Fe2P2O7 crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted FeO6 octahedra that share corners with six equivalent P(1)O4 tetrahedra and edges with three equivalent Fe(1)O6 octahedra. 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 six equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-60°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one P(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent P(1) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(1) atom.
Fe2P2O7 crystallizes in the monoclinic P2_1/c space group. Fe(1) is bonded to two equivalent O(1), two equivalent O(3), and two equivalent O(4) atoms to form distorted FeO6 octahedra that share corners with six equivalent P(1)O4 tetrahedra and edges with three equivalent Fe(1)O6 octahedra. There is one shorter (2.06 Å) and one longer (2.19 Å) Fe(1)-O(1) bond length. There is one shorter (1.99 Å) and one longer (2.45 Å) Fe(1)-O(3) bond length. There is one shorter (2.12 Å) and one longer (2.13 Å) Fe(1)-O(4) bond length. 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 six equivalent Fe(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 41-60°. The P(1)-O(1) bond length is 1.52 Å. The P(1)-O(2) bond length is 1.57 Å. The P(1)-O(3) bond length is 1.52 Å. The P(1)-O(4) bond length is 1.51 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one P(1) atom. In the second O site, O(2) is bonded in a linear geometry to two equivalent P(1) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to two equivalent Fe(1) and one P(1) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(1) atom.