Ca2Ni3O8 crystallizes in the hexagonal P6_3mc space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted CaO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, corners with three equivalent Ca(2)O4 tetrahedra, and edges with three equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. In the second Ca site, Ca(2) is bonded to one O(4) and three equivalent O(2) atoms to form CaO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and corners with nine equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 58-78°. 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 Ca(1)O6 octahedra, corners with three equivalent Ca(2)O4 tetrahedra,  an edgeedge with one Ca(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 Ca(1) and two equivalent Ni(1) atoms. In the second O site, O(2) is bonded to one Ca(1), one Ca(2), and two equivalent Ni(1) atoms to form distorted OCa2Ni2 tetrahedra that share  a cornercorner with one O(4)CaNi3 tetrahedra, corners with two equivalent O(2)Ca2Ni2 tetrahedra,  an edgeedge with one O(4)CaNi3 tetrahedra, and edges with two equivalent O(2)Ca2Ni2 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 Ca(2) and three equivalent Ni(1) atoms to form a mixture of distorted corner and edge-sharing OCaNi3 tetrahedra.
Ca2Ni3O8 crystallizes in the hexagonal P6_3mc space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted CaO6 octahedra that share corners with six equivalent Ni(1)O6 octahedra, corners with three equivalent Ca(2)O4 tetrahedra, and edges with three equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 53°. All Ca(1)-O(1) bond lengths are 2.26 Å. All Ca(1)-O(2) bond lengths are 2.40 Å. In the second Ca site, Ca(2) is bonded to one O(4) and three equivalent O(2) atoms to form CaO4 tetrahedra that share corners with three equivalent Ca(1)O6 octahedra and corners with nine equivalent Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 58-78°. The Ca(2)-O(4) bond length is 2.29 Å. All Ca(2)-O(2) bond lengths are 2.12 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 Ca(1)O6 octahedra, corners with three equivalent Ca(2)O4 tetrahedra,  an edgeedge with one Ca(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 1.92 Å. The Ni(1)-O(4) bond length is 1.95 Å. Both Ni(1)-O(1) bond lengths are 1.86 Å. Both Ni(1)-O(2) bond lengths are 1.89 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a trigonal planar geometry to one Ca(1) and two equivalent Ni(1) atoms. In the second O site, O(2) is bonded to one Ca(1), one Ca(2), and two equivalent Ni(1) atoms to form distorted OCa2Ni2 tetrahedra that share  a cornercorner with one O(4)CaNi3 tetrahedra, corners with two equivalent O(2)Ca2Ni2 tetrahedra,  an edgeedge with one O(4)CaNi3 tetrahedra, and edges with two equivalent O(2)Ca2Ni2 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 Ca(2) and three equivalent Ni(1) atoms to form a mixture of distorted corner and edge-sharing OCaNi3 tetrahedra.