FeP2O7 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)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(2), one O(4), and two equivalent O(3) atoms to form PO4 tetrahedra that share corners with four equivalent Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 25-50°. In the second P site, P(2) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 51°. There are five inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one Fe(1) and one P(2) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(1) 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 150 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one P(2) atom.
FeP2O7 crystallizes in the monoclinic C2/m space group. Fe(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form FeO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)O4 tetrahedra, and  an edgeedge with one Fe(1)O6 octahedra. Both Fe(1)-O(1) bond lengths are 2.02 Å. Both Fe(1)-O(2) bond lengths are 2.17 Å. Both Fe(1)-O(3) bond lengths are 1.93 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(4), and two equivalent O(3) atoms to form PO4 tetrahedra that share corners with four equivalent Fe(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 25-50°. The P(1)-O(2) bond length is 1.56 Å. The P(1)-O(4) bond length is 1.61 Å. Both P(1)-O(3) bond lengths are 1.51 Å. In the second P site, P(2) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form PO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 51°. The P(2)-O(4) bond length is 1.60 Å. The P(2)-O(5) bond length is 1.54 Å. Both P(2)-O(1) bond lengths are 1.53 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one Fe(1) and one P(2) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(1) 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 150 degrees geometry to one P(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a single-bond geometry to one P(2) atom.