Na3MgVO4 crystallizes in the monoclinic Cm space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 2-coordinate geometry to two equivalent O(2) atoms. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to one O(1), one O(3), and two equivalent O(2) atoms. Mg(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent V(1)O4 tetrahedra and  an edgeedge with one V(1)O4 tetrahedra. V(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form VO4 tetrahedra that share corners with two equivalent Mg(1)O4 tetrahedra and  an edgeedge with one Mg(1)O4 tetrahedra. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Na(2), one Mg(1), and one V(1) atom to form a mixture of distorted corner and edge-sharing ONa2MgV trigonal pyramids. In the second O site, O(2) is bonded to one Na(1), two equivalent Na(2), one Mg(1), and one V(1) atom to form distorted ONa3MgV trigonal bipyramids that share corners with three equivalent O(2)Na3MgV trigonal bipyramids, corners with two equivalent O(1)Na2MgV trigonal pyramids,  an edgeedge with one O(2)Na3MgV trigonal bipyramid, and  an edgeedge with one O(1)Na2MgV trigonal pyramid. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Na(2), one Mg(1), and one V(1) atom.
Na3MgVO4 crystallizes in the monoclinic Cm space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 2-coordinate geometry to two equivalent O(2) atoms. Both Na(1)-O(2) bond lengths are 2.55 Å. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to one O(1), one O(3), and two equivalent O(2) atoms. The Na(2)-O(1) bond length is 2.27 Å. The Na(2)-O(3) bond length is 2.27 Å. Both Na(2)-O(2) bond lengths are 2.41 Å. Mg(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form MgO4 tetrahedra that share corners with two equivalent V(1)O4 tetrahedra and  an edgeedge with one V(1)O4 tetrahedra. The Mg(1)-O(1) bond length is 1.97 Å. The Mg(1)-O(3) bond length is 1.97 Å. Both Mg(1)-O(2) bond lengths are 1.96 Å. V(1) is bonded to one O(1), one O(3), and two equivalent O(2) atoms to form VO4 tetrahedra that share corners with two equivalent Mg(1)O4 tetrahedra and  an edgeedge with one Mg(1)O4 tetrahedra. The V(1)-O(1) bond length is 1.96 Å. The V(1)-O(3) bond length is 1.96 Å. Both V(1)-O(2) bond lengths are 1.95 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Na(2), one Mg(1), and one V(1) atom to form a mixture of distorted corner and edge-sharing ONa2MgV trigonal pyramids. In the second O site, O(2) is bonded to one Na(1), two equivalent Na(2), one Mg(1), and one V(1) atom to form distorted ONa3MgV trigonal bipyramids that share corners with three equivalent O(2)Na3MgV trigonal bipyramids, corners with two equivalent O(1)Na2MgV trigonal pyramids,  an edgeedge with one O(2)Na3MgV trigonal bipyramid, and  an edgeedge with one O(1)Na2MgV trigonal pyramid. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Na(2), one Mg(1), and one V(1) atom.