Na4MgCr2O8 crystallizes in the monoclinic C2/m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(3) atoms. In the second Na site, Na(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form NaO6 octahedra that share corners with six equivalent Cr(1)O4 tetrahedra and edges with two equivalent Na(3)O6 octahedra. In the third Na site, Na(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form NaO6 octahedra that share corners with six equivalent Cr(1)O4 tetrahedra and edges with two equivalent Na(2)O6 octahedra. Mg(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(3) atoms. Cr(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form distorted CrO4 tetrahedra that share corners with three equivalent Na(2)O6 octahedra and corners with three equivalent Na(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 46-63°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), one Na(3), one Mg(1), and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Cr(1) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(1), one Na(2), one Na(3), one Mg(1), and one Cr(1) atom.
Na4MgCr2O8 crystallizes in the monoclinic C2/m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(3) atoms. The Na(1)-O(1) bond length is 2.46 Å. The Na(1)-O(2) bond length is 2.22 Å. Both Na(1)-O(3) bond lengths are 2.58 Å. In the second Na site, Na(2) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form NaO6 octahedra that share corners with six equivalent Cr(1)O4 tetrahedra and edges with two equivalent Na(3)O6 octahedra. Both Na(2)-O(2) bond lengths are 2.27 Å. All Na(2)-O(3) bond lengths are 2.78 Å. In the third Na site, Na(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form NaO6 octahedra that share corners with six equivalent Cr(1)O4 tetrahedra and edges with two equivalent Na(2)O6 octahedra. Both Na(3)-O(1) bond lengths are 2.50 Å. All Na(3)-O(3) bond lengths are 2.47 Å. Mg(1) is bonded in a 6-coordinate geometry to two equivalent O(1) and four equivalent O(3) atoms. Both Mg(1)-O(1) bond lengths are 2.21 Å. All Mg(1)-O(3) bond lengths are 2.15 Å. Cr(1) is bonded to one O(1), one O(2), and two equivalent O(3) atoms to form distorted CrO4 tetrahedra that share corners with three equivalent Na(2)O6 octahedra and corners with three equivalent Na(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 46-63°. The Cr(1)-O(1) bond length is 1.71 Å. The Cr(1)-O(2) bond length is 1.67 Å. Both Cr(1)-O(3) bond lengths are 1.78 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), one Na(3), one Mg(1), and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Cr(1) atom. In the third O site, O(3) is bonded in a 5-coordinate geometry to one Na(1), one Na(2), one Na(3), one Mg(1), and one Cr(1) atom.