Na2Al2B2O7 crystallizes in the monoclinic Cc space group. There are two inequivalent Na sites. In the first Na site, Na(1) 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 NaO6 octahedra that share corners with three equivalent Al(1)O4 tetrahedra and corners with three equivalent Al(2)O4 tetrahedra. In the second Na site, Na(2) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and three equivalent O(1) atoms. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 65-68°. In the second Al site, Al(2) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AlO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra and  a cornercorner with one Al(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 66-67°. There are two inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(5), one O(6), and one O(7) atom. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(2), one O(3), and one O(4) atom. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the third O site, O(7) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the fourth O site, O(1) is bonded in a distorted linear geometry to three equivalent Na(2), one Al(1), and one Al(2) atom. In the fifth O site, O(2) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom.
Na2Al2B2O7 crystallizes in the monoclinic Cc space group. There are two inequivalent Na sites. In the first Na site, Na(1) 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 NaO6 octahedra that share corners with three equivalent Al(1)O4 tetrahedra and corners with three equivalent Al(2)O4 tetrahedra. The Na(1)-O(2) bond length is 2.47 Å. The Na(1)-O(3) bond length is 2.49 Å. The Na(1)-O(4) bond length is 2.40 Å. The Na(1)-O(5) bond length is 2.50 Å. The Na(1)-O(6) bond length is 2.40 Å. The Na(1)-O(7) bond length is 2.46 Å. In the second Na site, Na(2) is bonded in a 9-coordinate geometry to one O(2), one O(3), one O(4), one O(5), one O(6), one O(7), and three equivalent O(1) atoms. The Na(2)-O(2) bond length is 2.88 Å. The Na(2)-O(3) bond length is 2.89 Å. The Na(2)-O(4) bond length is 3.00 Å. The Na(2)-O(5) bond length is 2.90 Å. The Na(2)-O(6) bond length is 2.99 Å. The Na(2)-O(7) bond length is 2.88 Å. There are a spread of Na(2)-O(1) bond distances ranging from 2.70-3.03 Å. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form AlO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra and  a cornercorner with one Al(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 65-68°. The Al(1)-O(1) bond length is 1.74 Å. The Al(1)-O(5) bond length is 1.77 Å. The Al(1)-O(6) bond length is 1.77 Å. The Al(1)-O(7) bond length is 1.77 Å. In the second Al site, Al(2) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form AlO4 tetrahedra that share corners with three equivalent Na(1)O6 octahedra and  a cornercorner with one Al(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 66-67°. The Al(2)-O(1) bond length is 1.74 Å. The Al(2)-O(2) bond length is 1.77 Å. The Al(2)-O(3) bond length is 1.77 Å. The Al(2)-O(4) bond length is 1.77 Å. There are two inequivalent B sites. In the first B site, B(1) is bonded in a trigonal planar geometry to one O(5), one O(6), and one O(7) atom. The B(1)-O(5) bond length is 1.38 Å. The B(1)-O(6) bond length is 1.38 Å. The B(1)-O(7) bond length is 1.38 Å. In the second B site, B(2) is bonded in a trigonal planar geometry to one O(2), one O(3), and one O(4) atom. The B(2)-O(2) bond length is 1.38 Å. The B(2)-O(3) bond length is 1.38 Å. The B(2)-O(4) bond length is 1.38 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the third O site, O(7) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(1), and one B(1) atom. In the fourth O site, O(1) is bonded in a distorted linear geometry to three equivalent Na(2), one Al(1), and one Al(2) atom. In the fifth O site, O(2) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom. In the sixth O site, O(3) is bonded in a 4-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), one Al(2), and one B(2) atom.