NaAsSe2 crystallizes in the monoclinic Pc space group. There are four inequivalent Na sites. In the first Na site, Na(1) is bonded to one Se(1), one Se(4), two equivalent Se(3), and two equivalent Se(7) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 11-16°. In the second Na site, Na(2) is bonded to one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(2) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 8-21°. In the third Na site, Na(3) is bonded to one Se(3), one Se(6), two equivalent Se(4), and two equivalent Se(8) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 7-17°. In the fourth Na site, Na(4) is bonded to one Se(2), one Se(8), two equivalent Se(5), and two equivalent Se(6) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 10-16°. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Se(2), one Se(3), and one Se(7) atom. In the second As site, As(2) is bonded in a trigonal non-coplanar geometry to one Se(4), one Se(7), and one Se(8) atom. In the third As site, As(3) is bonded in a 3-coordinate geometry to one Se(4), one Se(5), and one Se(6) atom. In the fourth As site, As(4) is bonded in a trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(5) atom. There are eight inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), two equivalent Na(2), and one As(4) atom. In the second Se site, Se(2) is bonded to one Na(4), two equivalent Na(2), one As(1), and one As(4) atom to form distorted SeNa3As2 trigonal bipyramids that share  a cornercorner with one Se(3)Na4As square pyramid, corners with two equivalent Se(2)Na3As2 trigonal bipyramids,  an edgeedge with one Se(3)Na4As square pyramid, and edges with two equivalent Se(6)Na4As square pyramids. In the third Se site, Se(3) is bonded to one Na(2), one Na(3), two equivalent Na(1), and one As(1) atom to form SeNa4As square pyramids that share corners with two equivalent Se(3)Na4As square pyramids, corners with two equivalent Se(6)Na4As square pyramids,  a cornercorner with one Se(2)Na3As2 trigonal bipyramid,  an edgeedge with one Se(2)Na3As2 trigonal bipyramid, and edges with two equivalent Se(4)Na3As2 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to one Na(1), two equivalent Na(3), one As(2), and one As(3) atom to form distorted SeNa3As2 trigonal bipyramids that share  a cornercorner with one Se(6)Na4As square pyramid, corners with two equivalent Se(4)Na3As2 trigonal bipyramids,  an edgeedge with one Se(6)Na4As square pyramid, and edges with two equivalent Se(3)Na4As square pyramids. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to two equivalent Na(4), one As(3), and one As(4) atom. In the sixth Se site, Se(6) is bonded to one Na(2), one Na(3), two equivalent Na(4), and one As(3) atom to form SeNa4As square pyramids that share corners with two equivalent Se(3)Na4As square pyramids, corners with two equivalent Se(6)Na4As square pyramids,  a cornercorner with one Se(4)Na3As2 trigonal bipyramid,  an edgeedge with one Se(4)Na3As2 trigonal bipyramid, and edges with two equivalent Se(2)Na3As2 trigonal bipyramids. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to two equivalent Na(1), one As(1), and one As(2) atom. In the eighth Se site, Se(8) is bonded in a distorted rectangular see-saw-like geometry to one Na(4), two equivalent Na(3), and one As(2) atom.
