Sr2MgMnO4 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 9-coordinate geometry to one O(2), four equivalent O(1), and four equivalent O(3) 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. Mn(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(1)Sr5Mg octahedra, and edges with four equivalent O(2)Sr5Mn 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 Mn(1) atom to form OSr5Mn octahedra that share corners with four equivalent O(2)Sr5Mn octahedra, edges with four equivalent O(1)Sr5Mg octahedra, and edges with four equivalent O(2)Sr5Mn octahedra. The corner-sharing octahedral tilt angles range from 3-4°. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Sr(2), two equivalent Mg(1), and two equivalent Mn(1) atoms.
Sr2MgMnO4 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 2.33 Å. There are a spread of Sr(1)-O(2) bond distances ranging from 2.74-2.79 Å. In the second Sr site, Sr(2) is bonded in a 9-coordinate geometry to one O(2), four equivalent O(1), and four equivalent O(3) 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 2.75-2.80 Å. There are two shorter (2.98 Å) and two longer (3.00 Å) Sr(2)-O(3) bond lengths. 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 2.02 Å. All Mg(1)-O(3) bond lengths are 2.02 Å. Mn(1) is bonded in a 5-coordinate geometry to one O(2) and four equivalent O(3) atoms. The Mn(1)-O(2) bond length is 2.10 Å. All Mn(1)-O(3) bond lengths are 2.14 Å. 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(1)Sr5Mg octahedra, and edges with four equivalent O(2)Sr5Mn 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 Mn(1) atom to form OSr5Mn octahedra that share corners with four equivalent O(2)Sr5Mn octahedra, edges with four equivalent O(1)Sr5Mg octahedra, and edges with four equivalent O(2)Sr5Mn octahedra. The corner-sharing octahedral tilt angles range from 3-4°. In the third O site, O(3) is bonded in a 4-coordinate geometry to two equivalent Sr(2), two equivalent Mg(1), and two equivalent Mn(1) atoms.