Na2U2O7 crystallizes in the monoclinic C2/m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a body-centered cubic geometry to two equivalent O(1), two equivalent O(4), and four equivalent O(3) atoms. In the second Na site, Na(2) is bonded to two equivalent O(5) and four equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with four equivalent U(1)O6 octahedra and corners with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 54°. In the third Na site, Na(3) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. There are two inequivalent U sites. In the first U site, U(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form UO6 octahedra that share corners with two equivalent Na(2)O6 octahedra, corners with two equivalent U(1)O6 octahedra, corners with two equivalent U(2)O7 pentagonal bipyramids, and edges with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 54°. In the second U site, U(2) is bonded to one O(1), one O(4), one O(5), and four equivalent O(2) atoms to form distorted UO7 pentagonal bipyramids that share  a cornercorner with one Na(2)O6 octahedra, corners with two equivalent U(1)O6 octahedra, edges with two equivalent U(1)O6 octahedra, and edges with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 37-66°. There are five inequivalent O sites. In the first O site, O(1) is bonded to one Na(1), one U(2), and two equivalent U(1) atoms to form distorted ONaU3 tetrahedra that share  a cornercorner with one O(4)Na3U tetrahedra, corners with three equivalent O(1)NaU3 tetrahedra, corners with four equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra, and edges with two equivalent O(2)NaU3 trigonal pyramids. In the second O site, O(2) is bonded to one Na(3), one U(1), and two equivalent U(2) atoms to form distorted ONaU3 trigonal pyramids that share corners with two equivalent O(4)Na3U tetrahedra, corners with two equivalent O(1)NaU3 tetrahedra, corners with six equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra,  an edgeedge with one O(1)NaU3 tetrahedra, and  an edgeedge with one O(2)NaU3 trigonal pyramid. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Na(3), and one U(1) atom. In the fourth O site, O(4) is bonded to one Na(1), two equivalent Na(3), and one U(2) atom to form distorted ONa3U tetrahedra that share  a cornercorner with one O(4)Na3U tetrahedra,  a cornercorner with one O(1)NaU3 tetrahedra, corners with four equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra,  an edgeedge with one O(1)NaU3 tetrahedra, and edges with two equivalent O(2)NaU3 trigonal pyramids. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Na(2), two equivalent Na(3), and one U(2) atom.
Na2U2O7 crystallizes in the monoclinic C2/m space group. There are three inequivalent Na sites. In the first Na site, Na(1) is bonded in a body-centered cubic geometry to two equivalent O(1), two equivalent O(4), and four equivalent O(3) atoms. Both Na(1)-O(1) bond lengths are 2.54 Å. Both Na(1)-O(4) bond lengths are 2.36 Å. All Na(1)-O(3) bond lengths are 2.78 Å. In the second Na site, Na(2) is bonded to two equivalent O(5) and four equivalent O(3) atoms to form distorted NaO6 octahedra that share corners with four equivalent U(1)O6 octahedra and corners with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 54°. Both Na(2)-O(5) bond lengths are 2.86 Å. All Na(2)-O(3) bond lengths are 2.32 Å. In the third Na site, Na(3) is bonded in a 8-coordinate geometry to two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. Both Na(3)-O(2) bond lengths are 2.91 Å. Both Na(3)-O(3) bond lengths are 2.53 Å. Both Na(3)-O(4) bond lengths are 2.57 Å. Both Na(3)-O(5) bond lengths are 2.45 Å. There are two inequivalent U sites. In the first U site, U(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form UO6 octahedra that share corners with two equivalent Na(2)O6 octahedra, corners with two equivalent U(1)O6 octahedra, corners with two equivalent U(2)O7 pentagonal bipyramids, and edges with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 54°. Both U(1)-O(1) bond lengths are 2.18 Å. Both U(1)-O(2) bond lengths are 2.19 Å. Both U(1)-O(3) bond lengths are 1.88 Å. In the second U site, U(2) is bonded to one O(1), one O(4), one O(5), and four equivalent O(2) atoms to form distorted UO7 pentagonal bipyramids that share  a cornercorner with one Na(2)O6 octahedra, corners with two equivalent U(1)O6 octahedra, edges with two equivalent U(1)O6 octahedra, and edges with two equivalent U(2)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles range from 37-66°. The U(2)-O(1) bond length is 2.33 Å. The U(2)-O(4) bond length is 1.86 Å. The U(2)-O(5) bond length is 1.85 Å. There are two shorter (2.20 Å) and two longer (2.53 Å) U(2)-O(2) bond lengths. There are five inequivalent O sites. In the first O site, O(1) is bonded to one Na(1), one U(2), and two equivalent U(1) atoms to form distorted ONaU3 tetrahedra that share  a cornercorner with one O(4)Na3U tetrahedra, corners with three equivalent O(1)NaU3 tetrahedra, corners with four equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra, and edges with two equivalent O(2)NaU3 trigonal pyramids. In the second O site, O(2) is bonded to one Na(3), one U(1), and two equivalent U(2) atoms to form distorted ONaU3 trigonal pyramids that share corners with two equivalent O(4)Na3U tetrahedra, corners with two equivalent O(1)NaU3 tetrahedra, corners with six equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra,  an edgeedge with one O(1)NaU3 tetrahedra, and  an edgeedge with one O(2)NaU3 trigonal pyramid. In the third O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Na(3), and one U(1) atom. In the fourth O site, O(4) is bonded to one Na(1), two equivalent Na(3), and one U(2) atom to form distorted ONa3U tetrahedra that share  a cornercorner with one O(4)Na3U tetrahedra,  a cornercorner with one O(1)NaU3 tetrahedra, corners with four equivalent O(2)NaU3 trigonal pyramids,  an edgeedge with one O(4)Na3U tetrahedra,  an edgeedge with one O(1)NaU3 tetrahedra, and edges with two equivalent O(2)NaU3 trigonal pyramids. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Na(2), two equivalent Na(3), and one U(2) atom.