Na4Mg(SiSe3)2 crystallizes in the monoclinic C2 space group. There are four inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Na(3)Se6 octahedra, corners with two equivalent Na(4)Se6 octahedra, edges with two equivalent Na(2)Se6 octahedra, edges with two equivalent Na(3)Se6 octahedra, edges with two equivalent Na(4)Se6 octahedra, and edges with three equivalent Mg(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. In the second Na site, Na(2) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(3)Se6 octahedra, and edges with three equivalent Na(4)Se6 octahedra. The corner-sharing octahedral tilt angles range from 7-13°. In the third Na site, Na(3) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, and edges with three equivalent Na(4)Se6 octahedra. The corner-sharing octahedral tilt angles are 6°. In the fourth Na site, Na(4) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, and edges with three equivalent Na(3)Se6 octahedra. The corner-sharing octahedral tilt angles range from 7-8°. Mg(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form MgSe6 octahedra that share corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Na(3)Se6 octahedra, corners with two equivalent Na(4)Se6 octahedra, edges with two equivalent Na(2)Se6 octahedra, edges with two equivalent Na(3)Se6 octahedra, edges with two equivalent Na(4)Se6 octahedra, and edges with three equivalent Na(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-8°. Si(1) is bonded in a distorted trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(3) atom. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-89°. In the second Se site, Se(2) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-89°. In the third Se site, Se(3) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-88°.
Na4Mg(SiSe3)2 crystallizes in the monoclinic C2 space group. There are four inequivalent Na sites. In the first Na site, Na(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Na(3)Se6 octahedra, corners with two equivalent Na(4)Se6 octahedra, edges with two equivalent Na(2)Se6 octahedra, edges with two equivalent Na(3)Se6 octahedra, edges with two equivalent Na(4)Se6 octahedra, and edges with three equivalent Mg(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-13°. Both Na(1)-Se(1) bond lengths are 2.98 Å. Both Na(1)-Se(2) bond lengths are 3.04 Å. Both Na(1)-Se(3) bond lengths are 2.97 Å. In the second Na site, Na(2) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(3)Se6 octahedra, and edges with three equivalent Na(4)Se6 octahedra. The corner-sharing octahedral tilt angles range from 7-13°. Both Na(2)-Se(1) bond lengths are 3.13 Å. Both Na(2)-Se(2) bond lengths are 3.00 Å. Both Na(2)-Se(3) bond lengths are 3.09 Å. In the third Na site, Na(3) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, and edges with three equivalent Na(4)Se6 octahedra. The corner-sharing octahedral tilt angles are 6°. Both Na(3)-Se(1) bond lengths are 2.96 Å. Both Na(3)-Se(2) bond lengths are 3.07 Å. Both Na(3)-Se(3) bond lengths are 2.91 Å. In the fourth Na site, Na(4) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form NaSe6 octahedra that share corners with two equivalent Na(1)Se6 octahedra, corners with two equivalent Mg(1)Se6 octahedra, edges with two equivalent Na(1)Se6 octahedra, edges with two equivalent Mg(1)Se6 octahedra, edges with three equivalent Na(2)Se6 octahedra, and edges with three equivalent Na(3)Se6 octahedra. The corner-sharing octahedral tilt angles range from 7-8°. Both Na(4)-Se(1) bond lengths are 2.94 Å. Both Na(4)-Se(2) bond lengths are 3.05 Å. Both Na(4)-Se(3) bond lengths are 3.15 Å. Mg(1) is bonded to two equivalent Se(1), two equivalent Se(2), and two equivalent Se(3) atoms to form MgSe6 octahedra that share corners with two equivalent Na(2)Se6 octahedra, corners with two equivalent Na(3)Se6 octahedra, corners with two equivalent Na(4)Se6 octahedra, edges with two equivalent Na(2)Se6 octahedra, edges with two equivalent Na(3)Se6 octahedra, edges with two equivalent Na(4)Se6 octahedra, and edges with three equivalent Na(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 6-8°. Both Mg(1)-Se(1) bond lengths are 2.77 Å. Both Mg(1)-Se(2) bond lengths are 2.80 Å. Both Mg(1)-Se(3) bond lengths are 2.82 Å. Si(1) is bonded in a distorted trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(3) atom. The Si(1)-Se(1) bond length is 2.30 Å. The Si(1)-Se(2) bond length is 2.28 Å. The Si(1)-Se(3) bond length is 2.27 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-89°. In the second Se site, Se(2) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-89°. In the third Se site, Se(3) is bonded to one Na(1), one Na(2), one Na(3), one Na(4), one Mg(1), and one Si(1) atom to form a mixture of edge and corner-sharing SeNa4MgSi octahedra. The corner-sharing octahedral tilt angles range from 3-88°.