Sr2MgFeO4 crystallizes in the monoclinic Cm space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(1) and four equivalent O(2) atoms to form SrO5 square pyramids that share corners with four equivalent Sr(1)O5 square pyramids,  a cornercorner with one Mg(1)O5 trigonal bipyramid, and edges with four equivalent Sr(1)O5 square pyramids. In the second Sr site, Sr(2) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. Mg(1) is bonded to one O(1) and four equivalent O(3) atoms to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Sr(1)O5 square pyramid and corners with four equivalent Mg(1)O5 trigonal bipyramids. Fe(1) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(3) atoms. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Sr(1), four equivalent Sr(2), and one Mg(1) atom to form distorted OSr5Mg octahedra that share corners with four equivalent O(1)Sr5Mg octahedra, edges with four equivalent O(2)Sr5Fe octahedra, and edges with four equivalent O(1)Sr5Mg octahedra. The corner-sharing octahedral tilt angles are 11°. In the second O site, O(2) is bonded to one Sr(2), four equivalent Sr(1), and one Fe(1) atom to form OSr5Fe octahedra that share corners with four equivalent O(2)Sr5Fe octahedra, edges with four equivalent O(2)Sr5Fe octahedra, and edges with four equivalent O(1)Sr5Mg octahedra. The corner-sharing octahedral tilt angles are 5°. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Mg(1) and two equivalent Fe(1) atoms.
Sr2MgFeO4 crystallizes in the monoclinic Cm space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(1) and four equivalent O(2) atoms to form SrO5 square pyramids that share corners with four equivalent Sr(1)O5 square pyramids,  a cornercorner with one Mg(1)O5 trigonal bipyramid, and edges with four equivalent Sr(1)O5 square pyramids. The Sr(1)-O(1) bond length is 2.35 Å. There are a spread of Sr(1)-O(2) bond distances ranging from 2.72-2.78 Å. In the second Sr site, Sr(2) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(1) atoms. The Sr(2)-O(2) bond length is 2.52 Å. There are a spread of Sr(2)-O(1) bond distances ranging from 2.75-2.77 Å. Mg(1) is bonded to one O(1) and four equivalent O(3) atoms to form distorted MgO5 trigonal bipyramids that share  a cornercorner with one Sr(1)O5 square pyramid and corners with four equivalent Mg(1)O5 trigonal bipyramids. The Mg(1)-O(1) bond length is 2.03 Å. All Mg(1)-O(3) bond lengths are 2.01 Å. Fe(1) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(3) atoms. The Fe(1)-O(2) bond length is 2.00 Å. There are two shorter (2.12 Å) and two longer (2.13 Å) Fe(1)-O(3) bond lengths. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Sr(1), four equivalent Sr(2), and one Mg(1) atom to form distorted OSr5Mg octahedra that share corners with four equivalent O(1)Sr5Mg octahedra, edges with four equivalent O(2)Sr5Fe octahedra, and edges with four equivalent O(1)Sr5Mg octahedra. The corner-sharing octahedral tilt angles are 11°. In the second O site, O(2) is bonded to one Sr(2), four equivalent Sr(1), and one Fe(1) atom to form OSr5Fe octahedra that share corners with four equivalent O(2)Sr5Fe octahedra, edges with four equivalent O(2)Sr5Fe octahedra, and edges with four equivalent O(1)Sr5Mg octahedra. The corner-sharing octahedral tilt angles are 5°. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Mg(1) and two equivalent Fe(1) atoms.