Mg2Ni3O8 crystallizes in the hexagonal P6_3mc space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, corners with three equivalent Mg(2)O4 tetrahedra, and edges with three equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. In the second Mg site, Mg(2) is bonded to one O(4) and three equivalent O(2) atoms to form MgO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with nine equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-62°. Ni(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Mg(1) and two equivalent Ni(1) atoms. In the second O site, O(2) is bonded to one Mg(1), one Mg(2), and two equivalent Ni(1) atoms to form distorted OMg2Ni2 tetrahedra that share  a cornercorner with one O(4)MgNi3 tetrahedra, corners with two equivalent O(2)Mg2Ni2 tetrahedra,  an edgeedge with one O(4)MgNi3 tetrahedra, and edges with two equivalent O(2)Mg2Ni2 tetrahedra. In the third O site, O(3) is bonded in a distorted T-shaped geometry to three equivalent Ni(1) atoms. In the fourth O site, O(4) is bonded to one Mg(2) and three equivalent Ni(1) atoms to form a mixture of distorted edge and corner-sharing OMgNi3 tetrahedra.
Mg2Ni3O8 crystallizes in the hexagonal P6_3mc space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form MgO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, corners with three equivalent Mg(2)O4 tetrahedra, and edges with three equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. All Mg(1)-O(1) bond lengths are 2.05 Å. All Mg(1)-O(2) bond lengths are 2.10 Å. In the second Mg site, Mg(2) is bonded to one O(4) and three equivalent O(2) atoms to form MgO4 tetrahedra that share corners with three equivalent Mg(1)O6 octahedra and corners with nine equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 60-62°. The Mg(2)-O(4) bond length is 2.03 Å. All Mg(2)-O(2) bond lengths are 1.91 Å. Ni(1) is bonded to one O(3), one O(4), two equivalent O(1), and two equivalent O(2) atoms to form NiO6 octahedra that share corners with two equivalent Mg(1)O6 octahedra, corners with three equivalent Mg(2)O4 tetrahedra,  an edgeedge with one Mg(1)O6 octahedra, and edges with four equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. The Ni(1)-O(3) bond length is 1.89 Å. The Ni(1)-O(4) bond length is 1.94 Å. Both Ni(1)-O(1) bond lengths are 1.86 Å. Both Ni(1)-O(2) bond lengths are 1.90 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Mg(1) and two equivalent Ni(1) atoms. In the second O site, O(2) is bonded to one Mg(1), one Mg(2), and two equivalent Ni(1) atoms to form distorted OMg2Ni2 tetrahedra that share  a cornercorner with one O(4)MgNi3 tetrahedra, corners with two equivalent O(2)Mg2Ni2 tetrahedra,  an edgeedge with one O(4)MgNi3 tetrahedra, and edges with two equivalent O(2)Mg2Ni2 tetrahedra. In the third O site, O(3) is bonded in a distorted T-shaped geometry to three equivalent Ni(1) atoms. In the fourth O site, O(4) is bonded to one Mg(2) and three equivalent Ni(1) atoms to form a mixture of distorted edge and corner-sharing OMgNi3 tetrahedra.