Li4FeP2O9 crystallizes in the monoclinic C2/m space group. There are four inequivalent Li sites. In the first Li site, Li(1) is bonded to two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms to form distorted LiO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, edges with two equivalent Li(1)O6 octahedra, and edges with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 59°. In the second Li site, Li(2) is bonded in a 5-coordinate geometry to one O(1), one O(3), one O(4), and two equivalent O(6) atoms. In the third Li site, Li(3) is bonded in a 5-coordinate geometry to one O(1), one O(5), one O(7), and two equivalent O(2) atoms. In the fourth Li site, Li(4) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(7) atoms to form distorted LiO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(2)O4 tetrahedra, edges with two equivalent Li(4)O6 octahedra, and edges with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 63°. Fe(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form FeO6 octahedra that share corners with two equivalent Li(1)O6 octahedra, corners with two equivalent Li(4)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-63°. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form PO4 tetrahedra that share corners with two equivalent Li(1)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Li(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-61°. In the second P site, P(2) is bonded to one O(5), one O(7), and two equivalent O(6) atoms to form PO4 tetrahedra that share corners with two equivalent Li(4)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Li(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-62°. There are seven inequivalent O sites. In the first O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Li(2), two equivalent Li(4), and one P(1) atom. In the second O site, O(5) is bonded in a rectangular see-saw-like geometry to one Li(3), two equivalent Li(1), and one P(2) atom. In the third O site, O(6) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Fe(1), and one P(2) atom. In the fourth O site, O(7) is bonded in a distorted rectangular see-saw-like geometry to one Li(3), two equivalent Li(4), and one P(2) atom. In the fifth O site, O(1) is bonded to one Li(2), one Li(3), and two equivalent Fe(1) atoms to form distorted OLi2Fe2 tetrahedra that share corners with two equivalent O(1)Li2Fe2 tetrahedra and  a cornercorner with one O(3)Li3P trigonal pyramid. In the sixth O site, O(2) is bonded in a 4-coordinate geometry to one Li(3), one Li(4), one Fe(1), and one P(1) atom. In the seventh O site, O(3) is bonded to one Li(2), two equivalent Li(1), and one P(1) atom to form distorted OLi3P trigonal pyramids that share  a cornercorner with one O(1)Li2Fe2 tetrahedra and  an edgeedge with one O(3)Li3P trigonal pyramid.
Li4FeP2O9 crystallizes in the monoclinic C2/m space group. There are four inequivalent Li sites. In the first Li site, Li(1) is bonded to two equivalent O(3), two equivalent O(5), and two equivalent O(6) atoms to form distorted LiO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, edges with two equivalent Li(1)O6 octahedra, and edges with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles are 59°. Both Li(1)-O(3) bond lengths are 2.20 Å. Both Li(1)-O(5) bond lengths are 2.42 Å. Both Li(1)-O(6) bond lengths are 2.11 Å. In the second Li site, Li(2) is bonded in a 5-coordinate geometry to one O(1), one O(3), one O(4), and two equivalent O(6) atoms. The Li(2)-O(1) bond length is 2.15 Å. The Li(2)-O(3) bond length is 1.97 Å. The Li(2)-O(4) bond length is 1.85 Å. Both Li(2)-O(6) bond lengths are 2.27 Å. In the third Li site, Li(3) is bonded in a 5-coordinate geometry to one O(1), one O(5), one O(7), and two equivalent O(2) atoms. The Li(3)-O(1) bond length is 2.36 Å. The Li(3)-O(5) bond length is 1.86 Å. The Li(3)-O(7) bond length is 1.94 Å. Both Li(3)-O(2) bond lengths are 2.26 Å. In the fourth Li site, Li(4) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(7) atoms to form distorted LiO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(2)O4 tetrahedra, edges with two equivalent Li(4)O6 octahedra, and edges with two equivalent P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 63°. Both Li(4)-O(2) bond lengths are 2.18 Å. Both Li(4)-O(4) bond lengths are 2.46 Å. Both Li(4)-O(7) bond lengths are 2.24 Å. Fe(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form FeO6 octahedra that share corners with two equivalent Li(1)O6 octahedra, corners with two equivalent Li(4)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, and corners with two equivalent P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 48-63°. Both Fe(1)-O(1) bond lengths are 1.86 Å. Both Fe(1)-O(2) bond lengths are 1.99 Å. Both Fe(1)-O(6) bond lengths are 2.03 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form PO4 tetrahedra that share corners with two equivalent Li(1)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Li(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-61°. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(4) bond length is 1.52 Å. Both P(1)-O(2) bond lengths are 1.58 Å. In the second P site, P(2) is bonded to one O(5), one O(7), and two equivalent O(6) atoms to form PO4 tetrahedra that share corners with two equivalent Li(4)O6 octahedra, corners with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Li(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-62°. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(7) bond length is 1.53 Å. Both P(2)-O(6) bond lengths are 1.58 Å. There are seven inequivalent O sites. In the first O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to one Li(2), two equivalent Li(4), and one P(1) atom. In the second O site, O(5) is bonded in a rectangular see-saw-like geometry to one Li(3), two equivalent Li(1), and one P(2) atom. In the third O site, O(6) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Fe(1), and one P(2) atom. In the fourth O site, O(7) is bonded in a distorted rectangular see-saw-like geometry to one Li(3), two equivalent Li(4), and one P(2) atom. In the fifth O site, O(1) is bonded to one Li(2), one Li(3), and two equivalent Fe(1) atoms to form distorted OLi2Fe2 tetrahedra that share corners with two equivalent O(1)Li2Fe2 tetrahedra and  a cornercorner with one O(3)Li3P trigonal pyramid. In the sixth O site, O(2) is bonded in a 4-coordinate geometry to one Li(3), one Li(4), one Fe(1), and one P(1) atom. In the seventh O site, O(3) is bonded to one Li(2), two equivalent Li(1), and one P(1) atom to form distorted OLi3P trigonal pyramids that share  a cornercorner with one O(1)Li2Fe2 tetrahedra and  an edgeedge with one O(3)Li3P trigonal pyramid.