NaAsSe2 crystallizes in the monoclinic Pc space group. There are four inequivalent Na sites. In the first Na site, Na(1) is bonded to one Se(1), one Se(4), two equivalent Se(3), and two equivalent Se(7) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 11-16°. The Na(1)-Se(1) bond length is 2.96 Å. The Na(1)-Se(4) bond length is 3.32 Å. There is one shorter (3.00 Å) and one longer (3.02 Å) Na(1)-Se(3) bond length. Both Na(1)-Se(7) bond lengths are 3.03 Å. In the second Na site, Na(2) is bonded to one Se(3), one Se(6), two equivalent Se(1), and two equivalent Se(2) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 8-21°. The Na(2)-Se(3) bond length is 2.98 Å. The Na(2)-Se(6) bond length is 2.95 Å. There is one shorter (2.99 Å) and one longer (3.01 Å) Na(2)-Se(1) bond length. There is one shorter (3.04 Å) and one longer (3.06 Å) Na(2)-Se(2) bond length. In the third Na site, Na(3) is bonded to one Se(3), one Se(6), two equivalent Se(4), and two equivalent Se(8) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 7-17°. The Na(3)-Se(3) bond length is 2.95 Å. The Na(3)-Se(6) bond length is 2.95 Å. There is one shorter (3.01 Å) and one longer (3.05 Å) Na(3)-Se(4) bond length. There is one shorter (3.00 Å) and one longer (3.01 Å) Na(3)-Se(8) bond length. In the fourth Na site, Na(4) is bonded to one Se(2), one Se(8), two equivalent Se(5), and two equivalent Se(6) atoms to form a mixture of corner and edge-sharing NaSe6 octahedra. The corner-sharing octahedral tilt angles range from 10-16°. The Na(4)-Se(2) bond length is 3.32 Å. The Na(4)-Se(8) bond length is 2.97 Å. There is one shorter (3.02 Å) and one longer (3.03 Å) Na(4)-Se(5) bond length. There is one shorter (3.00 Å) and one longer (3.02 Å) Na(4)-Se(6) bond length. There are four inequivalent As sites. In the first As site, As(1) is bonded in a 3-coordinate geometry to one Se(2), one Se(3), and one Se(7) atom. The As(1)-Se(2) bond length is 2.54 Å. The As(1)-Se(3) bond length is 2.34 Å. The As(1)-Se(7) bond length is 2.47 Å. In the second As site, As(2) is bonded in a trigonal non-coplanar geometry to one Se(4), one Se(7), and one Se(8) atom. The As(2)-Se(4) bond length is 2.45 Å. The As(2)-Se(7) bond length is 2.50 Å. The As(2)-Se(8) bond length is 2.33 Å. In the third As site, As(3) is bonded in a 3-coordinate geometry to one Se(4), one Se(5), and one Se(6) atom. The As(3)-Se(4) bond length is 2.55 Å. The As(3)-Se(5) bond length is 2.47 Å. The As(3)-Se(6) bond length is 2.34 Å. In the fourth As site, As(4) is bonded in a trigonal non-coplanar geometry to one Se(1), one Se(2), and one Se(5) atom. The As(4)-Se(1) bond length is 2.33 Å. The As(4)-Se(2) bond length is 2.46 Å. The As(4)-Se(5) bond length is 2.50 Å. There are eight inequivalent Se sites. In the first Se site, Se(1) is bonded in a distorted rectangular see-saw-like geometry to one Na(1), two equivalent Na(2), and one As(4) atom. In the second Se site, Se(2) is bonded to one Na(4), two equivalent Na(2), one As(1), and one As(4) atom to form distorted SeNa3As2 trigonal bipyramids that share  a cornercorner with one Se(3)Na4As square pyramid, corners with two equivalent Se(2)Na3As2 trigonal bipyramids,  an edgeedge with one Se(3)Na4As square pyramid, and edges with two equivalent Se(6)Na4As square pyramids. In the third Se site, Se(3) is bonded to one Na(2), one Na(3), two equivalent Na(1), and one As(1) atom to form SeNa4As square pyramids that share corners with two equivalent Se(3)Na4As square pyramids, corners with two equivalent Se(6)Na4As square pyramids,  a cornercorner with one Se(2)Na3As2 trigonal bipyramid,  an edgeedge with one Se(2)Na3As2 trigonal bipyramid, and edges with two equivalent Se(4)Na3As2 trigonal bipyramids. In the fourth Se site, Se(4) is bonded to one Na(1), two equivalent Na(3), one As(2), and one As(3) atom to form distorted SeNa3As2 trigonal bipyramids that share  a cornercorner with one Se(6)Na4As square pyramid, corners with two equivalent Se(4)Na3As2 trigonal bipyramids,  an edgeedge with one Se(6)Na4As square pyramid, and edges with two equivalent Se(3)Na4As square pyramids. In the fifth Se site, Se(5) is bonded in a rectangular see-saw-like geometry to two equivalent Na(4), one As(3), and one As(4) atom. In the sixth Se site, Se(6) is bonded to one Na(2), one Na(3), two equivalent Na(4), and one As(3) atom to form SeNa4As square pyramids that share corners with two equivalent Se(3)Na4As square pyramids, corners with two equivalent Se(6)Na4As square pyramids,  a cornercorner with one Se(4)Na3As2 trigonal bipyramid,  an edgeedge with one Se(4)Na3As2 trigonal bipyramid, and edges with two equivalent Se(2)Na3As2 trigonal bipyramids. In the seventh Se site, Se(7) is bonded in a rectangular see-saw-like geometry to two equivalent Na(1), one As(1), and one As(2) atom. In the eighth Se site, Se(8) is bonded in a distorted rectangular see-saw-like geometry to one Na(4), two equivalent Na(3), and one As(2) atom.