Ba2Ca3Tl2(NiO3)4 crystallizes in the tetragonal I4/mmm space group. Ba(1) is bonded in a 9-coordinate geometry to one O(3), four equivalent O(1), and four equivalent O(4) atoms. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a body-centered cubic geometry to four equivalent O(1) and four equivalent O(2) atoms. In the second Ca site, Ca(2) is bonded in a body-centered cubic geometry to eight equivalent O(2) atoms. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded in a rectangular see-saw-like geometry to four equivalent O(2) atoms. In the second Ni site, Ni(2) is bonded in a rectangular see-saw-like geometry to four equivalent O(1) atoms. Tl(1) is bonded to one O(4) and five equivalent O(3) atoms to form a mixture of distorted corner and edge-sharing TlO6 octahedra. The corner-sharing octahedral tilt angles are 9°. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), two equivalent Ca(1), and two equivalent Ni(2) atoms to form distorted OBa2Ca2Ni2 octahedra that share corners with two equivalent O(1)Ba2Ca2Ni2 octahedra, corners with two equivalent O(3)BaTl5 octahedra, corners with six equivalent O(2)Ca4Ni2 octahedra,  an edgeedge with one O(2)Ca4Ni2 octahedra, edges with two equivalent O(1)Ba2Ca2Ni2 octahedra, and faces with four equivalent O(1)Ba2Ca2Ni2 octahedra. The corner-sharing octahedral tilt angles range from 1-66°. In the second O site, O(2) is bonded to two equivalent Ca(1), two equivalent Ca(2), and two equivalent Ni(1) atoms to form OCa4Ni2 octahedra that share corners with six equivalent O(1)Ba2Ca2Ni2 octahedra, corners with eight equivalent O(2)Ca4Ni2 octahedra,  an edgeedge with one O(1)Ba2Ca2Ni2 octahedra, edges with three equivalent O(2)Ca4Ni2 octahedra, and faces with four equivalent O(2)Ca4Ni2 octahedra. The corner-sharing octahedral tilt angles range from 0-66°. In the third O site, O(3) is bonded to one Ba(1) and five equivalent Tl(1) atoms to form distorted OBaTl5 octahedra that share corners with four equivalent O(1)Ba2Ca2Ni2 octahedra, corners with four equivalent O(3)BaTl5 octahedra, and edges with eight equivalent O(3)BaTl5 octahedra. The corner-sharing octahedral tilt angles range from 9-43°. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to four equivalent Ba(1) and one Tl(1) atom.
Ba2Ca3Tl2(NiO3)4 crystallizes in the tetragonal I4/mmm space group. Ba(1) is bonded in a 9-coordinate geometry to one O(3), four equivalent O(1), and four equivalent O(4) atoms. The Ba(1)-O(3) bond length is 3.13 Å. All Ba(1)-O(1) bond lengths are 2.79 Å. All Ba(1)-O(4) bond lengths are 2.84 Å. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a body-centered cubic geometry to four equivalent O(1) and four equivalent O(2) atoms. All Ca(1)-O(1) bond lengths are 2.47 Å. All Ca(1)-O(2) bond lengths are 2.51 Å. In the second Ca site, Ca(2) is bonded in a body-centered cubic geometry to eight equivalent O(2) atoms. All Ca(2)-O(2) bond lengths are 2.50 Å. There are two inequivalent Ni sites. In the first Ni site, Ni(1) is bonded in a rectangular see-saw-like geometry to four equivalent O(2) atoms. All Ni(1)-O(2) bond lengths are 1.91 Å. In the second Ni site, Ni(2) is bonded in a rectangular see-saw-like geometry to four equivalent O(1) atoms. All Ni(2)-O(1) bond lengths are 1.91 Å. Tl(1) is bonded to one O(4) and five equivalent O(3) atoms to form a mixture of distorted corner and edge-sharing TlO6 octahedra. The corner-sharing octahedral tilt angles are 9°. The Tl(1)-O(4) bond length is 2.05 Å. There is one shorter (2.10 Å) and four longer (2.71 Å) Tl(1)-O(3) bond lengths. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), two equivalent Ca(1), and two equivalent Ni(2) atoms to form distorted OBa2Ca2Ni2 octahedra that share corners with two equivalent O(1)Ba2Ca2Ni2 octahedra, corners with two equivalent O(3)BaTl5 octahedra, corners with six equivalent O(2)Ca4Ni2 octahedra,  an edgeedge with one O(2)Ca4Ni2 octahedra, edges with two equivalent O(1)Ba2Ca2Ni2 octahedra, and faces with four equivalent O(1)Ba2Ca2Ni2 octahedra. The corner-sharing octahedral tilt angles range from 1-66°. In the second O site, O(2) is bonded to two equivalent Ca(1), two equivalent Ca(2), and two equivalent Ni(1) atoms to form OCa4Ni2 octahedra that share corners with six equivalent O(1)Ba2Ca2Ni2 octahedra, corners with eight equivalent O(2)Ca4Ni2 octahedra,  an edgeedge with one O(1)Ba2Ca2Ni2 octahedra, edges with three equivalent O(2)Ca4Ni2 octahedra, and faces with four equivalent O(2)Ca4Ni2 octahedra. The corner-sharing octahedral tilt angles range from 0-66°. In the third O site, O(3) is bonded to one Ba(1) and five equivalent Tl(1) atoms to form distorted OBaTl5 octahedra that share corners with four equivalent O(1)Ba2Ca2Ni2 octahedra, corners with four equivalent O(3)BaTl5 octahedra, and edges with eight equivalent O(3)BaTl5 octahedra. The corner-sharing octahedral tilt angles range from 9-43°. In the fourth O site, O(4) is bonded in a 5-coordinate geometry to four equivalent Ba(1) and one Tl(1) atom.