Ba4TiNb9O18 crystallizes in the tetragonal P4/mmm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1), four equivalent O(4), and four equivalent O(5) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Ba(1)O12 cuboctahedra, faces with five equivalent Ba(1)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O5 square pyramids. In the second Ba site, Ba(2) is bonded to four equivalent O(2), four equivalent O(3), and four equivalent O(6) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Ba(2)O12 cuboctahedra, faces with five equivalent Ba(2)O12 cuboctahedra, faces with four equivalent Ti(1)O6 octahedra, and faces with four equivalent Nb(4)O5 square pyramids. Ti(1) is bonded to two equivalent O(6) and four equivalent O(3) atoms to form TiO6 octahedra that share corners with four equivalent Ti(1)O6 octahedra, corners with two equivalent Nb(4)O5 square pyramids, and faces with eight equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a square co-planar geometry to one O(1), one O(2), and two equivalent O(7) atoms. In the second Nb site, Nb(2) is bonded to two equivalent O(5) and four equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, corners with two equivalent Nb(3)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the third Nb site, Nb(3) is bonded to one O(5) and four equivalent O(1) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(2)O6 octahedra, corners with four equivalent Nb(3)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the fourth Nb site, Nb(4) is bonded to one O(6) and four equivalent O(2) atoms to form NbO5 square pyramids that share  a cornercorner with one Ti(1)O6 octahedra, corners with four equivalent Nb(4)O5 square pyramids, and faces with four equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), one Nb(1), and two equivalent Nb(3) atoms to form a mixture of distorted edge and corner-sharing OBa2Nb3 trigonal bipyramids. In the second O site, O(2) is bonded in a distorted T-shaped geometry to two equivalent Ba(2), one Nb(1), and two equivalent Nb(4) atoms. In the third O site, O(3) is bonded to four equivalent Ba(2) and two equivalent Ti(1) atoms to form a mixture of distorted edge, corner, and face-sharing OBa4Ti2 octahedra. The corner-sharing octahedra are not tilted. In the fourth O site, O(4) is bonded in a distorted linear geometry to four equivalent Ba(1) and two equivalent Nb(2) atoms. In the fifth O site, O(5) is bonded in a distorted linear geometry to four equivalent Ba(1), one Nb(2), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to four equivalent Ba(2), one Ti(1), and one Nb(4) atom. In the seventh O site, O(7) is bonded in a rectangular see-saw-like geometry to four equivalent Nb(1) atoms.
Ba4TiNb9O18 crystallizes in the tetragonal P4/mmm space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded to four equivalent O(1), four equivalent O(4), and four equivalent O(5) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Ba(1)O12 cuboctahedra, faces with five equivalent Ba(1)O12 cuboctahedra, faces with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(3)O5 square pyramids. All Ba(1)-O(1) bond lengths are 2.89 Å. All Ba(1)-O(4) bond lengths are 2.96 Å. All Ba(1)-O(5) bond lengths are 2.98 Å. In the second Ba site, Ba(2) is bonded to four equivalent O(2), four equivalent O(3), and four equivalent O(6) atoms to form BaO12 cuboctahedra that share corners with eight equivalent Ba(2)O12 cuboctahedra, faces with five equivalent Ba(2)O12 cuboctahedra, faces with four equivalent Ti(1)O6 octahedra, and faces with four equivalent Nb(4)O5 square pyramids. All Ba(2)-O(2) bond lengths are 3.00 Å. All Ba(2)-O(3) bond lengths are 2.85 Å. All Ba(2)-O(6) bond lengths are 2.98 Å. Ti(1) is bonded to two equivalent O(6) and four equivalent O(3) atoms to form TiO6 octahedra that share corners with four equivalent Ti(1)O6 octahedra, corners with two equivalent Nb(4)O5 square pyramids, and faces with eight equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Ti(1)-O(6) bond lengths are 1.97 Å. All Ti(1)-O(3) bond lengths are 2.10 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a square co-planar geometry to one O(1), one O(2), and two equivalent O(7) atoms. The Nb(1)-O(1) bond length is 2.16 Å. The Nb(1)-O(2) bond length is 2.12 Å. Both Nb(1)-O(7) bond lengths are 2.11 Å. In the second Nb site, Nb(2) is bonded to two equivalent O(5) and four equivalent O(4) atoms to form NbO6 octahedra that share corners with four equivalent Nb(2)O6 octahedra, corners with two equivalent Nb(3)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Nb(2)-O(5) bond lengths are 2.01 Å. All Nb(2)-O(4) bond lengths are 2.10 Å. In the third Nb site, Nb(3) is bonded to one O(5) and four equivalent O(1) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(2)O6 octahedra, corners with four equivalent Nb(3)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. The Nb(3)-O(5) bond length is 2.17 Å. All Nb(3)-O(1) bond lengths are 2.11 Å. In the fourth Nb site, Nb(4) is bonded to one O(6) and four equivalent O(2) atoms to form NbO5 square pyramids that share  a cornercorner with one Ti(1)O6 octahedra, corners with four equivalent Nb(4)O5 square pyramids, and faces with four equivalent Ba(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. The Nb(4)-O(6) bond length is 2.14 Å. All Nb(4)-O(2) bond lengths are 2.11 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Ba(1), one Nb(1), and two equivalent Nb(3) atoms to form a mixture of distorted edge and corner-sharing OBa2Nb3 trigonal bipyramids. In the second O site, O(2) is bonded in a distorted T-shaped geometry to two equivalent Ba(2), one Nb(1), and two equivalent Nb(4) atoms. In the third O site, O(3) is bonded to four equivalent Ba(2) and two equivalent Ti(1) atoms to form a mixture of distorted edge, corner, and face-sharing OBa4Ti2 octahedra. The corner-sharing octahedra are not tilted. In the fourth O site, O(4) is bonded in a distorted linear geometry to four equivalent Ba(1) and two equivalent Nb(2) atoms. In the fifth O site, O(5) is bonded in a distorted linear geometry to four equivalent Ba(1), one Nb(2), and one Nb(3) atom. In the sixth O site, O(6) is bonded in a distorted linear geometry to four equivalent Ba(2), one Ti(1), and one Nb(4) atom. In the seventh O site, O(7) is bonded in a rectangular see-saw-like geometry to four equivalent Nb(1) atoms.