Na2P2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(2), one O(5), one O(6), and two equivalent O(3) atoms to form NaO5 square pyramids that share corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one Na(1)O5 square pyramid. In the second Na site, Na(2) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Na(1)O5 square pyramids and  a cornercorner with one P(2)O4 tetrahedra. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Na(1)O5 square pyramids and  a cornercorner with one P(1)O4 tetrahedra. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), and one P(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), and one P(2) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Na(2) and one P(2) atom. In the fourth O site, O(1) is bonded in a bent 150 degrees geometry to one Na(2) and one P(1) atom. In the fifth O site, O(2) is bonded in a distorted linear geometry to one Na(1) and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Na(2), two equivalent Na(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Na(2), one P(1), and one P(2) atom.
Na2P2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(2), one O(5), one O(6), and two equivalent O(3) atoms to form NaO5 square pyramids that share corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one Na(1)O5 square pyramid. The Na(1)-O(2) bond length is 2.32 Å. The Na(1)-O(5) bond length is 2.32 Å. The Na(1)-O(6) bond length is 2.45 Å. There is one shorter (2.41 Å) and one longer (2.44 Å) Na(1)-O(3) bond length. In the second Na site, Na(2) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), one O(6), and one O(7) atom. The Na(2)-O(1) bond length is 2.33 Å. The Na(2)-O(3) bond length is 2.61 Å. The Na(2)-O(4) bond length is 2.51 Å. The Na(2)-O(5) bond length is 2.67 Å. The Na(2)-O(6) bond length is 2.44 Å. The Na(2)-O(7) bond length is 2.35 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Na(1)O5 square pyramids and  a cornercorner with one P(2)O4 tetrahedra. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(2) bond length is 1.52 Å. The P(1)-O(3) bond length is 1.52 Å. The P(1)-O(4) bond length is 1.62 Å. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Na(1)O5 square pyramids and  a cornercorner with one P(1)O4 tetrahedra. The P(2)-O(4) bond length is 1.63 Å. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(6) bond length is 1.52 Å. The P(2)-O(7) bond length is 1.52 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), and one P(2) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one Na(2), and one P(2) atom. In the third O site, O(7) is bonded in a bent 150 degrees geometry to one Na(2) and one P(2) atom. In the fourth O site, O(1) is bonded in a bent 150 degrees geometry to one Na(2) and one P(1) atom. In the fifth O site, O(2) is bonded in a distorted linear geometry to one Na(1) and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Na(2), two equivalent Na(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a 3-coordinate geometry to one Na(2), one P(1), and one P(2) atom.