NaNb2AsO8 crystallizes in the monoclinic P2_1/c space group. Na(1) is bonded to one O(2), one O(3), one O(6), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NaO7 pentagonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, edges with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Na(1)O7 pentagonal bipyramid, and  an edgeedge with one As(1)O4 tetrahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 5-coordinate geometry to one O(1), one O(4), one O(5), one O(6), and one O(8) atom. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share corners with three equivalent As(1)O4 tetrahedra and edges with two equivalent Na(1)O7 pentagonal bipyramids. As(1) is bonded to one O(2), one O(3), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with two equivalent Na(1)O7 pentagonal bipyramids, and  an edgeedge with one Na(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 43-57°. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Na(1) and one Nb(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Nb(2), and one As(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Nb(2), and one As(1) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to one Nb(1) and one Nb(2) atom. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Na(1), one Nb(2), and one As(1) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one As(1) atom.
NaNb2AsO8 crystallizes in the monoclinic P2_1/c space group. Na(1) is bonded to one O(2), one O(3), one O(6), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NaO7 pentagonal bipyramids that share corners with two equivalent As(1)O4 tetrahedra, edges with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Na(1)O7 pentagonal bipyramid, and  an edgeedge with one As(1)O4 tetrahedra. The Na(1)-O(2) bond length is 2.79 Å. The Na(1)-O(3) bond length is 2.45 Å. The Na(1)-O(6) bond length is 2.66 Å. The Na(1)-O(7) bond length is 2.54 Å. The Na(1)-O(8) bond length is 2.75 Å. There is one shorter (2.47 Å) and one longer (2.58 Å) Na(1)-O(1) bond length. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a 5-coordinate geometry to one O(1), one O(4), one O(5), one O(6), and one O(8) atom. The Nb(1)-O(1) bond length is 1.77 Å. The Nb(1)-O(4) bond length is 2.06 Å. The Nb(1)-O(5) bond length is 2.02 Å. The Nb(1)-O(6) bond length is 2.01 Å. The Nb(1)-O(8) bond length is 2.03 Å. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), one O(5), one O(6), and one O(7) atom to form NbO6 octahedra that share corners with three equivalent As(1)O4 tetrahedra and edges with two equivalent Na(1)O7 pentagonal bipyramids. The Nb(2)-O(2) bond length is 2.14 Å. The Nb(2)-O(3) bond length is 2.12 Å. The Nb(2)-O(4) bond length is 1.95 Å. The Nb(2)-O(5) bond length is 1.87 Å. The Nb(2)-O(6) bond length is 1.94 Å. The Nb(2)-O(7) bond length is 2.15 Å. As(1) is bonded to one O(2), one O(3), one O(7), and one O(8) atom to form AsO4 tetrahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with two equivalent Na(1)O7 pentagonal bipyramids, and  an edgeedge with one Na(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 43-57°. The As(1)-O(2) bond length is 1.72 Å. The As(1)-O(3) bond length is 1.71 Å. The As(1)-O(7) bond length is 1.71 Å. The As(1)-O(8) bond length is 1.74 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Na(1) and one Nb(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Nb(2), and one As(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Na(1), one Nb(2), and one As(1) atom. In the fourth O site, O(4) is bonded in a 2-coordinate geometry to one Nb(1) and one Nb(2) atom. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Nb(1) and one Nb(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one Nb(2) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Na(1), one Nb(2), and one As(1) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one Na(1), one Nb(1), and one As(1) atom.