Na2Nb4O11 crystallizes in the monoclinic Cc space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), and one O(9) atom. In the second Na site, Na(2) is bonded in a 7-coordinate geometry to one O(1), one O(10), one O(2), one O(3), one O(4), one O(5), and one O(6) atom. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(10), one O(11), one O(3), one O(7), one O(8), and one O(9) atom to form distorted NbO7 pentagonal bipyramids that share corners with two equivalent Nb(4)O6 octahedra,  a cornercorner with one Nb(3)O7 pentagonal bipyramid, and edges with two equivalent Nb(3)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 43-48°. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(2), one O(4), one O(8), and one O(9) atom. In the third Nb site, Nb(3) is bonded to one O(10), one O(11), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form distorted NbO7 pentagonal bipyramids that share corners with two equivalent Nb(4)O6 octahedra,  a cornercorner with one Nb(1)O7 pentagonal bipyramid, and edges with two equivalent Nb(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 46-48°. In the fourth Nb site, Nb(4) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form corner-sharing NbO6 octahedra. There are eleven inequivalent O sites. In the first O site, O(5) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(3), and one Nb(4) atom. In the second O site, O(6) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(3), and one Nb(4) atom. In the third O site, O(7) is bonded in a 2-coordinate geometry to one Nb(1) and one Nb(3) atom. In the fourth O site, O(8) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the fifth O site, O(9) is bonded to one Na(1), one Nb(1), one Nb(2), and one Nb(3) atom to form distorted edge-sharing ONaNb3 tetrahedra. In the sixth O site, O(10) is bonded in a 4-coordinate geometry to one Na(2), one Nb(1), one Nb(2), and one Nb(3) atom. In the seventh O site, O(11) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the eighth O site, O(1) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(1), and one Nb(4) atom. In the ninth O site, O(2) is bonded to one Na(1), one Na(2), one Nb(2), and one Nb(4) atom to form distorted edge-sharing ONa2Nb2 trigonal pyramids. In the tenth O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(1), and one Nb(4) atom. In the eleventh O site, O(4) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(2), and one Nb(4) atom.
Na2Nb4O11 crystallizes in the monoclinic Cc space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded in a 7-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(5), one O(6), and one O(9) atom. The Na(1)-O(1) bond length is 2.73 Å. The Na(1)-O(2) bond length is 2.44 Å. The Na(1)-O(3) bond length is 2.48 Å. The Na(1)-O(4) bond length is 2.88 Å. The Na(1)-O(5) bond length is 2.68 Å. The Na(1)-O(6) bond length is 2.54 Å. The Na(1)-O(9) bond length is 2.65 Å. In the second Na site, Na(2) is bonded in a 7-coordinate geometry to one O(1), one O(10), one O(2), one O(3), one O(4), one O(5), and one O(6) atom. The Na(2)-O(1) bond length is 2.60 Å. The Na(2)-O(10) bond length is 2.77 Å. The Na(2)-O(2) bond length is 2.73 Å. The Na(2)-O(3) bond length is 2.70 Å. The Na(2)-O(4) bond length is 2.53 Å. The Na(2)-O(5) bond length is 2.50 Å. The Na(2)-O(6) bond length is 2.68 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(10), one O(11), one O(3), one O(7), one O(8), and one O(9) atom to form distorted NbO7 pentagonal bipyramids that share corners with two equivalent Nb(4)O6 octahedra,  a cornercorner with one Nb(3)O7 pentagonal bipyramid, and edges with two equivalent Nb(3)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 43-48°. The Nb(1)-O(1) bond length is 2.02 Å. The Nb(1)-O(10) bond length is 2.05 Å. The Nb(1)-O(11) bond length is 2.06 Å. The Nb(1)-O(3) bond length is 1.97 Å. The Nb(1)-O(7) bond length is 1.98 Å. The Nb(1)-O(8) bond length is 2.48 Å. The Nb(1)-O(9) bond length is 2.07 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(2), one O(4), one O(8), and one O(9) atom. The Nb(2)-O(10) bond length is 2.06 Å. The Nb(2)-O(11) bond length is 2.04 Å. The Nb(2)-O(2) bond length is 1.97 Å. The Nb(2)-O(4) bond length is 2.02 Å. The Nb(2)-O(8) bond length is 1.99 Å. The Nb(2)-O(9) bond length is 2.06 Å. In the third Nb site, Nb(3) is bonded to one O(10), one O(11), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form distorted NbO7 pentagonal bipyramids that share corners with two equivalent Nb(4)O6 octahedra,  a cornercorner with one Nb(1)O7 pentagonal bipyramid, and edges with two equivalent Nb(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 46-48°. The Nb(3)-O(10) bond length is 2.06 Å. The Nb(3)-O(11) bond length is 2.34 Å. The Nb(3)-O(5) bond length is 1.89 Å. The Nb(3)-O(6) bond length is 2.13 Å. The Nb(3)-O(7) bond length is 2.04 Å. The Nb(3)-O(8) bond length is 2.07 Å. The Nb(3)-O(9) bond length is 2.05 Å. In the fourth Nb site, Nb(4) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form corner-sharing NbO6 octahedra. The Nb(4)-O(1) bond length is 1.97 Å. The Nb(4)-O(2) bond length is 2.06 Å. The Nb(4)-O(3) bond length is 2.05 Å. The Nb(4)-O(4) bond length is 1.98 Å. The Nb(4)-O(5) bond length is 2.18 Å. The Nb(4)-O(6) bond length is 1.90 Å. There are eleven inequivalent O sites. In the first O site, O(5) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(3), and one Nb(4) atom. In the second O site, O(6) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(3), and one Nb(4) atom. In the third O site, O(7) is bonded in a 2-coordinate geometry to one Nb(1) and one Nb(3) atom. In the fourth O site, O(8) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the fifth O site, O(9) is bonded to one Na(1), one Nb(1), one Nb(2), and one Nb(3) atom to form distorted edge-sharing ONaNb3 tetrahedra. In the sixth O site, O(10) is bonded in a 4-coordinate geometry to one Na(2), one Nb(1), one Nb(2), and one Nb(3) atom. In the seventh O site, O(11) is bonded in a 3-coordinate geometry to one Nb(1), one Nb(2), and one Nb(3) atom. In the eighth O site, O(1) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(1), and one Nb(4) atom. In the ninth O site, O(2) is bonded to one Na(1), one Na(2), one Nb(2), and one Nb(4) atom to form distorted edge-sharing ONa2Nb2 trigonal pyramids. In the tenth O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(1), and one Nb(4) atom. In the eleventh O site, O(4) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Nb(2), and one Nb(4) atom.