Sr3MgFe2O7 crystallizes in the monoclinic Cm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(3) and four equivalent O(5) atoms to form distorted SrO5 square pyramids that share  a cornercorner with one Fe(1)O6 octahedra, corners with four equivalent Sr(1)O5 square pyramids, corners with four equivalent Fe(2)O5 trigonal bipyramids, and edges with four equivalent Sr(1)O5 square pyramids. The corner-sharing octahedra are not tilted. In the second Sr site, Sr(2) is bonded in a 9-coordinate geometry to one O(5), four equivalent O(1), and four equivalent O(3) atoms. In the third Sr site, Sr(3) is bonded in a 8-coordinate geometry to four equivalent O(1) and four equivalent O(4) atoms. Mg(1) is bonded in a 5-coordinate geometry to one O(4) and four equivalent O(2) atoms. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3), one O(4), and four equivalent O(1) atoms to form FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra and  a cornercorner with one Sr(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 2°. In the second Fe site, Fe(2) is bonded to one O(5) and four equivalent O(2) atoms to form distorted FeO5 trigonal bipyramids that share corners with four equivalent Sr(1)O5 square pyramids and corners with four equivalent Fe(2)O5 trigonal bipyramids. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(2), two equivalent Sr(3), and two equivalent Fe(1) atoms to form distorted OSr4Fe2 octahedra that share corners with two equivalent O(1)Sr4Fe2 octahedra, corners with two equivalent O(5)Sr5Fe octahedra, corners with four equivalent O(3)Sr5Fe octahedra, edges with two equivalent O(1)Sr4Fe2 octahedra, faces with two equivalent O(3)Sr5Fe octahedra, and faces with four equivalent O(1)Sr4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 2-52°. In the second O site, O(2) is bonded in a distorted see-saw-like geometry to two equivalent Mg(1) and two equivalent Fe(2) atoms. In the third O site, O(3) is bonded to one Sr(1), four equivalent Sr(2), and one Fe(1) atom to form distorted OSr5Fe octahedra that share corners with four equivalent O(3)Sr5Fe octahedra, corners with eight equivalent O(1)Sr4Fe2 octahedra, edges with four equivalent O(3)Sr5Fe octahedra, edges with four equivalent O(5)Sr5Fe octahedra, and faces with four equivalent O(1)Sr4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 18-52°. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to four equivalent Sr(3), one Mg(1), and one Fe(1) atom. In the fifth O site, O(5) is bonded to one Sr(2), four equivalent Sr(1), and one Fe(2) atom to form OSr5Fe octahedra that share corners with four equivalent O(1)Sr4Fe2 octahedra, corners with four equivalent O(5)Sr5Fe octahedra, edges with four equivalent O(3)Sr5Fe octahedra, and edges with four equivalent O(5)Sr5Fe octahedra. The corner-sharing octahedral tilt angles range from 4-48°.
Sr3MgFe2O7 crystallizes in the monoclinic Cm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(3) and four equivalent O(5) atoms to form distorted SrO5 square pyramids that share  a cornercorner with one Fe(1)O6 octahedra, corners with four equivalent Sr(1)O5 square pyramids, corners with four equivalent Fe(2)O5 trigonal bipyramids, and edges with four equivalent Sr(1)O5 square pyramids. The corner-sharing octahedra are not tilted. The Sr(1)-O(3) bond length is 2.28 Å. There are a spread of Sr(1)-O(5) bond distances ranging from 2.74-2.76 Å. In the second Sr site, Sr(2) is bonded in a 9-coordinate geometry to one O(5), four equivalent O(1), and four equivalent O(3) atoms. The Sr(2)-O(5) bond length is 2.80 Å. There are two shorter (2.63 Å) and two longer (2.64 Å) Sr(2)-O(1) bond lengths. There are a spread of Sr(2)-O(3) bond distances ranging from 2.77-2.79 Å. In the third Sr site, Sr(3) is bonded in a 8-coordinate geometry to four equivalent O(1) and four equivalent O(4) atoms. There are two shorter (2.60 Å) and two longer (2.61 Å) Sr(3)-O(1) bond lengths. There are a spread of Sr(3)-O(4) bond distances ranging from 2.79-2.81 Å. Mg(1) is bonded in a 5-coordinate geometry to one O(4) and four equivalent O(2) atoms. The Mg(1)-O(4) bond length is 1.93 Å. There are two shorter (2.12 Å) and two longer (2.13 Å) Mg(1)-O(2) bond lengths. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(3), one O(4), and four equivalent O(1) atoms to form FeO6 octahedra that share corners with four equivalent Fe(1)O6 octahedra and  a cornercorner with one Sr(1)O5 square pyramid. The corner-sharing octahedral tilt angles are 2°. The Fe(1)-O(3) bond length is 2.18 Å. The Fe(1)-O(4) bond length is 2.29 Å. All Fe(1)-O(1) bond lengths are 1.94 Å. In the second Fe site, Fe(2) is bonded to one O(5) and four equivalent O(2) atoms to form distorted FeO5 trigonal bipyramids that share corners with four equivalent Sr(1)O5 square pyramids and corners with four equivalent Fe(2)O5 trigonal bipyramids. The Fe(2)-O(5) bond length is 1.91 Å. All Fe(2)-O(2) bond lengths are 2.02 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(2), two equivalent Sr(3), and two equivalent Fe(1) atoms to form distorted OSr4Fe2 octahedra that share corners with two equivalent O(1)Sr4Fe2 octahedra, corners with two equivalent O(5)Sr5Fe octahedra, corners with four equivalent O(3)Sr5Fe octahedra, edges with two equivalent O(1)Sr4Fe2 octahedra, faces with two equivalent O(3)Sr5Fe octahedra, and faces with four equivalent O(1)Sr4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 2-52°. In the second O site, O(2) is bonded in a distorted see-saw-like geometry to two equivalent Mg(1) and two equivalent Fe(2) atoms. In the third O site, O(3) is bonded to one Sr(1), four equivalent Sr(2), and one Fe(1) atom to form distorted OSr5Fe octahedra that share corners with four equivalent O(3)Sr5Fe octahedra, corners with eight equivalent O(1)Sr4Fe2 octahedra, edges with four equivalent O(3)Sr5Fe octahedra, edges with four equivalent O(5)Sr5Fe octahedra, and faces with four equivalent O(1)Sr4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 18-52°. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to four equivalent Sr(3), one Mg(1), and one Fe(1) atom. In the fifth O site, O(5) is bonded to one Sr(2), four equivalent Sr(1), and one Fe(2) atom to form OSr5Fe octahedra that share corners with four equivalent O(1)Sr4Fe2 octahedra, corners with four equivalent O(5)Sr5Fe octahedra, edges with four equivalent O(3)Sr5Fe octahedra, and edges with four equivalent O(5)Sr5Fe octahedra. The corner-sharing octahedral tilt angles range from 4-48°.