Na2Se2O7 crystallizes in the triclinic P-1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(3), one O(4), one O(5), one O(6), and two equivalent O(2) atoms to form NaO6 octahedra that share corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra,  an edgeedge with one Na(1)O6 octahedra, and edges with two equivalent Na(2)O6 pentagonal pyramids. In the second Na site, Na(2) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form distorted NaO6 pentagonal pyramids that share corners with two equivalent Se(2)O4 tetrahedra, corners with four equivalent Se(1)O4 tetrahedra, edges with two equivalent Na(1)O6 octahedra, and edges with two equivalent Na(2)O6 pentagonal pyramids. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(2), one O(3), and one O(7) atom to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with four equivalent Na(2)O6 pentagonal pyramids, and  a cornercorner with one Se(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-56°. In the second Se site, Se(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with two equivalent Na(2)O6 pentagonal pyramids, and  a cornercorner with one Se(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-56°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Na(2) and one Se(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Na(1) and one Se(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), two equivalent Na(2), and one Se(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Se(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Na(1) and one Se(2) atom. In the seventh O site, O(7) is bonded in a bent 120 degrees geometry to one Se(1) and one Se(2) atom.
Na2Se2O7 crystallizes in the triclinic P-1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(3), one O(4), one O(5), one O(6), and two equivalent O(2) atoms to form NaO6 octahedra that share corners with three equivalent Se(1)O4 tetrahedra, corners with three equivalent Se(2)O4 tetrahedra,  an edgeedge with one Na(1)O6 octahedra, and edges with two equivalent Na(2)O6 pentagonal pyramids. The Na(1)-O(3) bond length is 2.38 Å. The Na(1)-O(4) bond length is 2.35 Å. The Na(1)-O(5) bond length is 2.37 Å. The Na(1)-O(6) bond length is 2.37 Å. There is one shorter (2.35 Å) and one longer (2.37 Å) Na(1)-O(2) bond length. In the second Na site, Na(2) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(3) atoms to form distorted NaO6 pentagonal pyramids that share corners with two equivalent Se(2)O4 tetrahedra, corners with four equivalent Se(1)O4 tetrahedra, edges with two equivalent Na(1)O6 octahedra, and edges with two equivalent Na(2)O6 pentagonal pyramids. The Na(2)-O(4) bond length is 2.37 Å. The Na(2)-O(5) bond length is 2.36 Å. There is one shorter (2.41 Å) and one longer (2.42 Å) Na(2)-O(1) bond length. There is one shorter (2.48 Å) and one longer (2.79 Å) Na(2)-O(3) bond length. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded to one O(1), one O(2), one O(3), and one O(7) atom to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with four equivalent Na(2)O6 pentagonal pyramids, and  a cornercorner with one Se(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 52-56°. The Se(1)-O(1) bond length is 1.63 Å. The Se(1)-O(2) bond length is 1.63 Å. The Se(1)-O(3) bond length is 1.64 Å. The Se(1)-O(7) bond length is 1.78 Å. In the second Se site, Se(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form SeO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra, corners with two equivalent Na(2)O6 pentagonal pyramids, and  a cornercorner with one Se(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 51-56°. The Se(2)-O(4) bond length is 1.62 Å. The Se(2)-O(5) bond length is 1.63 Å. The Se(2)-O(6) bond length is 1.62 Å. The Se(2)-O(7) bond length is 1.83 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to two equivalent Na(2) and one Se(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Na(1) and one Se(1) atom. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), two equivalent Na(2), and one Se(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Se(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Se(2) atom. In the sixth O site, O(6) is bonded in a bent 120 degrees geometry to one Na(1) and one Se(2) atom. In the seventh O site, O(7) is bonded in a bent 120 degrees geometry to one Se(1) and one Se(2) atom.