Na4Mg(NiO3)2 crystallizes in the monoclinic Cm space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 5-coordinate geometry to one O(1), two equivalent O(2), and two equivalent O(3) atoms. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to one O(1), one O(4), and two equivalent O(3) atoms. In the third Na site, Na(3) is bonded in a 1-coordinate geometry to one O(1), one O(4), two equivalent O(2), and four equivalent O(3) atoms. Mg(1) is bonded to one O(1), one O(4), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with six equivalent Ni(1)O5 trigonal bipyramids. Ni(1) is bonded to one O(4), two equivalent O(2), and two equivalent O(3) atoms to form NiO5 trigonal bipyramids that share corners with three equivalent Mg(1)O4 tetrahedra,  a cornercorner with one Ni(1)O5 trigonal bipyramid, and edges with two equivalent Ni(1)O5 trigonal bipyramids. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal bipyramidal geometry to one Na(2), one Na(3), two equivalent Na(1), and one Mg(1) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to one Na(3), two equivalent Na(1), one Mg(1), and two equivalent Ni(1) atoms. In the third O site, O(3) is bonded in a 7-coordinate geometry to one Na(2), two equivalent Na(1), two equivalent Na(3), and two equivalent Ni(1) atoms. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Na(2), one Na(3), one Mg(1), and two equivalent Ni(1) atoms.
Na4Mg(NiO3)2 crystallizes in the monoclinic Cm space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 5-coordinate geometry to one O(1), two equivalent O(2), and two equivalent O(3) atoms. The Na(1)-O(1) bond length is 2.28 Å. There is one shorter (2.36 Å) and one longer (2.73 Å) Na(1)-O(2) bond length. There is one shorter (2.43 Å) and one longer (2.72 Å) Na(1)-O(3) bond length. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to one O(1), one O(4), and two equivalent O(3) atoms. The Na(2)-O(1) bond length is 2.17 Å. The Na(2)-O(4) bond length is 2.46 Å. Both Na(2)-O(3) bond lengths are 2.30 Å. In the third Na site, Na(3) is bonded in a 1-coordinate geometry to one O(1), one O(4), two equivalent O(2), and four equivalent O(3) atoms. The Na(3)-O(1) bond length is 2.22 Å. The Na(3)-O(4) bond length is 2.68 Å. Both Na(3)-O(2) bond lengths are 2.74 Å. There are two shorter (2.56 Å) and two longer (2.92 Å) Na(3)-O(3) bond lengths. Mg(1) is bonded to one O(1), one O(4), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with six equivalent Ni(1)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 1.91 Å. The Mg(1)-O(4) bond length is 2.00 Å. Both Mg(1)-O(2) bond lengths are 2.04 Å. Ni(1) is bonded to one O(4), two equivalent O(2), and two equivalent O(3) atoms to form NiO5 trigonal bipyramids that share corners with three equivalent Mg(1)O4 tetrahedra,  a cornercorner with one Ni(1)O5 trigonal bipyramid, and edges with two equivalent Ni(1)O5 trigonal bipyramids. The Ni(1)-O(4) bond length is 1.97 Å. There is one shorter (1.93 Å) and one longer (2.06 Å) Ni(1)-O(2) bond length. There is one shorter (1.90 Å) and one longer (1.94 Å) Ni(1)-O(3) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal bipyramidal geometry to one Na(2), one Na(3), two equivalent Na(1), and one Mg(1) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to one Na(3), two equivalent Na(1), one Mg(1), and two equivalent Ni(1) atoms. In the third O site, O(3) is bonded in a 7-coordinate geometry to one Na(2), two equivalent Na(1), two equivalent Na(3), and two equivalent Ni(1) atoms. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to one Na(2), one Na(3), one Mg(1), and two equivalent Ni(1) atoms.