Na3AlSe3 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 4-coordinate geometry to one Se(1), two equivalent Se(2), and two equivalent Se(3) atoms. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to two equivalent Se(2) and two equivalent Se(3) atoms. In the third Na site, Na(3) is bonded in a 4-coordinate geometry to two equivalent Se(2) and two equivalent Se(3) atoms. Al(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form distorted edge-sharing AlSe4 tetrahedra. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Na(1) and two equivalent Al(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent Na(1), two equivalent Na(2), two equivalent Na(3), and one Al(1) atom. In the third Se site, Se(3) is bonded in a 7-coordinate geometry to two equivalent Na(1), two equivalent Na(2), two equivalent Na(3), and one Al(1) atom.
Na3AlSe3 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a 4-coordinate geometry to one Se(1), two equivalent Se(2), and two equivalent Se(3) atoms. The Na(1)-Se(1) bond length is 2.92 Å. There is one shorter (3.03 Å) and one longer (3.11 Å) Na(1)-Se(2) bond length. There is one shorter (2.92 Å) and one longer (3.32 Å) Na(1)-Se(3) bond length. In the second Na site, Na(2) is bonded in a 4-coordinate geometry to two equivalent Se(2) and two equivalent Se(3) atoms. There is one shorter (2.89 Å) and one longer (3.07 Å) Na(2)-Se(2) bond length. There is one shorter (2.87 Å) and one longer (3.03 Å) Na(2)-Se(3) bond length. In the third Na site, Na(3) is bonded in a 4-coordinate geometry to two equivalent Se(2) and two equivalent Se(3) atoms. There is one shorter (2.97 Å) and one longer (3.03 Å) Na(3)-Se(2) bond length. There is one shorter (2.97 Å) and one longer (3.03 Å) Na(3)-Se(3) bond length. Al(1) is bonded to one Se(2), one Se(3), and two equivalent Se(1) atoms to form distorted edge-sharing AlSe4 tetrahedra. The Al(1)-Se(2) bond length is 2.35 Å. The Al(1)-Se(3) bond length is 2.35 Å. There is one shorter (2.40 Å) and one longer (2.44 Å) Al(1)-Se(1) bond length. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded in a 3-coordinate geometry to one Na(1) and two equivalent Al(1) atoms. In the second Se site, Se(2) is bonded in a 7-coordinate geometry to two equivalent Na(1), two equivalent Na(2), two equivalent Na(3), and one Al(1) atom. In the third Se site, Se(3) is bonded in a 7-coordinate geometry to two equivalent Na(1), two equivalent Na(2), two equivalent Na(3), and one Al(1) atom.