Ba4ZrNb9O18 crystallizes in the tetragonal I4/mmm space group. Ba(1) is bonded to two equivalent O(2), two equivalent O(3), four equivalent O(1), 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 two equivalent Zr(1)O6 octahedra, faces with two equivalent Nb(4)O6 octahedra, faces with two equivalent Nb(1)O5 square pyramids, and faces with two equivalent Nb(2)O5 square pyramids. Zr(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form ZrO6 octahedra that share corners with four equivalent Nb(4)O6 octahedra, corners with two equivalent Nb(1)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(2) and four equivalent O(5) atoms to form NbO5 square pyramids that share  a cornercorner with one Zr(1)O6 octahedra, corners with four equivalent Nb(2)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the second Nb site, Nb(2) is bonded to one O(3) and four equivalent O(5) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(4)O6 octahedra, corners with four equivalent Nb(1)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the third Nb site, Nb(3) is bonded in a square co-planar geometry to two equivalent O(4) and two equivalent O(5) atoms. In the fourth Nb site, Nb(4) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form NbO6 octahedra that share corners with four equivalent Zr(1)O6 octahedra, corners with two equivalent Nb(2)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. There are five inequivalent O sites. In the first O site, O(2) is bonded in a distorted linear geometry to four equivalent Ba(1), one Zr(1), and one Nb(1) atom. In the second O site, O(3) is bonded in a distorted linear geometry to four equivalent Ba(1), one Nb(2), and one Nb(4) atom. In the third O site, O(4) is bonded in a square co-planar geometry to four equivalent Nb(3) atoms. In the fourth O site, O(5) is bonded to two equivalent Ba(1), one Nb(1), one Nb(2), and one Nb(3) atom to form a mixture of distorted corner and edge-sharing OBa2Nb3 trigonal bipyramids. In the fifth O site, O(1) is bonded in a distorted linear geometry to four equivalent Ba(1), one Zr(1), and one Nb(4) atom.
Ba4ZrNb9O18 crystallizes in the tetragonal I4/mmm space group. Ba(1) is bonded to two equivalent O(2), two equivalent O(3), four equivalent O(1), 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 two equivalent Zr(1)O6 octahedra, faces with two equivalent Nb(4)O6 octahedra, faces with two equivalent Nb(1)O5 square pyramids, and faces with two equivalent Nb(2)O5 square pyramids. Both Ba(1)-O(2) bond lengths are 3.00 Å. Both Ba(1)-O(3) bond lengths are 3.00 Å. All Ba(1)-O(1) bond lengths are 2.99 Å. All Ba(1)-O(5) bond lengths are 2.91 Å. Zr(1) is bonded to two equivalent O(2) and four equivalent O(1) atoms to form ZrO6 octahedra that share corners with four equivalent Nb(4)O6 octahedra, corners with two equivalent Nb(1)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Zr(1)-O(2) bond lengths are 2.07 Å. All Zr(1)-O(1) bond lengths are 2.17 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(2) and four equivalent O(5) atoms to form NbO5 square pyramids that share  a cornercorner with one Zr(1)O6 octahedra, corners with four equivalent Nb(2)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. The Nb(1)-O(2) bond length is 2.14 Å. All Nb(1)-O(5) bond lengths are 2.12 Å. In the second Nb site, Nb(2) is bonded to one O(3) and four equivalent O(5) atoms to form NbO5 square pyramids that share  a cornercorner with one Nb(4)O6 octahedra, corners with four equivalent Nb(1)O5 square pyramids, and faces with four equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. The Nb(2)-O(3) bond length is 2.20 Å. All Nb(2)-O(5) bond lengths are 2.12 Å. In the third Nb site, Nb(3) is bonded in a square co-planar geometry to two equivalent O(4) and two equivalent O(5) atoms. Both Nb(3)-O(4) bond lengths are 2.12 Å. Both Nb(3)-O(5) bond lengths are 2.14 Å. In the fourth Nb site, Nb(4) is bonded to two equivalent O(3) and four equivalent O(1) atoms to form NbO6 octahedra that share corners with four equivalent Zr(1)O6 octahedra, corners with two equivalent Nb(2)O5 square pyramids, and faces with eight equivalent Ba(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Nb(4)-O(3) bond lengths are 2.01 Å. All Nb(4)-O(1) bond lengths are 2.07 Å. There are five inequivalent O sites. In the first O site, O(2) is bonded in a distorted linear geometry to four equivalent Ba(1), one Zr(1), and one Nb(1) atom. In the second O site, O(3) is bonded in a distorted linear geometry to four equivalent Ba(1), one Nb(2), and one Nb(4) atom. In the third O site, O(4) is bonded in a square co-planar geometry to four equivalent Nb(3) atoms. In the fourth O site, O(5) is bonded to two equivalent Ba(1), one Nb(1), one Nb(2), and one Nb(3) atom to form a mixture of distorted corner and edge-sharing OBa2Nb3 trigonal bipyramids. In the fifth O site, O(1) is bonded in a distorted linear geometry to four equivalent Ba(1), one Zr(1), and one Nb(4) atom.