedra. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to two equivalent Ba(1), one Mg(1), one Ni(1), and one Ni(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to three equivalent Ba(1), one Ni(1), and one Ni(2) atom.
Ba2Mg(NiO3)2 crystallizes in the monoclinic C2/m space group. Ba(1) is bonded in a 7-coordinate geometry to three equivalent O(2) and four equivalent O(1) atoms. There is one shorter (2.65 Å) and two longer (2.92 Å) Ba(1)-O(2) bond lengths. There are two shorter (2.70 Å) and two longer (2.82 Å) Ba(1)-O(1) bond lengths. Mg(1) is bonded in a square co-planar geometry to four equivalent O(1) atoms. All Mg(1)-O(1) bond lengths are 1.99 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form face-sharing NiO6 octahedra. Both Ni(1)-O(2) bond lengths are 1.92 Å. All Ni(1)-O(1) bond lengths are 2.09 Å. In the second Ni site, Ni(2) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form distorted face-sharing NiO6 octahedra. Both Ni(2)-O(2) bond lengths are 2.32 Å. All Ni(2)-O(1) bond lengths are 1.89 Å. There are two inequivalent O sites. In the first O site, O(1) is bonded in a 5-coordinate geometry to two equivalent Ba(1), one Mg(1), one Ni(1), and one Ni(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to three equivalent Ba(1), one Ni(1), and one Ni(2) atom.