NaMgFe(SeO4)2 crystallizes in the monoclinic Cm space group. Na(1) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form NaO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra, and edges with two equivalent Fe(1)O6 octahedra. Mg(1) is bonded to one O(2), one O(5), and two equivalent O(1) atoms to form distorted MgO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, corners with two equivalent Se(1)O4 tetrahedra, and  an edgeedge with one Se(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 53-73°. Fe(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form distorted FeO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra, and edges with two equivalent Na(1)O6 octahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(2), one O(5), and two equivalent O(3) atoms to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and corners with two equivalent Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 40-78°. In the second Se site, Se(2) is bonded to one O(4), one O(6), and two equivalent O(1) atoms to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 20-67°. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Na(1), one Mg(1), one Fe(1), and one Se(2) atom to form distorted edge-sharing ONaMgFeSe tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Mg(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Fe(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Fe(1) and one Se(2) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one Mg(1), one Fe(1), and one Se(1) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Na(1) and one Se(2) atom.
NaMgFe(SeO4)2 crystallizes in the monoclinic Cm space group. Na(1) is bonded to one O(2), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form NaO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra, and edges with two equivalent Fe(1)O6 octahedra. The Na(1)-O(2) bond length is 2.66 Å. The Na(1)-O(6) bond length is 2.50 Å. Both Na(1)-O(1) bond lengths are 2.53 Å. Both Na(1)-O(3) bond lengths are 2.41 Å. Mg(1) is bonded to one O(2), one O(5), and two equivalent O(1) atoms to form distorted MgO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, corners with two equivalent Se(1)O4 tetrahedra, and  an edgeedge with one Se(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 53-73°. The Mg(1)-O(2) bond length is 1.95 Å. The Mg(1)-O(5) bond length is 1.93 Å. Both Mg(1)-O(1) bond lengths are 2.09 Å. Fe(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form distorted FeO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra, and edges with two equivalent Na(1)O6 octahedra. The Fe(1)-O(4) bond length is 2.25 Å. The Fe(1)-O(5) bond length is 1.97 Å. Both Fe(1)-O(1) bond lengths are 2.36 Å. Both Fe(1)-O(3) bond lengths are 2.13 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(2), one O(5), and two equivalent O(3) atoms to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and corners with two equivalent Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 40-78°. The Se(1)-O(2) bond length is 1.76 Å. The Se(1)-O(5) bond length is 2.11 Å. Both Se(1)-O(3) bond lengths are 1.72 Å. In the second Se site, Se(2) is bonded to one O(4), one O(6), and two equivalent O(1) atoms to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and  an edgeedge with one Mg(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 20-67°. The Se(2)-O(4) bond length is 1.66 Å. The Se(2)-O(6) bond length is 1.63 Å. Both Se(2)-O(1) bond lengths are 1.73 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Na(1), one Mg(1), one Fe(1), and one Se(2) atom to form distorted edge-sharing ONaMgFeSe tetrahedra. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Mg(1), and one Se(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Fe(1), and one Se(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Fe(1) and one Se(2) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one Mg(1), one Fe(1), and one Se(1) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Na(1) and one Se(2) atom.