MgFe4NiO8 is Spinel-derived structured and crystallizes in the trigonal R3m space group. Mg(1) is bonded to one O(3) and three equivalent O(4) atoms to form MgO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and corners with nine equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-59°. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Fe(2)O4 tetrahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. In the second Fe site, Fe(2) is bonded to one O(1) and three equivalent O(2) atoms to form FeO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and corners with nine equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-58°. Ni(1) is bonded to three equivalent O(2) and three equivalent O(4) atoms to form NiO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Fe(2)O4 tetrahedra, and edges with six equivalent Fe(1)O6 octahedra. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Fe(2) and three equivalent Fe(1) atoms to form a mixture of distorted corner and edge-sharing OFe4 trigonal pyramids. In the second O site, O(2) is bonded to one Fe(2), two equivalent Fe(1), and one Ni(1) atom to form distorted OFe3Ni trigonal pyramids that share  a cornercorner with one O(1)Fe4 trigonal pyramid, corners with two equivalent O(2)Fe3Ni trigonal pyramids,  an edgeedge with one O(1)Fe4 trigonal pyramid, and edges with two equivalent O(2)Fe3Ni trigonal pyramids. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1) and three equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), two equivalent Fe(1), and one Ni(1) atom.
MgFe4NiO8 is Spinel-derived structured and crystallizes in the trigonal R3m space group. Mg(1) is bonded to one O(3) and three equivalent O(4) atoms to form MgO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and corners with nine equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-59°. The Mg(1)-O(3) bond length is 2.00 Å. All Mg(1)-O(4) bond lengths are 1.98 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to one O(1), one O(3), two equivalent O(2), and two equivalent O(4) atoms to form FeO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Fe(2)O4 tetrahedra, edges with two equivalent Ni(1)O6 octahedra, and edges with four equivalent Fe(1)O6 octahedra. The Fe(1)-O(1) bond length is 2.09 Å. The Fe(1)-O(3) bond length is 2.04 Å. Both Fe(1)-O(2) bond lengths are 2.09 Å. Both Fe(1)-O(4) bond lengths are 2.01 Å. In the second Fe site, Fe(2) is bonded to one O(1) and three equivalent O(2) atoms to form FeO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and corners with nine equivalent Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-58°. The Fe(2)-O(1) bond length is 1.97 Å. All Fe(2)-O(2) bond lengths are 1.93 Å. Ni(1) is bonded to three equivalent O(2) and three equivalent O(4) atoms to form NiO6 octahedra that share corners with three equivalent Mg(1)O4 tetrahedra, corners with three equivalent Fe(2)O4 tetrahedra, and edges with six equivalent Fe(1)O6 octahedra. All Ni(1)-O(2) bond lengths are 2.11 Å. All Ni(1)-O(4) bond lengths are 2.08 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Fe(2) and three equivalent Fe(1) atoms to form a mixture of distorted corner and edge-sharing OFe4 trigonal pyramids. In the second O site, O(2) is bonded to one Fe(2), two equivalent Fe(1), and one Ni(1) atom to form distorted OFe3Ni trigonal pyramids that share  a cornercorner with one O(1)Fe4 trigonal pyramid, corners with two equivalent O(2)Fe3Ni trigonal pyramids,  an edgeedge with one O(1)Fe4 trigonal pyramid, and edges with two equivalent O(2)Fe3Ni trigonal pyramids. In the third O site, O(3) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1) and three equivalent Fe(1) atoms. In the fourth O site, O(4) is bonded in a rectangular see-saw-like geometry to one Mg(1), two equivalent Fe(1), and one Ni(1) atom.