Li2FeP2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a trigonal non-coplanar geometry to one O(2), one O(3), and one O(6) atom. In the second Li site, Li(2) is bonded to one O(1), one O(4), and two equivalent O(7) atoms to form distorted LiO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Fe(1)O6 octahedra, and  an edgeedge with one Li(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 66-78°. Fe(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(7) atom to form FeO6 octahedra that share corners with two equivalent Li(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one Li(2)O4 tetrahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra,  a cornercorner with one Li(2)O4 tetrahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-57°. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with three equivalent Li(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 31-55°. There are seven inequivalent O sites. In the first O site, O(7) is bonded to two equivalent Li(2), one Fe(1), and one P(2) atom to form distorted edge-sharing OLi2FeP trigonal pyramids. In the second O site, O(1) is bonded in a distorted trigonal planar geometry to one Li(2), one Fe(1), and one P(1) atom. In the third O site, O(2) is bonded in a trigonal planar geometry to one Li(1), one Fe(1), and one P(1) atom. In the fourth O site, O(3) is bonded in a distorted trigonal planar geometry to one Li(1), one Fe(1), and one P(2) atom. In the fifth O site, O(4) is bonded in a 3-coordinate geometry to one Li(2), one Fe(1), and one P(2) atom. In the sixth O site, O(5) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the seventh O site, O(6) is bonded in a trigonal planar geometry to one Li(1), one Fe(1), and one P(1) atom.
Li2FeP2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a trigonal non-coplanar geometry to one O(2), one O(3), and one O(6) atom. The Li(1)-O(2) bond length is 2.00 Å. The Li(1)-O(3) bond length is 1.99 Å. The Li(1)-O(6) bond length is 1.99 Å. In the second Li site, Li(2) is bonded to one O(1), one O(4), and two equivalent O(7) atoms to form distorted LiO4 tetrahedra that share corners with two equivalent Fe(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Fe(1)O6 octahedra, and  an edgeedge with one Li(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 66-78°. The Li(2)-O(1) bond length is 1.94 Å. The Li(2)-O(4) bond length is 1.94 Å. There is one shorter (1.99 Å) and one longer (2.09 Å) Li(2)-O(7) bond length. Fe(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(7) atom to form FeO6 octahedra that share corners with two equivalent Li(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one Li(2)O4 tetrahedra. The Fe(1)-O(1) bond length is 2.14 Å. The Fe(1)-O(2) bond length is 2.20 Å. The Fe(1)-O(3) bond length is 2.38 Å. The Fe(1)-O(4) bond length is 2.23 Å. The Fe(1)-O(6) bond length is 2.08 Å. The Fe(1)-O(7) bond length is 2.17 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra,  a cornercorner with one Li(2)O4 tetrahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-57°. The P(1)-O(1) bond length is 1.52 Å. The P(1)-O(2) bond length is 1.53 Å. The P(1)-O(5) bond length is 1.63 Å. The P(1)-O(6) bond length is 1.54 Å. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Fe(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with three equivalent Li(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 31-55°. The P(2)-O(3) bond length is 1.54 Å. The P(2)-O(4) bond length is 1.53 Å. The P(2)-O(5) bond length is 1.62 Å. The P(2)-O(7) bond length is 1.54 Å. There are seven inequivalent O sites. In the first O site, O(7) is bonded to two equivalent Li(2), one Fe(1), and one P(2) atom to form distorted edge-sharing OLi2FeP trigonal pyramids. In the second O site, O(1) is bonded in a distorted trigonal planar geometry to one Li(2), one Fe(1), and one P(1) atom. In the third O site, O(2) is bonded in a trigonal planar geometry to one Li(1), one Fe(1), and one P(1) atom. In the fourth O site, O(3) is bonded in a distorted trigonal planar geometry to one Li(1), one Fe(1), and one P(2) atom. In the fifth O site, O(4) is bonded in a 3-coordinate geometry to one Li(2), one Fe(1), and one P(2) atom. In the sixth O site, O(5) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the seventh O site, O(6) is bonded in a trigonal planar geometry to one Li(1), one Fe(1), and one P(1) atom.