NaNi3O8 crystallizes in the monoclinic Cm space group. Na(1) is bonded to one O(1), one O(2), two equivalent O(3), two equivalent O(4), and six equivalent O(5) atoms to form NaO12 cuboctahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with eight equivalent Ni(2)O6 octahedra, edges with six equivalent Na(1)O12 cuboctahedra,  an edgeedge with one Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and  a faceface with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 25-49°. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share  a cornercorner with one Na(1)O12 cuboctahedra,  an edgeedge with one Na(1)O12 cuboctahedra, edges with six equivalent Ni(2)O6 octahedra, and  a faceface with one Na(1)O12 cuboctahedra. In the second Ni site, Ni(2) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with four equivalent Na(1)O12 cuboctahedra,  an edgeedge with one Na(1)O12 cuboctahedra, edges with three equivalent Ni(1)O6 octahedra, and edges with three equivalent Ni(2)O6 octahedra. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the fifth O site, O(5) is bonded in a trigonal planar geometry to three equivalent Na(1) atoms.
NaNi3O8 crystallizes in the monoclinic Cm space group. Na(1) is bonded to one O(1), one O(2), two equivalent O(3), two equivalent O(4), and six equivalent O(5) atoms to form NaO12 cuboctahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with eight equivalent Ni(2)O6 octahedra, edges with six equivalent Na(1)O12 cuboctahedra,  an edgeedge with one Ni(1)O6 octahedra, edges with two equivalent Ni(2)O6 octahedra, and  a faceface with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 25-49°. The Na(1)-O(1) bond length is 2.91 Å. The Na(1)-O(2) bond length is 2.90 Å. Both Na(1)-O(3) bond lengths are 2.96 Å. Both Na(1)-O(4) bond lengths are 2.96 Å. There are two shorter (2.77 Å) and four longer (2.78 Å) Na(1)-O(5) bond lengths. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share  a cornercorner with one Na(1)O12 cuboctahedra,  an edgeedge with one Na(1)O12 cuboctahedra, edges with six equivalent Ni(2)O6 octahedra, and  a faceface with one Na(1)O12 cuboctahedra. The Ni(1)-O(1) bond length is 1.87 Å. The Ni(1)-O(2) bond length is 1.88 Å. Both Ni(1)-O(3) bond lengths are 1.87 Å. Both Ni(1)-O(4) bond lengths are 1.87 Å. In the second Ni site, Ni(2) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with four equivalent Na(1)O12 cuboctahedra,  an edgeedge with one Na(1)O12 cuboctahedra, edges with three equivalent Ni(1)O6 octahedra, and edges with three equivalent Ni(2)O6 octahedra. The Ni(2)-O(1) bond length is 1.87 Å. The Ni(2)-O(2) bond length is 1.87 Å. Both Ni(2)-O(3) bond lengths are 1.87 Å. There is one shorter (1.87 Å) and one longer (1.88 Å) Ni(2)-O(4) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Na(1), one Ni(1), and two equivalent Ni(2) atoms. In the fifth O site, O(5) is bonded in a trigonal planar geometry to three equivalent Na(1) atoms.