FeHP2O7 crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to one O(1), one O(3), one O(6), one O(7), and two equivalent O(2) atoms to form FeO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one Fe(1)O6 octahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(4), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 30-48°. In the second P site, P(2) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 33-52°. H(1) is bonded in a linear geometry to one O(1) and one O(5) atom. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Fe(1), one P(1), and one H(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(2) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one P(2) and one H(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(2) atom. In the seventh O site, O(7) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(1) atom.
FeHP2O7 crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to one O(1), one O(3), one O(6), one O(7), and two equivalent O(2) atoms to form FeO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one Fe(1)O6 octahedra. The Fe(1)-O(1) bond length is 2.01 Å. The Fe(1)-O(3) bond length is 1.97 Å. The Fe(1)-O(6) bond length is 1.99 Å. The Fe(1)-O(7) bond length is 1.95 Å. There is one shorter (2.15 Å) and one longer (2.16 Å) Fe(1)-O(2) bond length. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(4), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 30-48°. The P(1)-O(1) bond length is 1.55 Å. The P(1)-O(3) bond length is 1.51 Å. The P(1)-O(4) bond length is 1.62 Å. The P(1)-O(7) bond length is 1.52 Å. In the second P site, P(2) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 33-52°. The P(2)-O(2) bond length is 1.56 Å. The P(2)-O(4) bond length is 1.60 Å. The P(2)-O(5) bond length is 1.55 Å. The P(2)-O(6) bond length is 1.51 Å. H(1) is bonded in a linear geometry to one O(1) and one O(5) atom. The H(1)-O(1) bond length is 1.61 Å. The H(1)-O(5) bond length is 1.01 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal planar geometry to one Fe(1), one P(1), and one H(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(2) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one P(2) and one H(1) atom. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(2) atom. In the seventh O site, O(7) is bonded in a bent 150 degrees geometry to one Fe(1) and one P(1) atom.