MgFe3(P2O7)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2), one O(3), one O(5), one O(6), and two equivalent O(7) atoms to form FeO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra,  an edgeedge with one Fe(1)O6 octahedra, and  an edgeedge with one Fe(2)O6 octahedra. In the second Fe site, Fe(2) 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 P(1)O4 tetrahedra, corners with four equivalent P(2)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent 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(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share  a cornercorner with one Fe(2)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, 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 41-68°. In the second P site, P(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form PO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, 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 42-63°. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Fe(1) and one P(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Fe(1), one Fe(2), and one P(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Fe(2), and one P(2) atom. In the fifth O site, O(2) is bonded to one Mg(1), one Fe(1), one Fe(2), and one P(1) atom to form distorted corner-sharing OMgFe2P tetrahedra. In the sixth O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Fe(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom.
MgFe3(P2O7)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)O4 tetrahedra, edges with two equivalent Fe(1)O6 octahedra, and edges with two equivalent Fe(2)O6 octahedra. Both Mg(1)-O(1) bond lengths are 2.24 Å. Both Mg(1)-O(2) bond lengths are 2.29 Å. Both Mg(1)-O(3) bond lengths are 2.34 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(2), one O(3), one O(5), one O(6), and two equivalent O(7) atoms to form FeO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra,  an edgeedge with one Fe(1)O6 octahedra, and  an edgeedge with one Fe(2)O6 octahedra. The Fe(1)-O(2) bond length is 2.26 Å. The Fe(1)-O(3) bond length is 2.29 Å. The Fe(1)-O(5) bond length is 2.04 Å. The Fe(1)-O(6) bond length is 2.11 Å. There is one shorter (2.13 Å) and one longer (2.16 Å) Fe(1)-O(7) bond length. In the second Fe site, Fe(2) 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 P(1)O4 tetrahedra, corners with four equivalent P(2)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Fe(1)O6 octahedra. Both Fe(2)-O(1) bond lengths are 2.28 Å. Both Fe(2)-O(2) bond lengths are 2.33 Å. Both Fe(2)-O(6) bond lengths are 2.06 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share  a cornercorner with one Fe(2)O6 octahedra, corners with two equivalent Mg(1)O6 octahedra, 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 41-68°. The P(1)-O(2) bond length is 1.57 Å. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(4) bond length is 1.62 Å. The P(1)-O(5) bond length is 1.51 Å. In the second P site, P(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form PO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with two equivalent Fe(2)O6 octahedra, 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 42-63°. The P(2)-O(1) bond length is 1.53 Å. The P(2)-O(4) bond length is 1.60 Å. The P(2)-O(6) bond length is 1.55 Å. The P(2)-O(7) bond length is 1.53 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a bent 120 degrees geometry to one Fe(1) and one P(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Fe(1), one Fe(2), and one P(2) atom. In the third O site, O(7) is bonded in a distorted trigonal planar geometry to two equivalent Fe(1) and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Fe(2), and one P(2) atom. In the fifth O site, O(2) is bonded to one Mg(1), one Fe(1), one Fe(2), and one P(1) atom to form distorted corner-sharing OMgFe2P tetrahedra. In the sixth O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Fe(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom.