Hf2P2O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Hf sites. In the first Hf site, Hf(1) is bonded to one O(2), one O(3), one O(6), two equivalent O(1), and two equivalent O(4) atoms to form HfO7 pentagonal bipyramids that share  a cornercorner with one Hf(2)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Hf(2)O6 octahedra,  an edgeedge with one Hf(1)O7 pentagonal bipyramid, and  an edgeedge with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 36°. In the second Hf site, Hf(2) is bonded to one O(3), one O(7), two equivalent O(5), and two equivalent O(6) atoms to form HfO6 octahedra that share  a cornercorner with one Hf(1)O7 pentagonal bipyramid,  a cornercorner with one P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra,  an edgeedge with one Hf(2)O6 octahedra, and  an edgeedge with one Hf(1)O7 pentagonal bipyramid. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), and two equivalent O(5) atoms to form PO4 tetrahedra that share corners with three equivalent Hf(2)O6 octahedra,  a cornercorner with one Hf(1)O7 pentagonal bipyramid, and  an edgeedge with one Hf(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 23-29°. In the second P site, P(2) is bonded to one O(2), one O(7), and two equivalent O(4) atoms to form PO4 tetrahedra that share  a cornercorner with one Hf(2)O6 octahedra and corners with three equivalent Hf(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 39°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Hf(1) and one P(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Hf(1) and one P(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Hf(1), one Hf(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Hf(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Hf(2) and one P(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Hf(1) and two equivalent Hf(2) atoms. In the seventh O site, O(7) is bonded in a bent 150 degrees geometry to one Hf(2) and one P(2) atom.
Hf2P2O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Hf sites. In the first Hf site, Hf(1) is bonded to one O(2), one O(3), one O(6), two equivalent O(1), and two equivalent O(4) atoms to form HfO7 pentagonal bipyramids that share  a cornercorner with one Hf(2)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Hf(2)O6 octahedra,  an edgeedge with one Hf(1)O7 pentagonal bipyramid, and  an edgeedge with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 36°. The Hf(1)-O(2) bond length is 2.04 Å. The Hf(1)-O(3) bond length is 2.31 Å. The Hf(1)-O(6) bond length is 2.07 Å. There is one shorter (2.21 Å) and one longer (2.29 Å) Hf(1)-O(1) bond length. Both Hf(1)-O(4) bond lengths are 2.03 Å. In the second Hf site, Hf(2) is bonded to one O(3), one O(7), two equivalent O(5), and two equivalent O(6) atoms to form HfO6 octahedra that share  a cornercorner with one Hf(1)O7 pentagonal bipyramid,  a cornercorner with one P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra,  an edgeedge with one Hf(2)O6 octahedra, and  an edgeedge with one Hf(1)O7 pentagonal bipyramid. The Hf(2)-O(3) bond length is 2.13 Å. The Hf(2)-O(7) bond length is 2.01 Å. Both Hf(2)-O(5) bond lengths are 2.05 Å. There is one shorter (1.99 Å) and one longer (2.10 Å) Hf(2)-O(6) bond length. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), and two equivalent O(5) atoms to form PO4 tetrahedra that share corners with three equivalent Hf(2)O6 octahedra,  a cornercorner with one Hf(1)O7 pentagonal bipyramid, and  an edgeedge with one Hf(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles range from 23-29°. The P(1)-O(1) bond length is 1.55 Å. The P(1)-O(3) bond length is 1.56 Å. Both P(1)-O(5) bond lengths are 1.50 Å. In the second P site, P(2) is bonded to one O(2), one O(7), and two equivalent O(4) atoms to form PO4 tetrahedra that share  a cornercorner with one Hf(2)O6 octahedra and corners with three equivalent Hf(1)O7 pentagonal bipyramids. The corner-sharing octahedral tilt angles are 39°. The P(2)-O(2) bond length is 1.54 Å. The P(2)-O(7) bond length is 1.53 Å. Both P(2)-O(4) bond lengths are 1.53 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Hf(1) and one P(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Hf(1) and one P(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Hf(1), one Hf(2), and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 150 degrees geometry to one Hf(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Hf(2) and one P(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Hf(1) and two equivalent Hf(2) atoms. In the seventh O site, O(7) is bonded in a bent 150 degrees geometry to one Hf(2) and one P(2) atom.