NaAlSe2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), and two equivalent Se(4) atoms to form distorted NaSe6 pentagonal pyramids that share corners with four equivalent Al(1)Se4 tetrahedra, corners with six equivalent Al(2)Se4 tetrahedra, edges with two equivalent Na(2)Se6 pentagonal pyramids,  an edgeedge with one Al(1)Se4 tetrahedra,  a faceface with one Na(1)Se6 pentagonal pyramid, and faces with two equivalent Na(2)Se6 pentagonal pyramids. In the second Na site, Na(2) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), and two equivalent Se(4) atoms to form distorted NaSe6 pentagonal pyramids that share corners with four equivalent Na(2)Se6 pentagonal pyramids, corners with two equivalent Al(2)Se4 tetrahedra, corners with four equivalent Al(1)Se4 tetrahedra, edges with two equivalent Na(1)Se6 pentagonal pyramids,  an edgeedge with one Al(1)Se4 tetrahedra, edges with two equivalent Al(2)Se4 tetrahedra, and faces with two equivalent Na(1)Se6 pentagonal pyramids. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one Se(1), one Se(3), one Se(4), and one Se(5) atom to form AlSe4 tetrahedra that share corners with four equivalent Na(1)Se6 pentagonal pyramids, corners with four equivalent Na(2)Se6 pentagonal pyramids,  a cornercorner with one Al(1)Se4 tetrahedra, corners with three equivalent Al(2)Se4 tetrahedra,  an edgeedge with one Na(1)Se6 pentagonal pyramid, and  an edgeedge with one Na(2)Se6 pentagonal pyramid. In the second Al site, Al(2) is bonded to one Se(2), one Se(3), one Se(4), and one Se(5) atom to form AlSe4 tetrahedra that share corners with two equivalent Na(2)Se6 pentagonal pyramids, corners with six equivalent Na(1)Se6 pentagonal pyramids,  a cornercorner with one Al(2)Se4 tetrahedra, corners with three equivalent Al(1)Se4 tetrahedra, and edges with two equivalent Na(2)Se6 pentagonal pyramids. There are five inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), and two equivalent Al(1) atoms. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), and two equivalent Al(2) atoms. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Na(1), one Na(2), one Al(1), and one Al(2) atom. In the fourth Se site, Se(4) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), one Al(1), and one Al(2) atom. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Na(1), one Na(2), one Al(1), and one Al(2) atom.
NaAlSe2 crystallizes in the monoclinic C2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), and two equivalent Se(4) atoms to form distorted NaSe6 pentagonal pyramids that share corners with four equivalent Al(1)Se4 tetrahedra, corners with six equivalent Al(2)Se4 tetrahedra, edges with two equivalent Na(2)Se6 pentagonal pyramids,  an edgeedge with one Al(1)Se4 tetrahedra,  a faceface with one Na(1)Se6 pentagonal pyramid, and faces with two equivalent Na(2)Se6 pentagonal pyramids. The Na(1)-Se(1) bond length is 3.15 Å. The Na(1)-Se(2) bond length is 3.16 Å. The Na(1)-Se(3) bond length is 2.99 Å. The Na(1)-Se(5) bond length is 2.98 Å. There is one shorter (3.14 Å) and one longer (3.17 Å) Na(1)-Se(4) bond length. In the second Na site, Na(2) is bonded to one Se(1), one Se(2), one Se(3), one Se(5), and two equivalent Se(4) atoms to form distorted NaSe6 pentagonal pyramids that share corners with four equivalent Na(2)Se6 pentagonal pyramids, corners with two equivalent Al(2)Se4 tetrahedra, corners with four equivalent Al(1)Se4 tetrahedra, edges with two equivalent Na(1)Se6 pentagonal pyramids,  an edgeedge with one Al(1)Se4 tetrahedra, edges with two equivalent Al(2)Se4 tetrahedra, and faces with two equivalent Na(1)Se6 pentagonal pyramids. The Na(2)-Se(1) bond length is 3.17 Å. The Na(2)-Se(2) bond length is 3.17 Å. The Na(2)-Se(3) bond length is 3.08 Å. The Na(2)-Se(5) bond length is 3.02 Å. There is one shorter (3.16 Å) and one longer (3.18 Å) Na(2)-Se(4) bond length. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one Se(1), one Se(3), one Se(4), and one Se(5) atom to form AlSe4 tetrahedra that share corners with four equivalent Na(1)Se6 pentagonal pyramids, corners with four equivalent Na(2)Se6 pentagonal pyramids,  a cornercorner with one Al(1)Se4 tetrahedra, corners with three equivalent Al(2)Se4 tetrahedra,  an edgeedge with one Na(1)Se6 pentagonal pyramid, and  an edgeedge with one Na(2)Se6 pentagonal pyramid. The Al(1)-Se(1) bond length is 2.39 Å. The Al(1)-Se(3) bond length is 2.38 Å. The Al(1)-Se(4) bond length is 2.39 Å. The Al(1)-Se(5) bond length is 2.38 Å. In the second Al site, Al(2) is bonded to one Se(2), one Se(3), one Se(4), and one Se(5) atom to form AlSe4 tetrahedra that share corners with two equivalent Na(2)Se6 pentagonal pyramids, corners with six equivalent Na(1)Se6 pentagonal pyramids,  a cornercorner with one Al(2)Se4 tetrahedra, corners with three equivalent Al(1)Se4 tetrahedra, and edges with two equivalent Na(2)Se6 pentagonal pyramids. The Al(2)-Se(2) bond length is 2.39 Å. The Al(2)-Se(3) bond length is 2.38 Å. The Al(2)-Se(4) bond length is 2.39 Å. The Al(2)-Se(5) bond length is 2.37 Å. There are five inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), and two equivalent Al(1) atoms. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), and two equivalent Al(2) atoms. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Na(1), one Na(2), one Al(1), and one Al(2) atom. In the fourth Se site, Se(4) is bonded in a 6-coordinate geometry to two equivalent Na(1), two equivalent Na(2), one Al(1), and one Al(2) atom. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to one Na(1), one Na(2), one Al(1), and one Al(2) atom.