Nb2P2O9 crystallizes in the orthorhombic Cmce space group. Nb(1) is bonded to one O(2), one O(3), one O(4), and four equivalent O(1) atoms to form NbO7 pentagonal bipyramids that share  a cornercorner with one Nb(1)O7 pentagonal bipyramid, corners with four equivalent P(1)O4 tetrahedra, edges with two equivalent Nb(1)O7 pentagonal bipyramids, and  an edgeedge with one P(1)O4 tetrahedra. P(1) is bonded to one O(3), one O(4), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with four equivalent Nb(1)O7 pentagonal bipyramids and  an edgeedge with one Nb(1)O7 pentagonal bipyramid. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Nb(1) and one P(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Nb(1) atoms. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a linear geometry to one Nb(1) and one P(1) atom.
Nb2P2O9 crystallizes in the orthorhombic Cmce space group. Nb(1) is bonded to one O(2), one O(3), one O(4), and four equivalent O(1) atoms to form NbO7 pentagonal bipyramids that share  a cornercorner with one Nb(1)O7 pentagonal bipyramid, corners with four equivalent P(1)O4 tetrahedra, edges with two equivalent Nb(1)O7 pentagonal bipyramids, and  an edgeedge with one P(1)O4 tetrahedra. The Nb(1)-O(2) bond length is 1.91 Å. The Nb(1)-O(3) bond length is 2.05 Å. The Nb(1)-O(4) bond length is 2.16 Å. There are two shorter (2.18 Å) and two longer (2.25 Å) Nb(1)-O(1) bond lengths. P(1) is bonded to one O(3), one O(4), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with four equivalent Nb(1)O7 pentagonal bipyramids and  an edgeedge with one Nb(1)O7 pentagonal bipyramid. The P(1)-O(3) bond length is 1.52 Å. The P(1)-O(4) bond length is 1.50 Å. Both P(1)-O(1) bond lengths are 1.58 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Nb(1) and one P(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent Nb(1) atoms. In the third O site, O(3) is bonded in a linear geometry to one Nb(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a linear geometry to one Nb(1) and one P(1) atom.